OCS Study 

BOEM 2014-011 

Coastal Marine Institute 

U.S. Department of the Interior 

Bureau of Ocean Energy Management 

Gulf of Mexico OCS Region

Cooperative Agreement 

Coastal Marine Institute 

Louisiana State University

Determining the Geographical Distribution,

and Genetic Affinities of Corals on Offshore

Platforms, Northern Gulf of Mexico 
OCS Study 

BOEM 2014-011 

Published by 

U.S. Department of the Interior 

Bureau of Ocean Energy Management 

Gulf of Mexico OCS Region

Cooperative Agreement 

Coastal Marine Institute 

Louisiana State University

Coastal Marine Institute

Determining the Geographical Distribution 

and Genetic Affinities of Corals on Offshore

Platforms, Northern Gulf of Mexico 

Author 

Paul W. Sammarco 

March 2014 

Prepared under BOEM Cooperative Agreement 

M05AZ10668 (32806-36188) 

by 

Louisiana Universities Marine Consortium

8124 Hwy. 56

Chauvin, Louisiana 70344 

iii

DISCLAIMER

This report was prepared under contract between the Bureau of Ocean Energy Management 

(BOEM), previously the Minerals Managements Service (MMS) and Louisiana State University.

This report has been technically reviewed by BOEM, and it has been approved for publication. 

Approval does not necessarily signify that the contents reflect the views and policies of BOEM,

nor does mention of trade names or commercial products constitute endorsement or 

recommendation for use.

REPORT AVAILABILITY

To download a PDF file of this Gulf of Mexico OCS Region report, go to the U.S. Department of 

the Interior, Bureau of Ocean Energy Management, Environmental Studies Program Information 

System website and search on OCS Study BOEM 2014-011.

This report can be viewed at select Federal Depository Libraries. It can also be obtained from 

the National Technical Information Service; the contact information is below.

U.S. Department of Commerce

National Technical Information Service

5301 Shawnee Rd.

Springfield, Virginia 22312

Phone: (703) 605-6000, 1(800)553-6847

Fax: (703) 605-6900

Website: http://www.ntis.gov/

CITATION

Sammarco, Paul W. 2014. Determining the geographical distribution and genetic affinities of 

corals on offshore platforms, Northern Gulf of Mexico. U.S. Dept. of the Interior, 

Bureau of Ocean Energy Management, Gulf of Mexico OCS Region, New Orleans, LA. 

OCS Study BOEM 2014-011. 75 pp.

ABOUT THE COVER

Cover photo by Greg Speyrer © 2011 Greg Speyrer. All rights reserved. Used with permission.

v

CONTRIBUTING AUTHORS

• Amy D. Atchison–Department of Oceanography and Coastal Science, Louisiana State 

University A&M College, Baton Rouge, Louisiana

• Gregory S. Boland–Environmental Section, Bureau of Ocean Energy Management, U.S.

Department of the Interior, Herndon, Virginia

• Daniel A. Brazeau–Pharmaceutical Genetics Laboratory, Department of Pharmaceutical 

Sciences, University of Buffalo, Buffalo, New York

• Angela Lirette–Louisiana Universities Marine Consortium (LUMCON), Chauvin, Louisiana

• Paul W. Sammarco–Louisiana Universities Marine Consortium (LUMCON), Chauvin, 

Louisiana

• Yuasung (Fiona) Tung–Louisiana Universities Marine Consortium (LUMCON), Chauvin, 

Louisiana

ACKNOWLEDGEMENTS

Many thanks to those who assisted us with diving, sample collection, topside, laboratory, and 

other support: A. Atchison, A. Barker, G. Bunch, C. Chauffe, J. Collins, J. Conklin, J. 

Culbertson, B. DeFelize, T. Dempre, D. Dowdy, K. de Musert, T. Empey, C. Gentry, R. Harris, 

D. Hill-House, C. Horrell, S. Kolian, G. Kozlowski, H. Leedy, L. Logan, D. Marcel, Lt. Col. D. 

Perrenod, G. Rainey, C. Salmonsen, T. Sebastian, J. Sinclair, G. Speyrer, J. Van der Ham, K. 

Wheelock, and D. Woodall. H. Boudreaux, W. Hebert, C. Luke, S. Parrott, and B. Bourge

assisted with accounting; J. Fontenot, A. Lirette, and F. Tung with data analysis; M. Dear of 

Aeigir, Inc. with ROV Operations; T. Bachmann, T. Berbner, T. Cothren, and L. Ong of the 

ADR German TV. We also thank the crew of the M/V Fling (Gulf Diving, Inc.): K. Bush, J. 

Byron, P. Combs, J. Denton, L. DeWeese, J. Dibble, D. Fanning, A. Felth, K. Foster, G. 

Harding, J. Heiser, Jason, S. Kerr, T. Kilpatrick, M. McReynolds, S. Richman, J. Smale, R. 

Travis, R. Widaman; and the oil, gas, and other companies and their staff who provided us access 

to their offshore platforms and other assistance: Anadarko Petroleum: J. Davidson, S. Hathcock, 

S. Jensen; Apache Oil: J. Bordelon, G. Thibodaux; Chevron Texaco Exploration and Production: 

D. Lucas, K. Mire, S. Ulm; Coastal Oil and Gas, Devon Oil: B. Gary, R. Hebert, V. Mile, B. 

Moody; Dominion Exploration and Production: B. Ventura, K. Schlogel, M. Sledge; El Paso 

Energy: C. Thornton, S. Lesiker; Forcenergy/Forest Oil: W. Fontenot, W. Myers, G. Ruiz; KerrMcGee Oil and Gas: C. Bradford; Merit Energy: T. Lambert, P. Maiberger; Newfield 

Exploration: A. Comeaux, E. Haas, M. Prosper, J. Zernell; Noble Energy: S. Shuman; Offshore 

Energy; UNOCAL: D. Crusan, M. Hebert; Samedan Oil: R. Bemis, S. Berryhill, S. Mulbert, T. 

Spintzenberger, P. Tullos; and W&T Offshore: S. Schroeder; United Space Alliance: A. Knight. 

We thank J. Benzie for suggesting the use of AFLPs on coral tissue for the molecular genetics. 

We also thank L. Rouse of the Louisiana State University Coastal Marine Institute and the U.S.

Department of Interior Bureau of Ocean Energy Management for their support and for funding 

this study under Cooperative Agreement #32806/TO#36188 to PWS.

vii

TABLE OF CONTENTS

LIST OF FIGURES ....................................................................................................................... ix

LIST OF TABLES......................................................................................................................... xi

ABBREVIATIONS, ACRONYMS, AND SYMBOLS .............................................................. xiii

EXECUTIVE SUMMARY ...........................................................................................................xv

1.0 INTRODUCTION....................................................................................................................1

1.1 Expansion of Coral Communities throughout the northern Gulf of Mexico 

through Offshore Oil and Gas Platforms.......................................................................1

1.1.1 General Background.........................................................................................1

1.1.2 Coral Larval Dispersal and Gene Flow ............................................................3

1.1.3 Objectives.........................................................................................................5

1.2 Genetic Connectivity in Corals across the Northern Gulf of Mexico, with 

Respect to Oil and Gas Platforms and the Flower Garden Banks .................................5

1.2.1 Background: Genetic Affinity in Coral Populations........................................5

1.2.2 Objectives.........................................................................................................8

1.3 Duration .........................................................................................................................9

2.0 MATERIALS AND METHODS ...........................................................................................11

2.1 Adult Coral Community Surveys on the Platforms.....................................................11

2.2 Technical Considerations and Alteration of Original Plan..........................................13

2.3 Determining Genetic Connectivity between Coral Populations on Platforms in 

the Northern Gulf of Mexico and on the Flower Garden Banks..................................13

2.3.1 Study Site .......................................................................................................13

2.3.2 Sample Collection ..........................................................................................14

2.3.3 Molecular Technique used to Determine Genetic Affinity, Amplified 

Fragment Length Polymorphism....................................................................14

2.3.4 Preparation of Coral Tissue Lysates for Genetic Analysis.............................15

2.3.5 Statistical Analyses.........................................................................................17

3.0 RESULTS...............................................................................................................................19

3.1 Adult Coral Communities on Platforms throughout the Northern Gulf of 

Mexico .........................................................................................................................19

3.1.1 Coral Species Composition ............................................................................19

3.1.2 Coral Species Richness: Number of Species..................................................19

3.1.3 Coral Distribution and Abundance.................................................................20

3.2 Genetic Connectivity in Corals in the Northern Gulf of Mexico.................................30

3.2.1 Relationship between Genetic Distance and Geographical Distance 

for Madracis decactis, Using STRUCTURE.................................................30

3.2.2 Relationship between Genetic Distance and Geographical Distance 

for Madracis decactis, Using AFLPOP .........................................................32
viii

3.2.3 Relationship between Genetic Distance and Geographical Distance 

for Tubastraea coccinea, Using STRUCTURE.............................................33

3.2.4 Relationship between Genetic Distance and Geographical Distance 

for Tubastraea coccinea, Using AFLPOP .....................................................34

4.0 DISCUSSION ........................................................................................................................37

4.1 Patterns in Coral Species Richness, Composition, and Abundance on the 

Northern GOM Shelf ...................................................................................................37

4.1.1 Hermatypic Corals..........................................................................................37

4.1.2 Ahermatypic Corals........................................................................................39

4.2 Genetic Affinities of Corals in the northern Gulf of Mexico.......................................39

5.0 CONCLUSIONS....................................................................................................................43

5.1 Adult Coral Communities on Platforms throughout the Northern Gulf of 

Mexico .........................................................................................................................43

5.2 Genetic Affinities of Corals in the Northern Gulf of Mexico......................................44

REFERENCES ..............................................................................................................................45
ix

LIST OF FIGURES

Figure 1. Map of the northern GOM, indicating locations of oil and gas platforms in 2003.. .. 3

Figure 2. Map depicting location of the East and West FGB, and other major banks on the 

continental shelf of the northern GOM (from Rezak et al., 1985).............................. 3

Figure 3. Map of the GOM depicting the locations of the FGB and their nearest 

neighboring reefs. Inset: The FGB and a sub-set of the numerous oil and gas 

production platforms around them (see Sammarco et al., 2004)................................ 7

Figure 4. Map of the oil and gas platforms in the northern GOM, indicating the four crossshelf transects. The oval represents the study area sampled for Sammarco et al., 

2004. ......................................................................................................................... 12

Figure 5. Species diversity of hermatypic scleractinian corals in number of species on oil 

and gas platforms in the GOM and on the FGB....................................................... 20

Figure 6. Total coral density of hermatypic scleractinian corals in no.per 1,000 m-2 on oil 

and gas platforms in the GOM.................................................................................. 21

Figure 7. Density of the hermatypic scleractinian coral Madracis decactis on oil and gas 

platforms in the northern GOM, shown in no. per 1,000 m-2. ................................. 22

Figure 8. Density of the hermatypic scleractinian coral Diploria strigosa on oil and gas 

platforms in the northern GOM. ............................................................................... 23

Figure 9. Density of the hermatypic scleractinian coral Montastraea cavernosa on oil and 

gas platforms in the northern GOM.......................................................................... 24

Figure10. Total coral density of all scleractinian corals, hermatypic and ahermatypic, on oil 

and gas platforms in the GOM.................................................................................. 25

Figure 11. Density of all ahermatypic scleractinian corals in the GOM.................................... 26

Figure 12. Coral diversity of ahermatypic corals in number of species on oil and gas 

platforms across the northern GOM. ........................................................................ 27

Figure 13. Density of Tubastraea coccinea, the dominant ahermatypic coral on oil and gas 

platforms in the northern GOM. ............................................................................... 28

Figure 14. Density of Oculina diffusa on oil and gas platforms in the northern GOM. ............ 29

Figure 15. Density of Phyllangia americana, on oil and gas platforms in the northern GOM. 30

Figure 16. Madracis decactis: Genetic affinity in coral populations on oil and gas platforms 

across the continental shelf in the northern GOM. ................................................... 31

Figure 17. Tubastraea coccinea: Genetic affinity in coral populations on oil and gas 

platforms across the continental shelf in the northern GOM.................................... 34

Figure 18. Map of the GOM, depicting examples of general currents known to exist. Note 

the general westerly current across the continental shelf in the vicinity of the 

FGB........................................................................................................................... 40

xi

LIST OF TABLES

Table 1. Details of the 41 oil and gas production platforms studied in the northern GOM 

along four cross-continental shelf transects from Matagorda Island, Texas to 

Mobile, Alabama, including the 14 sampled in a previous study............................. 13

Table 2 Sequences of the adapters and arimers used in the AFLP protocol.......................... 16

Table 3 Madracis Decactis: Genetic affinities in populations of corals on oil and gas 

platforms in the northern GOM along two transects on either side of the 

Mississippi River ...................................................................................................... 32

Table 4. Madracis Decactis: Genetic affinities in populations of corals on oil and gas 

platforms in the northern GOM along two transects on either side of the 

Mississippi River mouth ........................................................................................... 33

Table 5. Tubastraea coccinea: Genetic affinities in populations of corals on oil and gas 

platforms in the northern GOM along two transects on either side of the 

Mississippi River mouth ........................................................................................... 35

Table 6. Tubastraea Coccinea: Genetic affinities in populations of corals on oil and gas 

platforms in the northern GOM along two transects, on either side of the 

Mississippi River mouth ........................................................................................... 36

xiii

ABBREVIATIONS, ACRONYMS, AND SYMBOLS

AFLP Amplified Fragment Length Polymorphism

AFLPOP Amplified Fragment Length Polymorphism Population Allocation 

Analysis

AMOVA Analysis of Molecular Variance

ANOVA Analysis of Variance

BOEM

FGB

Bureau of Ocean Energy Management

Flower Garden Banks

GOM Gulf of Mexico

km kilometers

m meters

nmi nautical miles

ft feet

PCR

pg

s

SST

Polymerase Chain Reaction

picograms

second(s)

Sea surface temperature

STRUCTURE

x g

Molecular genetic statistical analysis which analyzes for population 

genetic structure

times gravity

xv

EXECUTIVE SUMMARY

Cross-Shelf Platform Coral Study

There are approximately 3,200 oil and gas platforms in the northern Gulf of Mexico (GOM). 

These platforms provide hard substratum that extends from the bottom of the continental shelf up 

through the sea surface, in a region where such substratum has been rare in recent geological 

time. Major exceptions to this are the Flower Garden Banks (FGB), coral reefs which occur ~190

km S-SE of Galveston, Texas. Our previous Phase 1 study of 13 platforms surrounding the FGB 

provided evidence that suggests that the presence of these platforms were associated with 

substantial amounts of Caribbean reef fauna (Sammarco et al., 2004).

In the current study, we have determined the range of occurrence of corals on oil and gas 

platforms in the northern GOM, their distribution and abundance over this broad geographic 

range, and their species richness patterns. Thirty-five platforms along four transects, spanning 

from 20 km offshore to the edge of the continental shelf or beyond, were examined (Figure 4). 

Transect I was in the Matagorda Island (MI) lease area, extending S-SE from Corpus Christi, 

Texas; Transect II bisected the High Island (HI) lease area, running S from Lake Sabine, Texas 

to 10 km beyond the shelf edge; Transect III ran across the South Timbalier (ST) lease area, S of 

Terrebonne Bay, Louisiana; and Transect IV extended across the Main Pass (MP) lease area, SSW from Mobile, Alabama. Transect II was closest to the FGB. Coral density and species 

richness data were also included from our previous Phase 1 study (Sammarco et al., 2004), 

encompassing an additional 13 platforms in the FGB area to provide as complete a picture as 

possible of these variables in the northern GOM. Visual surveys of corals were conducted down 

to a maximum depth of 37 m by teams of SCUBA divers. We attempted to determine whether 

extensive scleractinian coral populations have colonized these platforms by quantifying the 

distribution and abundance of scleractinian corals (hermatypic and ahermatypic) and determining

population characteristics.

Corals occurred on many of the platforms in the northern GOM. Coral species richness (number 

of species) in the northern GOM peaked on the coral reefs of the FGB and was three times higher 

than the maximum species richness observed on any individual oil and gas platform. Coral 

species richness was very low on inshore platforms. Twelve species of coral were found, 

encompassing nine hermatypic corals and three ahermatypic corals. In order of abundance, the 

most abundant zooxanthellate hermatypic corals found on platforms were Madracis decactis, 

Diploria strigosa, and Montastraea cavernosa. The other hermatypes observed on platforms 

were Porites astreoides, Madracis formosa, Colpophyllia natans, Stephanocoenia intercepta, 

Stephanocoenia michelinii, and Millepora alcicornis. All were members of the Scleractinia, 

except for Millepora, which is a member of the Hydrozoa. The three ahermatypic corals found 

on platforms were Tubastraea coccinea, Oculina diffusa, and Phyllangia americana. All of 

these three were scleractinians. T. coccinea was the most abundant, by far.

The total density of hermatypic/zooxanthellate corals peaked on platforms in Transect III, south

of Terrebonne Bay, Louisiana. These high densities extended to the north, toward shore in this 

region. Higher densities were also noted in the vicinity of the FGB, in Transect II. Coral 

densities on platforms dropped to low levels outside of this region. In no case were hermatypic 

corals observed within ~50km of shore. Densities of M. decactis were found to drive overall 

density of hermatypic corals, peaking on platforms in the north central GOM at the edge of the 
xvi

continental shelf, south of Terrebonne Bay, Louisiana, in Transect III. Densities of D. strigosa, 

the second dominant in the platform coral community, peaked around the FGB. Its densities 

extended northward towards Port Arthur-Lake Sabine (Transect II). Densities of M. cavernosa

were bimodal, with a higher peak near the FGB (Transect II) and a lower peak on east side of the 

Mississippi River mouth off Mobile, Alabama in Transect IV.

Density of all corals combined, including hermatypic/zooxanthellate and ahermatypic/ 

azooxanthellate types, was four orders of magnitude higher than density of hermatypes alone. 

This density peaked on platforms in the central and eastern regions of the northern GOM, with 

tens of thousands of colonies occurring south of Terrebonne Bay, Louisiana and Mobile, 

Alabama (Transects III and IV). This density pattern was driven entirely by 

ahermatypic/azooxanthellate corals, which far surpassed hermatypic corals in density. The 

pattern of total ahermatypic/ azooxanthellate corals in the northern GOM was driven entirely by 

densities of T. coccinea. Densities of this species were tens of thousands per 1,000 m2

.

Species richness (S, number of species) of ahermatypic corals was highest in the far western 

sector of the northern GOM, south of Matagorda Island, Texas (Transect I). In three of the 

transect areas, unlike the hermatypes, species richness peaked in mid-shelf and inshore areas. 

The peak in Transect I was further offshore. Richness generally decreased from west to east. The 

peak in Transect I indicated that the FGB were most likely not the source of ahermatypic corals 

in the northern GOM and that the ahermatypic corals are most likely derived from the southern 

GOM off Mexico or from the Caribbean.

Molecular Genetics Study

A previous study of 13 platforms surrounding the FGB (Sammarco et al. 2004) demonstrated 

that the presence of these platforms has facilitated the biogeographic extension of Caribbean reef 

fauna. However, there was a question about the origin of those corals and their genetic affinity 

with coral populations on the platforms and the FGB (Atchison et al. 2008). Earlier studies 

suggested that hermatypic coral populations were highly independent, most likely due to 

Founder Effect (Futuyma, 1998). Patterns of genetic affinity among coral populations 

throughout the entire northern GOM were not known. Here, we ask, “What are the patterns of 

genetic affinity in these populations for both hermatypic and ahermatypic corals?”

To determine the genetic affinity between coral populations across the northern GOM, we 

sampled coral tissue from populations of two species occurring on oil and gas platforms there: 

one hermatype, M. decactis, and one ahermatype, T. coccinea. The same transects used for the 

cross-shelf platform coral study were examined for the molecular genetics study (Figure 4; see 

section 2.1 for a description of the transects). Samples of M. decactis were taken from every 

colony encountered by teams of SCUBA divers, up to 30 samples per platform; thus, the entire 

population was sampled between depths of 8 m and 37 m. Because of the extraordinarily high 

abundances of T. coccinea, populations of this species were sub-sampled. Divers collected 

samples haphazardly, placed them in pre-labeled ZipLock® bags, and recorded the platform and 

depth from which the collection was made. The bags were returned to the support vessel and 

logged. A minimum of 30 samples of T. coccinea was collected from each platform.
xvii

Two cm2 tissue samples were preserved in SED buffer (saturated NaCl, 250 mM EDTA, pH 7.5, 

20% DMSO) to preserve the DNA and were returned to the laboratory for processing. Genetic 

variation was assessed using the DNA replication/polymerase chain reaction (PCR) technique of 

Amplified Fragment Length Polymorphisms (AFLPs). This is a DNA-fingerprinting technique 

that detects polymorphisms based upon the selective PCR amplification of a subset of numerous 

restriction fragments. These fragments are generated by two different restriction enzymes.

All samples were checked for zooxanthellar DNA contamination using PCR techniques 

developed by laboratories at the University of Buffalo and LUMCON (Brazeau et al. 2005; 

Atchison et al. 2005, 2008; Sammarco et al. 2012). Extra caution was taken in processing 

samples through all procedural steps to maximize repeatability of results. Samples were 

processed in large lots containing members from all populations to distribute any error 

potentially introduced by reaction conditions uniformly between populations in an unbiased 

fashion. All PCR reactions were run using one machine and the same thermal cycle profiles. The 

large number of polymorphic markers generated by AFLPs allowed for the use of highly 

sensitive statistical analytical techniques. We used the population allocation techniques AFLPOP 

and STRUCTURE.

STRUCTURE revealed that genetic affinity between M. decactis populations on the platforms 

was highest south of Port Arthur-Lake Sabine, Texas (Transect II), which was closest to the 

FGB, and decreased primarily with distance to the east. A slight increase in genetic affinity in the 

eastern sector (Transect IV) indicated some isolation between the populations from different 

sides of the Mississippi River (Transects III and IV).

STRUCTURE also revealed a similar genetic affinity pattern between populations of T.

coccinea, which was highest in the west, off Matagorda Island, Texas (Transect I). Genetic 

affinity generally decreased to the east, although only slightly. A precipitous drop in genetic 

affinity off of Mobile, Alabama indicated a dramatic difference in populations between that area 

and other populations to the west of the Mississippi River, indicating that this hydrographic 

feature most likely represents a formidable barrier to coral larval dispersal in this region.

STRUCTURE identified a “point-drop” in genetic affinity for both M. decactis and T. coccinea

off Terrebonne Bay, Louisiana. The point-drop was more pronounced for M. decactis than for T. 

coccinea, but indicated that a population very different from the others in the northern GOM

exists here. It is possible that these populations could have been seeded from sources other than 

the FGB, possibly by the Loop Current derived from the Caribbean Current, or a jet current 

originating from the southern GOM.

AFLPOP analyses revealed that M. decactis populations off Terrebonne Bay, Louisiana and off 

Mobile, Alabama were almost 100% distinct from each other, exhibiting almost no crosspopulation recognition. Home population recognition on platforms within a transect was 

extremely high, but cross-site recognition between transects on either side of the Mississippi 

River was extremely low.

These results indicate that larval dispersal across the river mouth, and even between populations 

on platforms within a transect on either side of the river, was highly limited. This same trend of 
xviii

no population recognition across the river mouth was even stronger in T.coccinea. However, 

self-allocation to home sites on platforms was more highly variable, and cross-site recognition 

between the platforms was higher than in M. decactis. This indicates that T. coccinea has higher 

effective larval dispersal and recruitment capabilities than M. decactis and therefore exhibits 

greater gene flow.
1

1.0 INTRODUCTION

1.1 EXPANSION OF CORAL COMMUNITIES THROUGHOUT THE NORTHERN GULF 

OF MEXICO THROUGH OFFSHORE OIL AND GAS PLATFORMS

1.1.1 General Background

Since the 1940s, approximately 40,000 oil and gas wells have been drilled in the northern GOM

(Francois, 1993). There were 3,600 production platform structures servicing these wells in 2003, 

(Francois, 1993, Knott, 1995, Dauterive, 2000; Figure 1), and ~3,200 platforms remaining in 

2011 (USDOI, BOEM, 2012). These platforms extend from the bottom of the continental shelf

through the water’s surface, and provide hard substratum for colonization by a variety of reef 

organisms (Gallaway and Lewbel, 1981; Driessen, 1989; Bright et al., 1991; Adams, 1996; 

Boland, 2002; K. Deslarzes, pers. comm.). The northern GOM has historically possessed little 

hard substratum in shallow water in recent geological time (Curray, 1965a,b; Blum et al., 2001; 

Frost, 1977, Schroeder et al., 1995, Blum et al., 1998). The Flower Garden Banks (FGB) is an 

important exception to this rule, because they have developed on top of large salt diapirs (Gross 

and Gross, 1995), rising to ~18 m of the water’s surface. They possess a well-developed set of 

coral reefs, the only ones in the northern GOM (Rezak et al., 1985, Bright et al., 1992, Gittings,

1992, 1998; Sammarco et al., 2002, Sammarco and Atchison, 2002, 2003).

The FGB are considered to be among the most isolated coral reefs in the western Atlantic 

(Bright, 1981; Rezak et al,. 1985, Snell et al., 1998; Sammarco et al., 2004). They occur ~110 

nmi south-southwest of Galveston, Texas (Rezak et al., 1985) and have a healthy coral 

community, as has been inferred from observations of high coral cover (Aronson et al., 2005; 

Precht et al., 2008), mass coral spawnings (Bright et al., 1992; Gittings et al., 1992), and high 

recruitment levels (Gittings, 1992; Sammarco and Brazeau, 2001; Brazeau et al., 2011

Sammarco et al., 2012b).

There are 37 named banks protected by BOEM on the continental shelf of the northern GOM 

(Hickerson et al., 2006; Schmahl and Hickerson, 2006), including the FGB (Figure 2). Most 

banks, other than the FGB, occurring offshore in “blue water” (e.g., Rankin-1, Rankin-2, Bright, 

Geyer, Elvers, Claypile, etc; Rezak et al. 1985, Lugo-Fernandez et al., 2001, G. Boland, pers. 

obs.) are too deep or lack other appropriate environmental conditions for development of true 

coral reefs (actively growing reefs built entirely of calcium-carbonate, secreted by hermatypic 

corals). However, some banks do possess populations of reef-building corals, including Stetson, 

Bright, Sonnier and MacNeil Banks (Schmahl et al., 2005; Hickerson et al., 2006).

Sea level in the GOM has been variable throughout recent geological history (Curray, 1965 a,b;

Blum et al., 2001; Frost, 1977, Schroeder et al., 1995, Blum et al., 1998). When sea level was 30 

m below the present level, during the late Pleistocene and the Holocene epochs, banks known to 

exist now may well have supported coral reefs. It is possible that the northern GOM supported 

dozens of reefs at the edge of the continental shelf since that time (Rezak et al., 1985).

The FGB now share the northern GOM with thousands of oil and gas production platforms, 

providing thousands of artificial islands with substrate suitable for settlement that otherwise 

would not exist. Over a period of decades, benthic communities, including hermatypic 
2

scleractinian corals, have developed on these platforms (Bright et al., 1991; Sammarco et al., 

2004; G. Boland, pers. comm., unpub. data; K. Deslarzes, pers. comm.).

There is no question now that these oil and gas platforms harbor hermatypic scleractinian corals 

(Bright et al, 1991; Boland, 2002; Sammarco, 2002a,b, 2003, 2005; Sammarco et al., 2004; K. 

Deslarzes, pers. comm.; pers. obs.). In fact, it is possible that these platforms have increased the 

stability of the coral community on the Banks and on the platforms themselves in this region 

(Sammarco et al., 2004; Atchison et al., 2006, 2008). Observations of scleractinian corals on 

platforms were originally made in the 1980s (T. Bright, Q. Dokken) and 1990s (Bright et al.,

1991, Boland 2002, K. Deslarzes, pers. comm.; pers. obs. all authors). Quantitative data were 

collected in later studies (Sammarco et al., 2004). Platforms have clearly assisted in the 

biogeographic expansion of corals in the northern GOM (Sammarco et al., 2004; Atchison et al., 

2008; Brazeau et al., 2011). Population expansions of this sort have been well-documented in 

both terrestrial and marine environments, by “leap-frogging” or “stepping stone” mechanisms 

(Elton, 1958, Futuyma, 1998; Gold et al., 2001). A good example is the expansion of reef fish 

populations in association with the introduction of oil and gas platforms (Shinn, 1973; Winfield,

1973; Sonnier et al., 1976, Boland et al., 1983, Shinn and Wicklund, 1989, Love et al., 2000, 

Pattengill et al., 1997, Rooker et al., 1997, Childs, 1998; Schroeder et al., 2000).

Coral reefs are experiencing a severe decline in health globally. Mass coral mortalities caused by 

bleaching, disease, poor fishing techniques, nutrient enrichment, etc. (Sammarco, 1996, Souter 

and Linden, 2000, Hughes et al., 2003, McClanahan et al., 2008, Sammarco, 2009, Sammarco 

and Strychar, 2009, 2010, Strychar and Sammarco, 2010). We now have evidence that 

demonstrates that at least in one system, the GOM, coral populations are expanding their 

distributions through the colonization of oil and gas platforms.
3

Figure 1. Map of the northern GOM, indicating locations of oil and gas platforms in 2003.

Figure 2. Map depicting location of the East and West FGB, and other major banks on the continental 

shelf of the northern GOM (from Rezak et al., 1985).

1.1.2 Coral Larval Dispersal and Gene Flow

Exchange of coral larvae would probably have been relatively high in the late Pleistocene and 

Holocene epochs because of the broad distribution of the many shallow, submerged banks in the 
4

northern GOM at that time. Original expansion of these coral populations would most likely 

have followed the “Island Model” of gene flow (one way gene flow from a large continental

“mainland” population to smaller island populations; see Futuyma, 1998 for description). As sea 

level rose over the next 6,000–10,000 years, the reefs drowned. The FGB became two of the few 

true coral reefs in the northern GOM remaining. Some of the shallowest reefs nearest to the 

FGB are those off the northern coast of Yucatan Peninsula (Alacran and associated reefs, 

650 km); Tampico, Mexico (700 km); and the Florida Keys (1,090 km).

Adult corals are sessile; therefore, colonization of remote habitats such as the FGB by corals is 

achieved by larval dispersal (Sammarco, 1994). Because the FGB are isolated by hundreds to 

thousands of kilometers from neighboring coral reefs (Rezak et al., 1985), the probability of 

colonization by larvae from neighboring reefs is small (Sammarco and Brazeau, 2001). Coral 

larvae can travel hundreds and possibly thousands of kilometers to successfully settle on remote 

reefs (Richmond, 1981, 1987), but the probability of actually reaching a distant target site and of 

settling successfully there is low. A single platform in 33 m of water can present ~8,100 sq m of 

hard surface to the surrounding waters (Shinn, 1974). In the absence of these platforms in the 

northern GOM, survivorship of coral larvae derived from the FGB or other reefs in the region 

would be lower due to larval mortality after extensive dispersal, attempts to settle on soft bottom, 

or being transported into nearshore habitats unsuitable for settlement (low salinity, low 

temperature, high turbidity, etc.).

Gittings (1992) conjectured that because of self-seeding of coral larvae within the Bank system, 

the FGB were self-sustaining. Sammarco and Brazeau (2001), Atchison (2003, 2005, 2008), 

Brazeau et al. (2005, 2011), Atchison et al. (2008), and Sammarco et al. (2012) have presented 

molecular genetic data to support this hypothesis, suggesting that self-seeding is dominant there

at multiple spatial scales. Brazeau et al. (2005) presented evidence for self-seeding based on 

comparisons between adult populations of Agaricia agaricites from the FGB, the Florida Keys, 

and the Bahamas, and also using newly-settled spat collected from the FGB (Brazeau et al. 2008, 

2011). We have found similar results by examining 13 platforms for corals and attempting to 

determine genetic population structure and genetic affinities in Madracis decactis, Diploria 

strigosa, and Montastraea cavernosa from the FGB and those platforms bearing these species 

within a 45 km radius of that site (Atchison et al., 2008; Brazeau et al., 2008). Lugo-Fernandez 

(1998; Lugo-Fernandez et al., 2001) has described meso-scale circulation patterns near the FGB 

which could support self-seeding there (also see Sammarco and Heron, 1994 for a general 

discussion).

External sources for seeding the FGB have yet to be confirmed. Larvae may be derived from the

Tampico region or Bahia de Campeche, Mexico (Salas-de-Leon et al., 1998) by transportation by 

the Western Boundary Current (Vidal et al., 1999); or possibly from the Cozumel and Cancun 

regions by transportation north by the Caribbean Current into the GOM or by jets traveling north 

from the Alacran region and then west by the Loop Current (Sturges and Blama, 1976, Hamilton 

et al., 1999; Lugo-Fernandez and Gravois, 2010). It is also possible that currents may transport 

larvae from the Florida Keys, including the Tortugas Bank and Pulley Ridge (Jarrett et al., 2000; 

also see Meyers et al., 2001) to the western GOM (see Section 4.1.1 below for discussion and 

Figure 18 for graphic representation of currents). However, our data on the molecular genetics 

of coral spat at the FGB suggest that corals are not seeded from the eastern GOM (Brazeau et al., 
5

2005). Our earlier studies indicated that the FGB spat are 100% differentiable from adults from 

the Bahamas and Florida Keys.

A high degree of self-seeding is not necessarily a sign of long-term ecosystem stability, for selfseeding carries with it risk. For example, in the late 1970s and early 1980s, reefs of the Pacific 

coast of Panama–well isolated from those of the central and western Pacific–suffered mass coral 

mortality because of bleaching caused by El Niño and associated high sea surface temperatures 

(SSTs) (Glynn, 1994). Two hydrocoral species were driven locally extinct (Glynn and de 

Weerdt, 1991). Re-seeding of these communities must come from neighboring communities and 

may require long periods of time, particularly if remote. Time required for population 

regeneration is determined, in large part, by the density of adults. It is also determined by the 

balance between cross-seeding (connectedness between coral communities) and self-seeding.

1.1.3 Objectives

The objectives of this study were as follows:

• To determine the extent to which coral communities have developed on oil 

and gas platforms in the northern GOM;

• To determine population densities of the coral species on those platforms;

• To determine the geographic pattern of distribution of those populations; and

• To examine patterns of coral species richness on platforms along and across 

the continental shelf of the northern GOM.

1.2 GENETIC CONNECTIVITY IN CORALS ACROSS THE NORTHERN GULF OF 

MEXICO, WITH RESPECT TO OIL AND GAS PLATFORMS AND THE FLOWER 

GARDEN BANKS

1.2.1 Background: Genetic Affinity in Coral Populations

Before the 1940s, the bottom of the GOM was characterized by terrigenous, sandy muds with 

little habitat diversity (Rezak et al. 1983, Scarborough-Bull 1989). During that decade, offshore 

drilling for oil and gas began and the number of production platforms increased steadily, 

spreading southward across the continental shelf of the GOM. Those platforms served as 

substrate for colonization of numerous marine organisms, and this has been continuing. The 

platforms extend up from the seafloor through the euphotic zone and above the water’s surface, 

creating “islands” and providing hard substrate in open water (Shinn, 1974) that would 

otherwise not be available to benthic, demersal, and pelagic marine organisms. It has been 

estimated that a 200 ft. platform jacket can provide acres of hard substrate, supporting algae, 

barnacles, mussels, and other sessile epibenthic invertebrates (Driessen, 1989; ScarboroughBull, 1989). In earlier studies, we and others have documented the presence of both hermatypic 

(zooxanthellate, reef-building) and ahermatypic (azooxanthellate, non-reef-building) 

scleractinian corals on many of these platforms (Sammarco et al., 2002, 2003, 2004, 2006; 

Sammarco et al., 2004; also see Bright et al., 1991).
6

The only true coral reefs in the northern GOM are the FGB NOAA Flower Garden Banks 

National Marine Sanctuary (Dokken et al., 2002), located ~190 km SE of Galveston, Texas. 

These banks are now surrounded by oil and gas platforms. The FGB are defined by two banks: 

the East Bank (27o54’32” N, 93o36’ W) and the West Bank (27o52’27” N, 93o48’47” W; Bright 

et al., 1984; Rezak et al., 1983; see Figure 3). They approach the water’s surface to within 18m 

(Lugo-Fernandez et al., 2001). Calcium carbonate reefs have developed on the caps of the FGB 

(Bright et al., 1984; Dokken et al., 1999) which are productive (Rezak et al., 1985) and healthy, 

being characterized by 24 species of hermatypic corals (Bright et al,. 1984; Gittings et al., 1992, 

Lugo-Fernandez et al. 2001; Dokken et al., 2002). The closest reefs to the FGB are the LobosTuxpan system that are located 13 km off Cabo Rojo, Mexico, > 640 km away (Hagman et al.,

1998; Dokken et al., 2002; Sammarco et al., 2004; Figure 2). Other banks do exist on the 

northwestern GOM shelf, such as Stetson, Sonnier, 32 Fathom, etc. (Figure 2), but they do not 

qualify as true coral reefs because they are not biogenic in origin (i.e., composed of calcium 

carbonate that has been accreted by corals). Although corals exist there (Schmahl et al., 2005; 

Hickerson et al., 2006), they occur in deeper waters and are responsible for only a low percentcover of the bottom (Rezak et al., 1985; Lugo-Fernandez et al., 2001; Schmahl, 2003; Sammarco 

et al., 2004; G. Boland, pers. obs.). It is possible that coral populations on the deeper banks 

could be a source of larvae for platform colonization, but the densities of corals are much lower 

than those of the FGB; and thus their potential as a larval source for recruitment on platforms in 

the region would be comparatively low.

Here, we focused on one hermatypic and one ahermatypic scleractinian coral species which 

occur on the platforms in the northern GOM and also on the FGB. We attempted to determine 

the degree of genetic connectedness among the natural and platform populations on a large 

geographic scale, covering most of the northern GOM. Through earlier surveys, we found that 

two species were abundant enough to provide sample sizes sufficient for meaningful comparative 

molecular genetic analyses. The corals we studied were M. decactis (Lyman, 1859; 

Pocilloporidae; hermatype) and Tubastraea coccinea (Lesson, 1829; Dendrophylliidae; 

ahermatype). Both of these corals reproduce by brooding. M.decactis is a simultaneous

hermaphrodite and planulates monthly from March to December with a peak occurring from 

Sept. to Nov. (Vermeij et al., 2003). T. coccinea generally produces planulae sexually (Glynn et 

al., 2008) but can also produce them asexually (Ayre and Resing, 1986; Shearer, 2008). In 

addition, T. coccinea is highly effective at producing new colonies asexually through the 

formation of “runners” (Vermeij, 2005; Pagad, 2007).
7

Figure 3. Map of the GOM depicting the locations of the FGB and their nearest neighboring reefs. Inset: 

The FGB and a sub-set of the numerous oil and gas production platforms around them (see 

Sammarco et al., 2004).

Regarding reproduction in these two coral species, fully-developed larvae derived from brooding 

corals have the capability to settle in ≥ 4 hours (Harrison and Wallace, 1990). Such species can 

become reproductively mature at ages of 1–2 years. Brooders generally planulate on a monthly 

cycle and can release larvae up to 8–10 times per year (e.g., Porites astreoides; McGuire, 1998). 

Planulae are released during a spawning event that may extend over a period of days. Data from 

some coral studies suggest that brooders are adapted for short-distance dispersal, while broadcast 

spawners (release of sperm and eggs into the water column for fertilization and larval 

development) are more effective at longer-distance dispersal (Baums et al., 2005). Nevertheless, 

the potential for long-range dispersal between planulae produced by these two types of corals is 

most likely comparable once the planulae have become fully developed (Sammarco and

Andrews 1989, Sammarco 1994).

In the western Atlantic, T. coccinea is considered to be one of the most successful species to 

invade the Atlantic Ocean from the Indo-Pacific and is the single, most successful invasive coral 

in this ocean. The only other known invasive corals of species in the Caribbean are Fungia 

scutaria (Lajeunesse et al., 2005; J.C. Lang, pers. comm.; P.W. Sammarco, pers. obs. 1973) and 
8

Tubastraea micranthus (Ehrenberg, 1834), a much more recent introduction to the region just 

south of the Mississippi River mouth (the Grand Isle [GI] lease area; Sammarco et al., 2010). 

Figueira de Paula and Creed (2004) have also reported the introduction of Tubastraea tagusensis

to Brazil, along with that of T. coccinea. Thus, the total number of introduced species to the 

western Atlantic Ocean is now four.

T. coccinea by far represents the most successful invasion. It was first recorded in Puerto Rico 

in 1943 and then in Curacao, Netherlands Antilles in 1948, occurring on ships’ hulls (Cairns, 

2000). This coral appeared in Belize and Mexico in the late 1990s and early 2000s (Fenner, 

1999; Humann and DeLoach, 2002). Its spread progressed to Venezuela, northern GOM, and the 

Florida Keys (Fenner, 2001; Fenner and Banks, 2004; Sammarco et al., 2004); Brazil (Figueira 

de Paula and Creed, 2004); and Colombia, Panama, the Bahamas, and throughout the Lesser and 

Greater Antilles (Humann and DeLoach, 2002). In terms of sheer numbers, T. coccinea is 

clearly the most abundant scleractinian coral, hermatypic or ahermatypic, in the northern GOM 

on artificial substrata (Sammarco et al., 2004, 2007a,b,c). Hundreds of thousands of colonies 

may be found on a single platform (e.g., 28/m2

; Sammarco et al., 2007a; Sammarco, 2008). It 

has also been found on some of the deeper banks of the northern GOM, but its abundances were 

low there (Schmahl, 2003; Hickerson et al., 2006; Schmahl and Hickerson, 2006).

The genetic affinities among populations of the scleractinian corals M. decactis and T. coccinea, 

were examined by sampling populations derived from the FGB and a large number of platforms 

across the continental shelf of the northern GOM from Matagorda Island, Texas to Mobile, 

Alabama. The FGB occur west of center of the range sampled. We will examine the degree of 

connectedness between coral populations on the platforms in this region, and between those 

platforms and the natural reefs. We will attempt to determine the degree of genetic distinctness

for each population on a platform or reef, and thus the degree of connectivity or affinity between 

the platforms, and between the platforms and the FGB. We will also attempt to infer information 

about the comparative effectiveness of dispersal and colonization by these two brooding species, 

and relate it to an island-hopping strategy of colonization as described by MacArthur and Wilson 

(1967; also see Maltagliati et al., 2002; Atchison, 2005; Baums et al., 2005; Atchison et al. 

2008). Here, we expand the analyses of Atchison et al. (2006, 2008), who worked on M.

decactis and D. strigosa (a broadcasting coral) in a more restricted region around the FGB. We 

include the ahermatypic, invasive brooding species T. coccinea, and analyze data covering a 

much broader geographic range. We also use a more conservative statistical analytical approach 

to analyze the data, based on extensive simulations.

1.2.2 Objectives

The objectives of this portion of the study were as follows:

• To determine the degree of genetic affinity between adult coral populations of 

the hermatypic scleractinian coral species M. decactis on oil and gas platforms 

throughout most of the northern GOM and the FGB;

• To determine the degree of genetic affinity between adult coral populations of 

the ahermatypic scleractinian coral species T. coccinea on oil and gas

platforms throughout most of the northern GOM and the FGB;
9

• To compare variation in genetic affinity between adult coral populations of 

the same species on different platforms;

• To determine the degree of genetic affinity between adult coral populations of 

the same species on the platforms compared with the FGB; and

• To use data on patterns of genetic variation to infer comparative colonization 

potentials for the two target species.

1.3 DURATION

This initial phase of the study ran for four years, from 2004 through 2008. A variety of issues, 

including hurricanes, resulted in cruise delays and extended the study two years beyond the 

projected original date of completion. The first three years were spent surveying the platforms

and sampling coral populations on the selected platforms. Distribution and abundance data about 

adult coral populations were analyzed. Tissues from adult colonies were genetically fingerprinted and statistical analyses of the data were performed. Various reports were prepared 

during the course of the study and submitted to BOEM.

11

2.0 MATERIALS AND METHODS

2.1 ADULT CORAL COMMUNITY SURVEYS ON THE PLATFORMS

To address questions about the distribution of adult coral populations on platforms throughout 

the northern GOM on oil and gas platforms, we initiated a set of field surveys and experiments, 

followed by laboratory analyses. These surveys were conducted on offshore platforms between 

Madagorda Island, Texas and Mobile, Alabama, a distance of ~3,000 km. We sampled an area 

from ~20 km offshore to the edge of the continental shelf. Platforms were sampled along four 

cross-shelf transects spaced at approximately equal intervals along the GOM coast (Figure 4). A 

number of study platforms were examined along each of these transects, as follows:

1. Transect I From Corpus Christi, Texas; bearing = 125° SE;

2. Transect II From Port Arthur-Lake Sabine, Texas; bearing = 170°S;

3. Transect III From Timbalier Island, Louisiana; bearing = 180°S;

4. Transect IV From Mobile Point, Alabama; bearing = 19°S.

There were several reasons for choosing the position of these transects. First, they cover the 

breadth and width of the shelf where platforms exist, particularly the shelf edge. Second, they

cover enough of the shelf to potentially provide northern boundary information about coral

colonization and survival. They also provide information on coral colonization and growth on 

platforms near the edge of the shelf. The second transect was designed to bisect our previous 

study area and provide additional data for comparison. In fact, coral density and species richness 

data from the first phase of this study have been included in the analyses and graphs to provide 

as complete a picture as possible of geographic patterns in these variables in the northern GOM 

(see Sammarco et al., 2004 for details). We chartered a dive vessel (M/V Fling, Freeport, Texas) 

and surveyed a total of 28 oil and gas platforms over a period of three years (Table 1). Surveys 

were conducted with teams of SCUBA divers during the summer and fall seasons of 2004–2007. 

Divers examined the platform jacket from the surface to a depth of 37 m. Data were collected on 

numbers of scleractinian and hydrozoan corals, depth of occurrence, and species identification.

Abundance data were standardized to density using approximate area surveyed. Architectural 

structural drawings of the platform jackets were obtained from a number of oil and gas 

companies whose platforms were sampled. Platform jackets (support structures) were placed 

into categories, based on number of primary pilings (3, 4, 6, 8, etc.). Total surface area was 

estimated for each platform. The surface area for each 3 m interval of depth was also 

determined, in order to compensate for additional surface area added by horizontal support 

structures that typically occurred at 10–15 m and 24–27 m. The molecular genetics of corals 

from another thirteen platforms around the FGB were considered in an earlier study and also 

used here. The details of these results may be found in Atchison (2005) and Atchison et al. 

(2008). Unfortunately, the techniques used in that study were updated here and precluded direct 

joint analysis and comparison of results. The current study used the most up-to-date AFLP 

techniques available at the time of the study.

Standard parametric univariate statistical analyses were performed on the data using BIOMStat 

V3.2 and V3.3, SigmaPlot 10.0, and SAS. Data were transformed by square-root where 

necessary for purposes of normalization (Sokal and Rohlf 1994). Only significant results are 
12

discussed. Surfer V.8.6 was used to generate three-dimensional graphs from coral density and 

species richness data collected in the field. In some cases, kriging and interpolation of values 

resulted in the generation of negative numbers in the graphs. These were generally restricted to 

land areas.

Figure 4. Map of the oil and gas platforms in the northern GOM, indicating the four cross-shelf transects. 

The oval represents the study area sampled for Sammarco et al., 2004.
13

Table 1.

Details of the 41 oil and gas production platforms studied in the northern GOM along four crosscontinental shelf transects from Matagorda Island, Texas to Mobile, Alabama, including the 14 

sampled in a previous study

2.2 TECHNICAL CONSIDERATIONS AND ALTERATION OF ORIGINAL PLAN

Because of a variety of reasons, survey plans needed to be altered. Poor or severe weather 

conditions frequently caused truncation or cancellation and re-scheduling of cruises. This 

resulted in changes in survey dates and number of platforms to be surveyed per transect. 

Statistical analyses were adjusted accordingly to accommodate changes in experimental design.

2.3 DETERMINING GENETIC CONNECTIVITY BETWEEN CORAL POPULATIONS ON 

PLATFORMS IN THE NORTHERN GULF OF MEXICO AND ON THE FLOWER 

GARDEN BANKS

2.3.1 Study Site

Twenty-eight oil and gas platforms were surveyed for hermatypic corals off the coasts of Texas,

Louisiana, and Alabama. The same transects used for the cross-shelf platform coral study were 

examined for the molecular genetics study; (see section 2.1 for a description of the transects and 

Figure 4 for their location). Madracis decactis and Tubastraea coccinea were the target species 

for the coral genetic connectivity study. All platforms surveyed had been deployed for 15–26

years, because it has been determined that a minimum of 15 years is associated with the 

development of substantial adult coral populations (Sammarco et al., 2004).
14

2.3.2 Sample Collection

Tissue was collected from all M. decactis coral colonies on the platforms between 5 and 37 m 

depth. This represented a total population for these depths. Sub-samples were taken for T.

coccinea, since, in most cases, the populations were very abundant. Divers collected samples 

randomly, placed them in pre-labeled ZipLock® bags, and recorded the platform and depth from 

which the collection was made on slates with underwater data-log sheets. The bags were 

returned to the support vessel and logged. A minimum of 30 samples of T. coccinea were 

collected from each platform. At the East and West FGB, coral tissue samples of both M.

decactis and T. coccinea were also collected by SCUBA divers randomly within a 50 m radius of 

Buoy #2.

Tissue samples, two cm2 in area, were collected by SCUBA divers from the growing edge of 

adult corals of the two target species using small hammers and chisels. Total population sizes of 

M. decactis on the platforms were small, and sample sizes represent the total populations of this 

species on each platform between 5 and 37 m. The samples returned to the ship and sealed in 

plastic freezer bags containing SED buffer (saturated NaCl, 250 mM EDTA, pH 7.5, 20% 

DMSO) to preserve the DNA. This preservative allows tissue samples to be stored at room 

temperature, eliminating the need for storage in liquid nitrogen. The samples were then placed 

in additional SED buffer, placed in ice chests, and returned to the laboratory.

2.3.3 Molecular Technique used to Determine Genetic Affinity, Amplified 

Fragment Length Polymorphism

The technique we used to conduct molecular genetic analysis was Amplified Fragment Length 

Polymorphism (AFLP). This is a DNA-fingerprinting technique (Sunnucks, 2000) that detects 

polymorphisms based upon the selective PCR amplification of a subset of numerous restriction 

fragments generated by two different restriction enzymes (Vos et al., 1995; Mueller and 

Wolfenbarger, 1999). AFLPs tend to be highly polymorphic, but they are not co-dominantly 

expressed. They are commonly used in studies of commercial crop species and other 

economically important species; but they have not been widely use in animal studies (Bensch 

and Akesson, 2005). However, AFLPs can provide something that many other approaches

cannot because alternate techniques are limited by the number of polymorphic markers 

produced.

AFLPs have been used successfully to estimate migration rates (He et al., 2004), species 

boundaries (Lopez et al., 1999; Fukami et al., 2004), and degree of parental contributions to 

populations (VanToai et al., 1997). AFLPs are not ideal for all population genetic applications 

(Sunnucks, 2000). They do, however, perform extraordinarily well for population assignment or 

allocation studies (Blears et al., 1998; Mueller and Wolfenbarger, 1999; Brazeau et al., 2005, 

2011), where the number of polymorphic loci is more important than allelic diversity 

(Bernatchez and Duchesne, 2000).

It is possible that some genetic variation detected using AFLPs may not be derived from the 

target organism (Sunnucks, 2000). This has been an area of concern with corals, which possess 

endosymbiotic zooxanthellae. Here, however, we have used zooxanthella-specific PCR primers 

to confirm for each sample that any contamination by zooxanthellar DNA is at levels far below 

those necessary for AFLP (i.e., 5–10 pg of zooxanthellar DNA in a background of coral DNA; 
15

Brazeau et al., 2005). In a previous study, Brazeau et al., 2005 assessed this potential problem. 

We spiked coral DNA preparations with zooxanthellar DNA and tested for detection. The 

amount of zooxanthella DNA detectable by our zoozanthella-specific PCR assay (4.8 pg) is five 

to six orders of magnitude less than the usual amount of coral DNA present in each sample 

(minimally 50–100 ng). More important, the detection limit of our assay for zooxanthellae 

contamination (0.48 pg) is well below (~200,000 fold) the amount of DNA necessary to generate 

AFLP bands. Attempts to generate AFLP bands using pure zooxanthellar DNA in amounts 

ranging from 0.48 to 480 pg were unsuccessful. Thus, we determined that coral samples 

showing no detectable zooxanthellae-PCR product would surely have too little zooxanthellar

DNA to contribute any substantial numbers of AFLP bands, confounding the coral bands.

2.3.4 Preparation of Coral Tissue Lysates for Genetic Analysis

DNA was isolated by macerating samples lightly in SED buffer and spinning at 16g for 5 min in 

a centrifuge min to pellet the zooxanthellae from the homogenate. The DNA was then purified 

using the Wizard® SV Genomic DNA Purification System (Promega Corporation), following the 

manufacturer’s instructions for animal tissue. All samples were checked for zooxanthellae DNA 

contamination using the PCR techniques described in Brazeau et al. (2005) and Atchison et al. 

(2006, 2008).

AFLPs, like other similar molecular genetic techniques, generate a subset of markers from a 

large population of markers. Of the subset obtained from a given AFLP experiment, a portion is 

often sensitive to specific reaction conditions. Thus, extra caution is required in processing 

samples through all procedural steps to maximize repeatability of results. Here, we processed 

samples in large lots containing members from all populations. This helped to distribute any 

error potentially introduced by reaction conditions uniformly between populations in an unbiased 

fashion. Also, all PCR reactions were done using one machine and the same thermal cycle 

profiles.

2.3.4.1 Genomic Coral DNA digestion and adapter ligation

A restriction-ligation “master mix” was prepared using the following reagents (measures are per 

sample): 1.1 μl T4 DNA ligase 10X buffer (30 mM Tris-HCl, pH 7.8 / 10 mM MgCl2 / 10mM 

dithiothreitol (DTT ) / 1mM ATP), 1.1 μl of 0.5 M NaCl, 0.5 μl bovine serum albumin (BSA; 1 

mg/ml), 1.0 μl Mse I adapters (50 μM), 1.0 μl EcoRI adapter (5 μM), 0.25 μl Mse I (4U/μl; New 

England BioLabs), 0.25 μl of EcoRI (20U/μL; New England BioLabs), and 0.33 μl of T4 ligase 

(3 U/μl; 10 mM Tris-HCl, pH 7.0 / 50mM KCl / 1mM DTT / 0.1 mM EDTA / 50% glycerol). 

Sequences for the Mse I and EcoRI adapters and PCR primers are listed in Table 2. To each new 

1.7 ml tube, 5.5 μl of the restriction-ligation mixture plus 5.5 μl (500 ng genomic DNA) of the 

purified genomic was added, centrifuged for 15 s, and incubated at room temperature overnight. 

At the end of the restriction-ligation reaction, 189 μl of TE buffer (10 mM Tris-HCl, pH 8.0 / 0.1 

mM EDTA) was added (10-fold dilution), serving as the template for the next-step, pre-selective 

amplification.

2.3.4.2 Pre-selective (PS) Amplification of Coral DNA

A second “master mix” was made for pre-selection (PS) amplification, using the following 

reagents (per sample measure given): 8.1 μl of nuclease-free water, 2.0 μl of 10X PCR buffer 

(15 mM Mg++ in buffer), 0.8 μl of 5 mM dNTP’s, 2.0 μl of EcoRI PS primer (2.75 μM), 2.0 μl of 
16

MseI PS primer (2.75 μM), and 0.1 μl of Thermostable (Taq) DNA polymerase (5U/μl), for a 

total volume of 15.0 μl. 15 μl of the pre-selective amplification master mix was added to each 

0.5 ml tube, and 5 μl of each of the diluted restriction ligation reactions samples was vortexed 

and centrifuged for 15 s. Amplification was performed using a 2-min initial incubation at 72oC, 

followed by 20 cycles of 20 s denaturation at 94oC, 30 s annealing at 56oC, and 2 min extension 

at 72oC. Last steps were 2 min final extension at 72oC, and 30 min final incubation at 60oC. 

After the cycling was completed, 180 μl of TE buffer was added to each tube, which consisted of 

the templates for the final step, selective amplification.

Table 2

Sequences of the adapters and arimers used in the AFLP protocol 

Name Sequence

Adapters EcoRI EcoF 5’-CTCGTAGACTGCGTACC

EcoR 5’-AATTGGTACGCAGTCTAC

Adapters MseI MseF 5’-GACGATGAGTCCTGAG

MseR 5’-TACTCAGGACTCAT

Pre-selective primer EcoRI A 5’-GACTGCGTACC AATTC A

Pre-selective primer MseI C 5’-GATGAGTCCTGAG TAA C

Selective primers EcoRI 5’-GACTGCGTACCAATTC ACT

(Set 1) MseI 5’-GATGAGTCCTGAGTAA CAG

Selective primers EcoRI 5’-GACTGCGTACCAATTC ACC

(Set 2) MseI 5’-GATGAGTCCTGAGTAA CTT

Pre-selective and selective nucleotides are indicated in bold.

2.3.4.3 Selective Amplification of Coral DNA

In the final step, a selective amplification “master mix” was made, containing the following 

components: 8.1 μl of nuclease-free water, 2.0 μl of 10X PCR buffer (with Mg++ at 15mM), 0.8 

μl of 5mM dNTP’s, 2.0 μl of EcoRI selective primer (0.46 μM), 2.0 μl of MseI selective primer 

(2.75 μM), and 0.1 μl of Taq DNA polymerase (5U/μl) for a total volume of 15.0 μl. To each 0.5 

ml micro-centrifuge tube, 5 μl of the diluted pre-selection PCR reaction was added to each 

corresponding tube, mixed, and centrifuged for 15 s. Samples were placed in the thermocycler, 

and the cycle profile was performed as indicated: 2 min initial denaturation at 94oC, followed by 

1 cycle of 20 s denaturation at 94oC, 30 s annealing at 66oC, and 2 min extension at 72oC. Next, 

there were 9 cycles: 20 s at 94oC, initial 30 s at 66oC (reduced 1oC/cycle), and 2 min at 72oC. 

Final cycle consisted of 20 cycles: 20 s at 94oC, 30s at 56oC, and 2 min at 72oC, followed by a 30

min final incubation at 60oC.
17

The products of the selective PCR were separated on a 5% polyacrylamide (sequencing) gel. 

Banding patterns were analyzed using Kodak Digital Image Analysis software (Eastman Kodak 

Co. Scientific Imaging Systems; Bonin et al., 2004). Bands were assigned to bins based upon 20 

base-pair (bp) size intervals. The Selective PCR reactions were repeated three times for each 

sample. These “repeat” reactions were run on different days with populations mixed in each run. 

Bands were considered present if they appeared in two of the three runs; conversely, bands were 

scored as absent if two out of the three reactions yielded no band. Of the bands included in the 

study, >90% yielded the same result in all three PCR runs. These inclusion criteria helped to 

exclude bands that were overly sensitive to reaction conditions.

2.3.5 Statistical Analyses

With respect to the molecular genetics section of the study, two statistical analyses were used to 

assess population differentiation: AFLPOP population allocation analysis (Version 1.1; 

Duchesne and Bernatchez, 2002), a statistical analytical procedure designed particularly to 

analyze data generated by AFLPs, and STRUCTURE Version 2.0 (Pritchard et al., 2000). We 

have reported the development of these techniques and their application to the analysis of 

Caribbean corals earlier (Brazeau et al., 2005).

AFLPOP uses AFLP presence/absence data to calculate log-likelihood values for any 

individual’s membership in a reference population, based upon their banding patterns. The 

reference population is that target population (e.g., from one platform) against which all other 

colonies from other sites are compared for genetic affinity. Each individual is allocated to the 

population showing the highest likelihood for that genotype (Duchesne and Bernatchez, 2002;

He et al., 2004). Population assignment tests for individuals, based on genetic differentiation 

among populations, have provided the most promising statistical methods used to estimate 

contemporary long-distance dispersal (He et al., 2004). When the individual is assigned to a 

population different from the site from which it was collected, it is interpreted as evidence of 

dispersal. One major advantage of using these types of assignment methods is that populations 

do not have to be sampled exhaustively (He et al., 2004). In an AFLPOP simulation, an 

individual was chosen randomly from the entire population, population marker frequencies were 

then calculated without that individual, and then the individual is assigned to the new data set. 

For each simulation run, this was repeated 500 times. Average assignments to a given site were 

subsequently calculated as a percent value, based on 10 repeats of these 500 iterations.

The AFLPOP program allows the user to set a log-likelihood threshold for each assignment. A 

log-likelihood threshold set to 0.0 will result in the assignment of a colony to the population with 

the highest likelihood value. Atchison (2005) and Atchison et al. (2006, 2008) found that this 

may yield potentially misleading results. This is because there may be more than one population 

with nearly-equal likelihood values. Here, we also performed simulations using 1.0 as the 

comparative log-likelihood threshold in the analysis. This approach is more conservative and 

removes potentially spurious groupings. With the threshold set to 1.0, assignment of a colony to 

a population was not made unless the probability of the given assignment was 10 times more 

likely than the next most probable assignment. If this threshold was not met, the individual was 

not assigned to any population; in that case, it was designated “Criteria for Allocation Not Met” 

(CANM). This does not necessarily imply that the sample could not be assigned to any 

population with high probability; it only means that there may have been at least two populations 
18

with nearly equivalent probabilities of assignment. It could also mean that the individual fits 

none of the populations well; in that case it could have been derived from an outside population 

(Atchison et al., 2006, 2008). In this study, we focus primarily on cases where clear assignments 

have been made, including self-allocations to sites of origin, and cross-population allocations.

STRUCTURE uses Bayesian techniques and Monte Carlo simulations to assign samples to 

populations. Unlike AFLPOP, in which assignment is based solely upon marker frequencies,

STRUCTURE makes assignments that minimize deviations from the Hardy-Weinberg (H-W) 

equilibrium, which assumes that the population giving rise to the recruits constitutes a large, 

randomly mating population. Using this approach, the program calculates probabilities of 

individual assignment, estimates of FST (Wright’s measure of population subdivision – a measure 

of genetic distance between populations; Neigel, 2002), and probable paternity, grand-paternity, 

etc., relationships. The program can accommodate dominant marker data such as those 

generated by our AFLP technique. The entire data set was subjected to several preliminary runs 

in order to evaluate parameter estimates used by Markov Chain Monte Carlo (MCMC) iterations 

for stability. STRUCTURE also called for definition of the parameter MIGPRIOR before 

running. This parameter is the prior probability of a spat being identified as coming from an 

external source. It was run at two levels, for comparative purposes, 0.05 and 0.50, taking into 

account different potential estimated migration rates. Once these parameters were set, data were 

analyzed using a burn-in period of 500,000 iterations, followed by another 100,000 MCMC 

repetitions.

The genetic variables derived from this study were analyzed using additional parametric and 

non-parametric statistical techniques. The software used for analyses was BIOMStat V. 3.2 and 

SigmaPlot V. 10.0. Percent data were transformed by arcsine (square-root of Y) before analysis 

for normalization purposes, if necessary.
19

3.0 RESULTS

3.1 ADULT CORAL COMMUNITIES ON PLATFORMS THROUGHOUT THE 

NORTHERN GULF OF MEXICO

3.1.1 Coral Species Composition

Twelve species of scleractinian corals were found on the platforms surveyed. Nine were 

hermatypic/zooxanthellate corals, and three were ahermatypic/azooxanthellate species, as 

follows:

Hermatypic/Zooxanthellate

- Madracis decactis (Lyman, 1859)

- Diploria strigosa (Dana, 1848)

- Montastraea cavernosa (Linnaeus, 1766)

- Porites astreoides (Lamarck, 1816)

- Madracis formosa (Wells, 1973)

- Colpophyllia natans (Houttyn, 1772)

- Stephanocoenia intercepta (Lamarck, 1816)

- Stephanocoenia michelinii (Milne Edwards et Haime, 1848)

- Millepora alcicornis (Linnaeus, 1758)

Ahermatypic/Azooxanthellate

- Tubastraea coccinea (Lesson, 1829)

- Oculina diffusa (Lamarck, 1816)

- Phyllangia americana (Milne Edwards and Haime, 1849)

For comparative purposes, the FGB are reported to have 24 species of hermatypic corals and 2 

spp. of ahermatypic corals (Rezak et al., 1985).

3.1.2 Coral Species Richness: Number of Species

To facilitate interpretation of geographic trends in the data, results are presented in threedimensional graphic format.

When one considered the distribution of number of coral species on oil and gas platforms in the 

northern GOM, particularly including the number of species on the FGB, an interesting pattern 

emerged. First, a clear peak in hermatypic coral species richness emerged at the FGB which fell 

off precipitously in all directions with respect to the number of corals on the platforms (Figure

5). Note the peak of diversity at the FGB and how it slopes away in all directions, but is 

extended towards the north, towards shore, off Port Arthur-Lake Sabine, Texas (Transect II). 

Unexpectedly, the distribution was skewed to the west, towards Transect I, in the far western 

sector near Matagorda Island, as opposed to the east, where prevailing currents flow. That is, 

there appeared to be more species to the west of the FGB than to the east. Three other features 

were apparent in this figure. The higher species richness appeared to be limited to the edge of 

the continental shelf for all transects, and fell off gradually as one moved closer to the coast.
20

Also, coral species richness peaked in the western sector (in Transect II off Lake Sabine-Port 

Arthur, Texas), near the FGB, with some additional minor peaks on platforms at the mid-shelf. 

In addition, there were no hermatypic coral species on the northern half of the shelf in the central 

or western sectors of the northern GOM (Transects I, II, and III). This “no coral zone” was 

compressed, however, in the Main Pass lease area, to the east of the Mississippi River mouth 

(Transect IV).

Figure 5. Species richness of hermatypic scleractinian corals in 

number of species on oil and gas platforms in the GOM and 

on the FGB.

3.1.3 Coral Distribution and Abundance

3.1.3.1 Hermatypic/Zooxanthellate Scleractinian Corals

The distribution and abundance patterns of hermatypic corals in the northern GOM did not 

mimic the species richness pattern (Figure 5 compared with Figure 6). In the case of overall 

hermatypic coral density, there is a clear peak south of Terrebonne Bay in Transect III, which 

extends to some degree inshore, but otherwise in all directions. There are some other minor 

peaks around the FGB off Lake Sabine-Port Arthur, in Transect II. Hermatypic corals are 

present throughout the northern GOM on platforms, albeit at very low densities in most cases, 

and except at the inner coastline. This report focuses on the three dominant species found on 

platforms in the GOM: M. decactis, D. strigosa, and M. cavernosa.
21

Figure 6. Total coral density of hermatypic scleractinian corals in no.per

1,000 m-2 on oil and gas platforms in the GOM.

3.1.3.1.1 Madracis decactis

The dominant coral in the hermatypic scleractinian coral community found on platforms in the 

northern GOM was M. decactis. Its pattern of distribution was similar to that of all hermatypic 

corals and was responsible for driving that pattern. No peak was observed around the FGB 

(Figure 7).

Rather, it occurred on platforms near the shelf edge east of the FGB, off of Terrebonne Bay, 

Louisiana (west of the mouth of the Mississippi River; Transect III). At this longitude, its 

distribution was skewed to the north, reaching within 40 km of the coast. Its distribution also 

was skewed to the east into Main Pass lease area (Transect IV), again with highest densities 

occurring at the edge of the shelf, but reaching moderate densities towards the coast. Densities 

of M. decactis were lower in the western half of the study area. Densities on platforms around 

the FGB (Transect II) were moderate, and only 1–2 colonies were found in the far western 

sector, and only at the shelf edge (Transect I).
22

Note: Negative numbers exhibited on the Z-axis are due to interpolation 

calculations performed by SURFER; these occur generally over land areas.

Figure 7. Density of the hermatypic scleractinian coral M. decactis on 

oil and gas platforms in the northern GOM, shown in no. per

1,000 m-2.

3.1.3.1.2 Diploria strigosa

D. strigosa, was the second dominant species in the hermatypic scleractinian coral community 

found on platforms, although its densities were relatively low. Its individual densities peaked at 

~2/1,000 m2 on a platform near the FGB and on several platforms at and beyond the outer edge 

of the continental shelf (Figure 8). This peak was at the southern end of the cross-shelf transect 

in the west, off Lake Sabine-Port Arthur, Texas (Transect II). Coral densities fell dramatically to 

zero with distance from that area. It was absent in all other sectors.
23

Figure 8. Density of the hermatypic scleractinian coral D. strigosa on 

oil and gas platforms in the northern GOM.

3.1.3.1.3 Montastraea cavernosa

The third-ranking dominant coral in the hermatypic scleractinian coral community on platforms 

in the northern GOM was M. cavernosa. Its average densities, overall, were low and bimodal in 

nature, peaking at ~0.25/1,000 m2 on platforms just to the east of the FGB off Lake Sabine-Port 

Arthur, Texas (Transect II; Figure 9). A second minor peak occurred in the eastern Main Pass 

lease area, off Mobile Bay, Alabama, in Transect IV. However, because only one colony was 

found, densities were negligible. This species was absent in the far western and central sectors 

(Transects I and III), off Matagorda Island, Texas and Terrebonne Bay, Louisiana. In the west 

(Transect II), however, their distribution did reach northward, more than halfway across the 

shelf. Note that, similar to that of D. strigosa, the density peaks east of the FGB, at the edge of 

the continental shelf.
24

Figure 9. Density of the hermatypic scleractinian coral M. cavernosa on oil 

and gas platforms in the northern GOM.

3.1.3.2 Total Scleractinian Corals

If one combines the densities of all scleractinian corals on the platforms, including both 

hermatypic/zooxanthellate and ahermatypic/azooxanthellate corals, the distribution varies greatly 

from that of the scleractinian corals alone (Figure 6; Figure 10). There is a clear density peak in 

Transects III and IV in the central and eastern regions. The highest densities occur near the shelf 

edge, but they also extend northward to within 35 kms of the coastline off Mobile, Alabama. 

The densities are skewed to the west, where they show a small peak again at the shelf edge near 

the FGB. Note that the peak average density of all corals occurring on platforms near the shelf 

edge in the central sector, off Terrebonne Bay, Louisiana, is approximately 35,000 corals/1,000 

m

2

, a much greater density than observed for the hermatypic corals.
25

Note: See scale on Y-axis, compared to earlier graphs (Figs. 7-10).

Figure10. Total coral density of all scleractinian corals, hermatypic and 

ahermatypic, on oil and gas platforms in the GOM.

3.1.3.3 Ahermatypic/Azooxanthellate Corals

The density of ahermatypic/azooxanthellate corals in the northern GOM mimicked precisely the 

pattern exhibited by all scleractinian corals (Figure 11; compare with Figure 10). This implies 

that the ahermatypic corals are by far driving the overall coral distribution and abundance pattern 

in this region. The pattern of distribution mimics almost precisely that of all scleractinian corals, 

shown in Figure 9. Peaks occur in the central and near-eastern sectors, off Terrebonne Bay, 

Louisiana and Mobile, Alabama, on either side of the Mississippi River.
26

Note: See scale on Y-axis, compared to earlier graphs. Negative numbers exhibited 

on the Z-axis are due to interpolation calculations performed by SURFER; these 

occur generally over land areas.

Figure 11. Density of all ahermatypic scleractinian corals in the GOM.

3.1.3.3.1 Species Richness of Ahermatypic Compared with Hermatypic Corals

The distribution of ahermatypic/azooxanthellate coral species – Species Richness (S) - on 

platforms across the northern GOM did not follow the pattern exhibited by the 

hermatypic/scleractinian corals. Rather than showing a distinct peak at the FGB, it exhibited a 

series of peaks, one on each of the four cross-shelf transects (Figure 12). Considering the 

number of species under consideration here (three), these peaks are relatively low compared to 

the distribution of hermatypic species, implying a much more equitable distribution of species 

across the shelf. In addition, they generally did not occur at the shelf edge, but peaked 

approximately at the mid-shelf. The peaks decreased in size from west to east. The widest 

representation of all three ahermatypes was observed in the west, off Matagorda Island 

(Transect I).
27

Figure 12. Species richness (number of species - S) of ahermatypic corals in number of 

species on oil and gas platforms across the northern GOM.

3.1.3.3.2 Tubastraea coccinea

The densities of the ahermatypic/azooxanthellate T. coccinea on platforms were orders of 

magnitude higher than any other coral analyzed in this study. The individual densities of this 

species reached upwards of 40,000 colonies per 1,000 m2

. This density occurred on one platform 

in the central region off Terrebonne Bay, Louisiana, at the shelf edge (Transect III). This coral 

species clearly dominated the northern GOM, whether considering hermatypes, ahermatypes, or 

both groups combined.

When one considers the specific distribution and abundance of T. coccinea alone, it is quite clear 

that this species followed the precise pattern of all ahermatypic corals and indeed all corals 

combined, whether hermatypic or ahermatypic (Figure 13). T. coccinea is clearly driving the 

distribution and abundance on platforms in the northern GOM of ahermatypic corals as well as 

all corals combined through its sheer numbers. Its densities peaked on the outer half of the 

continental shelf in the central region (Transect III), off Terrebonne Bay, Louisiana, occupying 

the outer half of the shelf. Its second highest peak was in the eastern region (Transect IV), in the 

Main Pass area, off Mobile, Alabama. It was present in the west (Transect II, off Lake Sabine), 

on the outer half of the shelf, with densities reaching ~3,000/1,000 m2. T. coccinea densities 

dwindled dramatically, however, in the west (Transect I), occurring in tens to hundreds of 

colonies per 1,000 m2, being sparsely distributed only on the outer half of the shelf.
28

Note: Negative numbers exhibited on the Z-axis are due to interpolation calculations 

performed by SURFER; these occur generally over land areas.

Figure 13. Density of T. coccinea, the dominant ahermatypic coral on oil 

and gas platforms in the northern GOM.

3.1.3.3.3 Oculina diffusa

The ahermatype O. diffusa was present in all sectors of the GOM sampled. Its densities were 

highest on platforms just beyond the shelf edge in the western sector off Lake Sabine-Port 

Arthur, Texas (Transect II), where individual densities almost reached 6,000 per 1,000 m2

(Figure 14). This species also occurred there at the mid-shelf, but at much lower densities. The 

second highest density of this species was observed offshore in the far-western sector off 

Matagorda Island (Transect I), but densities reached no higher than ~150/1,000 m2. Although 

this species was present in the central and far-eastern sectors, the densities there were quite low. 

The pattern of distribution follows that of D. strigosa (Figure 9) and M. cavernosa (Figure 10), 

indicating that the FGB may be the source of larvae.
29

Figure 14. Density of O. diffusa on oil and gas platforms in the northern 

GOM.

3.1.3.3.4 Phyllangia americana

P. americana was the least abundant ahermatypic coral encountered on platforms. Its densities 

were generally low, never surpassing ~10/1,000 m2. Its densities were highest in the western 

GOM (Figure 15). In the far western sector, off Matagorda Island (Transect I), they were 

abundant on platforms across the continental shelf. In the west, off Lake Sabine-Port Arthur, 

Texas (Transect II), they occurred inshore, peaked at the midshelf, and were absent off the edge 

of the shelf. In the central region, off Terrebonne Bay, Louisiana, they occurred in varying 

densities over the continental shelf. In the eastern sector, they only occurred towards the outer 

edge of the shelf. The pattern of distribution is unique to those of other species encountered in 

this study, decreasing towards the east and indicating that the source of larvae may be to the 

south or south west of the study area.
30

Figure 15. Density of P. americana, on oil and gas platforms in the northern 

GOM.

3.2 GENETIC CONNECTIVITY IN CORALS IN THE NORTHERN GULF OF MEXICO

3.2.1 Relationship between Genetic Distance and Geographical Distance for 

Madracis decactis, Using STRUCTURE

In examining M. decactis on platforms in the northern GOM, the analyses derived from 

STRUCTURE revealed a clear pattern. The highest levels of genetic affinity in this species 

occurred between those coral populations on platforms in Transect II, off of Port Arthur-Lake 

Sabine, Texas, and particularly on platforms around the FGB (Figure 16). (Transect I had an 

insufficient sample size to be included in this analysis.) Genetic affinity clearly peaked in the 

west (Transect II, off Lake Sabine) and then decreased steadily and smoothly to the east, to 

Transect III, south of Terrebonne Bay. The genetic affinity then increased slightly in Transect 

IV, off Mobile, Alabama. There was also an anomalous point depression in genetic affinity 

which occurred near the edge of the continental shelf in Transect III (off Terrebonne Bay, 

Louisiana). The peak in the west implies that corals on the platforms were most likely derived 

from the Flower Garden Banks. Population differentiation is also in the east, on either side of 

the Mississippi River mouth. The point-depression south of Terrebonne Bay, Louisiana may 

represent a population drawn from outside of the northern GOM.
31

Note: Population differentiation determined by STRUCTURE. Reference population was the West FGB

Figure 16. M. decactis: Genetic affinity in coral populations on oil and gas platforms across 

the continental shelf in the northern GOM. Transect I (Matagorda Island) not 

shown because of lack of M. decactis there.
32

3.2.2 Relationship between Genetic Distance and Geographical Distance for 

Madracis decactis, Using AFLPOP

We then focused on comparing genetic affinities of M. decactis populations on either side of the 

Mississippi River mouth (Transects III and IV), to determine whether this large-scale geographic 

feature could be acting as a geographic barrier to dispersal. Enough samples were collected on 

the east side of the Mississippi River (Transect IV) that individual platforms were analyzed. 

Samples were combined on the east side of the River (Transect III) to have a sample size large 

enough for analysis. When analyzing the genetic data using AFLPOP, with a log-likelihood 

threshold value of “0” (all colonies must be assigned to a population), it became clear that not 

only was there no recognition between the two sets of populations, but there was also little to no 

recognition between platform populations within a transect in this region (Table 3). When this 

analysis was repeated using a log-likelihood threshold value of 1.0 (a colony must be 10X more 

likely to belong to a given populations than another before being assigned there), a much more 

conservative approach, there was very little difference in the results (Table 4). Both analyses 

indicated minimal levels of dispersal between these two sets of populations, i.e., between 

transects on either side of the Mississippi River plume, and also between platforms within a 

transect.

Table 3

M. Decactis: Genetic affinities in populations of corals on oil and gas platforms 

in the northern GOM along two transects on either side of the Mississippi River

Madracis decactis

Transects III & IV

AFLPOP Analysis, Log-Likelihood = 0

Percentage (%) of Colonies

Transect III Transect IV

W. of Miss. 

River E. of Miss. River

Allocated to ST-295 SS-277 ST-292 MP-All

ST-295 100 0 0.3 0

SS-277 0 100 0 0

ST-292 0.2 0 99.7 0

MP - All 0 0 0 100

Note: Platforms on the east side of the Mississippi River were combined to provide sufficient 

sample size for comparison. Note extraordinarily levels of high self-assignment to home 

populations and lack of recognition of neighboring populations. This indicates geographic 

isolation of these coral populations and possibly different larval sources. All of the MP 

populations were combined to provide sample sizes large enough to accommodate analysis.
33

Table 4.

M. Decactis: Genetic affinities in populations of corals on oil and gas 

platforms in the northern GOM along two transects on either side of the 

Mississippi River mouth

Madracis decactis

Transects III & IV

AFLPOP Analysis, Log-Likelihood = 1

Percentage (%) of Colonies

Transect III Transect IV

W. of Miss. 

River E. of Miss. River

Allocated to ST-295 SS-277 ST-292 MP-All

ST-295 99 0 0 0

SS-277 0 100 0 0

ST-292 0.1 0 99.4 0

MP - All 0 0 0 100

CNM 0.6 0 0.6 0

Note: Platforms on the east side of the Mississippi River were combined to provide sufficient 

sample size for comparison. Data analyzed with AFLPOP, with a log-likelihood value set at 

1. The resulting pattern is almost identical to analysis performed with more liberal 0 settingSee Table 3. All of the MP populations were combined to provide sample sizes large 

enough to accommodate analysis.

3.2.3 Relationship between Genetic Distance and Geographical Distance for Tubastraea

coccinea, Using STRUCTURE

The analytical program STRUCTURE yielded clear results for T. coccinea. In general, the 

genetic affinity values in this species were higher across the continental shelf than those for M. 

decactis (Figure 17). Affinities exhibited more dramatic changes across the continental shelf for 

T. coccinea than for M. decactis. Although levels of genetic affinity generally decreased from 

west to east in this species, there were two substantial local variations in this pattern. First, 

genetic affinities dropped precipitously east of the Mississippi River. Second, an anomalous 

point depression in genetic affinity in T. coccinea occurred at the edge of the continental shelf 

off Terrebonne Bay, Louisiana. The relative flatness of the curve indicates no major local larval 

source; i.e., it is unlikely that the FGB are a source of larvae for the region for this species. The 

steep drop-off in the east indicates major population differences in genetic population structure 

between the two sides of the Mississippi River. The point depression south of Terrebonne Bay 

indicates a population from a very different source than the rest of the western populations. 
34

Genetic affinity values were calculated as maximal genetic distance value minus individual 

genetic distance value for each point, yielding an estimate of affinity.

Note: Population differentiation determined by STRUCTURE. Reference population was 

the West FGB.

Figure 17. T. coccinea: Genetic affinity in coral populations on oil and gas 

platforms across the continental shelf in the northern GOM.

3.2.4 Relationship between Genetic Distance and Geographical Distance for 

Tubastraea coccinea, Using AFLPOP

The AFLPOP was used to compare genetic affinities of T. coccinea populations on either side of 

the Mississippi River. Note that the sample size was large enough to compare individual 

platforms east of the River, but that all samples were combined west of the River to obtain 

sufficient sample size for the analysis. When one makes a direct comparison of genetic affinities 

of populations occurring off Terrebonne Bay, Louisiana vs. off Mobile, Alabama using AFLPOP 

with a log-likelihood threshold value of 0.0 (forcing assignment), it becomes apparent that, once 

again, there was little recognition of populations across the mouth of the Mississippi River 

(Table 5). A slightly higher degree of recognition occurred between populations within transects 

for T. coccinea than occurred for M. decactis. This indicates some genetic similarities between 

populations on different platforms within a transect. When this analysis was repeated using a 

log-likelihood threshold value of 1.0 (much more conservative), self-assignment to home 

populations decreased and more colonies fell into the CNM (Criteria Not Met) category (Table 

6). In general, these analyses indicate that dispersal is higher in T. coccinea than in M. decactis.
35

Table 5.

T. coccinea: Genetic affinities in populations of corals on oil and gas platforms 

in the northern GOM along two transects on either side of the Mississippi River 

mouth

Note: Platforms on the western side combined to provide sufficient sample size for 

comparison. Only those populations for which there were sufficient sample sizes were 

analyzed here.
36

Table 6.

T. Coccinea: Genetic affinities in populations of corals on oil and gas platforms in the 

northern GOM along two transects, on either side of the Mississippi River mouth

Note: Platforms on the western side combined to provide sufficient sample size for 

comparison.

 
37

4.0 DISCUSSION

4.1 PATTERNS IN CORAL SPECIES RICHNESS, COMPOSITION, AND ABUNDANCE 

ON THE NORTHERN GOM SHELF

4.1.1 Hermatypic Corals

The fact that species richness of hermatypic corals on the northern GOM shelf peaks at the FGB 

indicates several things. First, in accordance with MacArthur and Wilson’s (1967) and 

Simberloff and Wilson’s (1969) Theory of Island Biogeography, the platforms represent islands, 

although much smaller islands than the FGB. Second, they are reticulate and do not offer a 

substantial barrier to far-field flow. Currents flow generally through them, rather than around 

them. Third, they are very young communities, both in ecological and geological time. The 

oldest platforms sampled were < 30 years old. Even for a coral reef, this can represent a short 

period of time for community development (Hughes and Connell, 1999). By contrast, in their 

current configuration and under current sea level conditions, the FGB are estimated to be 

~14,000 years old (Rezak et al., 1985), allowing a climax community to develop, even 

considering that some of its coral species can live for hundreds of years (Boto and Isdale, 1985; 

Hough-Guldberg, 1999). Fourth, because the species richness of hermatypic corals on the 

platforms clearly dissipates rapidly with increasing distance from the FGB, following a negative 

logarithmic pattern, this is the first indication in the study that the hermatypic corals on the 

platforms are derived primarily from the FGB, not from elsewhere in the GOM or the Caribbean. 

This is further supported by the pattern of Diploria strigosa’s density around the FGB, which 

mimics very closely that of hermatypic coral species richness, peaking at the FGB and 

dissipating rapidly with distance (to be discussed further below).

Next, the fact that there is a distinct extension of species richness and density of D. strigosa

inshore towards Port Arthur and Lake Sabine, Texas lends support to the hypothesis proposed by 

Lugo-Hernandez et al. (2001). Their drifter study revealed currents that dominate during the 

spawning season which move east along the edge of the continental shelf, across the FGB, then 

flow north, follow a rapid long-shore current to the west and then move south, looping back

towards the FGB again. They propose that such a current would allow coral planulae released by 

FGB corals to be drawn north towards land and then be brought back out to the FGB region in 

time for settlement. This would provide a mechanism for enhanced larval recruitment on those 

platforms near, or to the north of, the FGB. It would also allow for a self-seeding mechanism for 

the FGB, as proposed by Lugo-Hernandez et al. (2001).

As in our earlier study (Sammarco et al., 2004), the dominance of three hermatypic species on 

the platforms–Madracis decactis, D. strigosa, and M. cavernosa–was somewhat unexpected. 

First, these are the dominant, late-sere corals on the FGB. This is an anomaly. Corals which 

generally colonize young reef communities or substratum which has become newly available are 

usually pioneer species or opportunistic species. Among these for the Caribbean are the 

brooding species Agaricia agaricites, Agaricia spp., Porites spp., and Favia fragum, and some 

other species. The species found to be dominant in the GOM platform communities are 

characteristic of later series in community development (Odum 1969, 1971; Smith and Smith,

1999). Also, two of them (Diploria and Montastraea) are broadcast spawners. M. decactis, 

however, is a brooder. This emphasizes the point that platform coral communities are not typical 
38

of natural coral reefs in the region or in the Caribbean. They have their own unique character as 

artificial reef communities.

The fact that the density of the primary dominant hermatypic coral, M. decactis, was not centered 

on platforms around the FGB was unexpected. The peak in its density was south of Terrebonne 

Bay, Louisiana, implying that platforms in this region were receiving a larger supply of this 

species’ larvae than platforms directly around the FGB. The center of this highdensity region 

lies east of the FGB. It falls on the edge of the continental shelf and is skirted by a westerly

current which travels from the western GOM to the east. This current could carry coral larvae 

from the FGB to this center within their viable period. The bathymetry of the edge of the 

continental shelf would be important to consider here, regarding eddies and overall flow. Coral 

planulae released by brooding corals are nearly fully developed upon release and capable of 

settling in as few as four hours. They can remain competent to settle, however, for ~60–90 days, 

maximum. Most planulae settle, however, within two to four days (Harrison and Wallace, 

1990). Thus, the window for peak settlement in this region by planulae of M. decactis is 

approximately correct to create such an abundance peak; (see discussion below regarding major 

currents in this region).

The density pattern of D. strigosa on the northern GOM, on the other hand, which was centered 

on platforms around the FGB, varied greatly from that of M. and implied a different pattern of 

larval dispersal. The fact that the center of distribution of Diploria is on platforms around the 

FGB implies that the fertilized eggs and larvae generally remain in that vicinity. This is 

consistent with our earlier findings that this species, which is a broadcaster, recruits more locally 

than its brooding associate, M. decactis, which has peak densities on platforms to the east of the 

FGB (south of Terrebonne Bay, Louisiana). The finding is again counter-intuitive, as spawned

eggs must be fertilized in the water column, and they require 24–72 hours for development. 

Thus, they must spend a minimum of this amount of time in the water column before even being 

competent to settle, unlike larvae from the brooders. It is possible that the larvae of both species 

may be retained by eddies around the FGB, but that those of M. decactis spend a longer period of 

time in the water column than those of D. strigosa.

M. cavernosa’s recruitment pattern partially mimics that of D. strigosa, in that its peak occurs 

in the vicinity of the FGB. Thus, one may draw some similar inferences between them. The fact 

that this species also exhibits a somewhat smaller peak on the east side of the Mississippi River, 

however, in the Main Pass area, suggests that it may be receiving larvae from a distant source 

there, perhaps from the Caribbean through the Loop Current (Figure 18; Sturges and Blama,

1976; Hamilton et al., 1999; Lugo-Fernandez and Gravois, 2010) or from the southern GOM in 

the Alacran area off the Yucatan Peninsula. The molecular genetics portion of this study has 

demonstrated that adult coral populations of M. and Tubastraea on the two sides of the 

Mississippi River mouth are clearly distinct. It is also possible that the Montastraea populations 

on either side of the Mississippi River may are distinct as well. Lugo-Fernandez et al. (2010) has

proposed that there may be a jet current that is capable of carrying larvae N-NE directly from the 

Caribbean to the northeastern GOM Main Pass area within a time period which encompasses the 

period of larval competence for the species. This has yet to be confirmed via a comparative 

molecular genetics study of the Yucatan coral populations.
39

4.1.2 Ahermatypic Corals

Although the species richness of ahermatypic corals was greatest in the western GOM, their

densities were highest in the east. This density of ahermatypic corals caused the extraordinary 

peak of total coral density (hermatypic and ahermatypic) in the northeastern GOM. The 

densities of T. coccinea were clearly driving the total coral density pattern.

T. coccinea’s densities were orders of magnitude higher than any other coral in the study. This 

invasive species was clearly the dominant coral on platforms throughout the northern GOM. It is 

believed to have entered the western Atlantic through the Panama Canal in the 1940s and spread

through the Americas and the Antilles over the next 70 years. During this period of time, it has 

extended its biogeographic range from the Florida Keys to Brazil. The density data presented 

here demonstrates that the greatest coral densities occur in the eastern half of the study area and 

suggest that this population of T. coccinea may have entered through the Port of New Orleans 

(29°54'51" N, -90°5'26" W) or Port Fourchon (29°37'27" N, -90°11'40" W). Densities were 

substantially higher east of the Mississippi River, off Mobile, Alabama, than densities observed 

to the west of the River. However, our molecular genetics data indicate that the populations on 

the east compared with the west side of the Mississippi are not related. If nothing else, we may 

be observing the results of two independent introductions.

The molecular genetics data for T. coccinea also indicate that there is a gradual decrease in 

genetic affinity in populations from the west to the east, although it is very subtle. The 

populations on the eastern side of the Mississippi are totally different, however. It is possible 

that the western population was introduced to the GOM from Mexico and the Caribbean through 

the clockwise current around the GOM or through another port. It might also be possible that a 

separate seeding occurred at another time through direct input from the Loop Current or, indeed, 

through the hull or ballast water of a ship, to the eastern side of the Mississippi River. The 

Mississippi River discharge appears to be sufficient to keep the two populations separate. Only 

further molecular genetics studies will be able to shed further light on this situation.

The pattern of decreasing richness in ahermatypic coral species from west to east implies that the 

source of larvae for this group as a whole is derived from somewhere in the GOM. Once again, 

the decrease in species richness follows the clockwise current direction around the GOM, and it 

is likely that, on the average, the larval source is the southern GOM or the Caribbean. There is 

variance in this pattern, of course. For example, Oculina diffusa shows evidence of being 

derived directly from the FGB. The fact that richness peaks more on the mid-shelf than at the 

shelf-edge implies that this group is better adapted to more variable salinities and temperatures 

than its hermatypic counterparts.

4.2 GENETIC AFFINITIES OF CORALS IN THE NORTHERN GULF OF MEXICO

The high degree of genetic affinity in M. decactis between populations on the platforms in the 

west (Transect II), as determined by STRUCTURE, confirms that populations of this coral 

species in the northern GOM most likely originated from the FGB. Populations on those 

platforms show high affinity for the populations on the FGB and for those populations on other 

platforms around them. Also, as the populations become more distant from Transect II and the 

FGB, they exhibit greater isolation and less genetic affinity to each other. The slight increase in 

local genetic affinity in the eastern sector (Transect IV), beyond the mouth of the Mississippi 
40

River mouth, underscores the lack of affinity between the populations on the eastern and western 

sides of the river. It is likely that the Mississippi River is creating a strong geographic barrier to 

larval dispersal via lower salinities, high sedimentation, and high nutrient discharges in this 

region.

The anomalous point depression in genetic affinity exhibited by M. decactis off Terrebonne Bay, 

Louisiana, raises an important question. It is possible that some M. decactis larvae are being 

introduced to this region by more than one means. In the first case, they may be introduced from 

the Caribbean through the Loop Current entering the GOM through the Yucatan Straits and 

proceeding north to the Mobile, Alabama region (Transect IV compared with Transects I, II, and 

III; see Figure 18).

Figure 18. Map of the GOM, depicting examples of general currents known to 

exist. Note the general westerly current across the continental shelf 

in the vicinity of the FGB.

Alternatively, M. decactis larvae may have been introduced via a jet current from the northern 

portion of the Yucatan Peninsula (Lugo-Hernandez, 2006). In either case, it is clear that this M. 

population is different from all other M. decactis populations sampled in the northern GOM.

The geographic pattern exhibited in genetic affinity by T. coccinea in the northern GOM, as 

determined by STRUCTURE, had general similarities to that of M. decactis, in that there was a 

general decrease in affinity from west to east. The variations in this pattern that were observed 

in M. decactis, however, were much more dramatic in T. coccinea. Firstly, the highly 

precipitous drop-off in genetic affinity near the mouth of the Mississippi River indicates a sharp 

differentiation between the population off Mobile, Alabama, and all of those to the west of the 

Mississippi River. In addition, a deep point depression in genetic affinity of T. coccinea was 

noted south of Terrebonne Bay, Louisiana. It was similar to that observed in M. decactis, but 
41

much more dramatic. On that platform, there was clearly a population of this invasive species 

which was unrelated to the others in the northern GOM. This may represent an additional 

introduction of this species to the northern GOM. The source might be the Caribbean, as 

described above; but, it is also possible that this could represent a third successful introduction to 

the western Atlantic from a commercial vessel from the Indo-Pacific, its native habitat. In either 

case, this population is unrelated to the other populations in this large region.

Judging from the data collected here, it is clear that the Mississippi River represents a formidable 

east-west barrier to coral larval dispersal in this region. This was evidenced through the 

dichotomy of, and lack of genetic affinity between, coral populations on either side. Also, this 

pattern was clearly evident in two coral species: M. decactis and T. coccinea.

There are two possible explanations for the differences in differential population structure on 

either side of the Mississippi River, and they are not mutually exclusive. The first is that the 

lower salinity, higher sediment content, and higher nutrient concentrations associated with the 

Mississippi River plume (Rabalais et al., 1996; Dagg and Breed, 2003; Lohrenz et al., 2008; 

Dagg et al., 2008) may decrease coral larval survivorship levels in this region preventing the 

larvae from successfully crossing the Mississippi River plume and promoting genetic drift (see 

Bassim et al., 2002; Bassim and Sammarco, 2003). The second is that larvae from corals in the 

Caribbean Sea may be seeding platforms to the east of the Mississippi River plume as they are 

carried up by the Caribbean Current into the northern GOM by the Loop Current. Eddies that 

break off from the Loop Current can traverse the Main Pass region (eastern sector) in the northeastern GOM, dispersing larvae to the east of the Mississippi River plume before moving to the 

W-SW over the next six months or so (Oey et al., 2005). The two different sources of larvae, the 

FGB for the west and the Caribbean Sea for the east of the Mississippi River plume, could 

explain the lack of genetic affinity in Madracis and Tubastraea on either side of the Mississippi 

River plume.

Another point which emerged from this part of the study is that the invasive ahermatypic T. 

coccinea has higher larval dispersal and recruitment capabilities than even the most abundant 

and widely dispersed native hermatypic species encountered in this study: M. decactis. M

decactis populations on either side of the river mouth showed almost no genetic affinity to each 

other. The same was true for T. coccinea. Population recognition was stronger in the latter, 

however, between transects on either side for T. coccinea.

The results of this study support the findings from our earlier study (Sammarco et al., 2004; 

Brazeau et al., 2005; Atchison et al., 2008) that the M. decactis are highly isolated on these 

platforms and that their variation from each other most likely resulted from Founder Effect (also 

see Hellberg, 2007 for a discussion of similar patterns in deep-sea corals). That is, the local 

population was the product of the arrival of a small group of larvae from the mother FGB 

population to the west or another source to the east, which then expanded to create a subpopulation of that mother population with a different genetic signature characteristic of only a 

portion of the mother population. It was originally a sample-size effect. This is consistent with 

the Island Model of gene flow (Futuyma, 1998; Gold et al., 2001), as explained above, whereby 

small islands are seeded by a large mother population and then can also seed each other.
42

Another conclusion may be drawn from comparisons between M. decactis and T. coccinea. The 

higher dispersal rates exhibited by the latter species may be the reason it has been so successful 

in its distribution throughout the Greater Caribbean region over the past 70 years. Its 

reproductive, larval dispersal and recruitment capabilities are simply very good, better than the 

dominant native species in this study. Its dispersal capabilities are also better than those of one 

of the only other successfully introduced coral species, Fungia scutaria (P.W. Sammarco, pers. 

obs., 1973; J.C. Lang, pers. obs., 1972; LaJeunesse et al., 2005). Indeed, with reproductive 

characteristics like these (including rapid asexual reproduction and growth), the only reason that 

this species has not over-run our natural coral reefs is that it apparently does not compete well in 

natural systems, only in artificial habitats (breakwaters, platforms, bridge pilings, etc.).

It is possible that some other Indo-Pacific species may be similarly better adapted to reproduce 

and disperse than our native species, and that all precautions should be taken to eliminate them 

should they be successfully introduced in our waters. Sammarco et al. (2010) recently reported 

the new introduction of a closely related species–Tubastraea micranthus–into the northern GOM 

in the Grand Isle lease area, S-SW of the mouth of the Mississippi River. Once again, this area is 

close to the commercial shipping lanes (safety fairways) for the Mississippi River and Port 

Fourchon. We are currently attempting to determine the extent of the invasion. Nonetheless, a 

rapid eradication would be fitting for such species because of a) the possibility of success of such 

an invasion (Fitzhugh and Rouse, 1999), b) the decreasing probability of success of eradication if 

such is delayed (Simberloff, 2000; Hewitt et al., 2005), and c) major problems that can arise if 

one waits too long to proceed with eradication (Bergstrom et al., 2009; Casey, 2009).
43

5.0 CONCLUSIONS

5.1 ADULT CORAL COMMUNITIES ON PLATFORMS THROUGHOUT THE 

NORTHERN GULF OF MEXICO

− The peak in hermatypic coral species richness in the northern GOM occurs on the FGB. The 

number of species observed there is approximately three times higher than the maximum 

observed on any individual oil and gas platform. Coral species richness is very low inshore.

− The three dominant hermatypic/zooxanthellate coral species observed on the platforms were, 

in order of abundance, Madracis decactis, Diploria strigosa, and Montastraea cavernosa.

− The other hermatypic/zooxanthellate species observed on platforms were Porites astreoides,

Madracis formosa, Colpophyllia natans, Stephanocoenia intercepta, Stephanocoenia 

michelinii, and Millepora alcicornis.

− The dominant ahermatypic/azooxanthellate coral species on platforms in the northern GOM 

was Tubastraea coccinea.

− The other two species of ahermatypic corals on platforms were, in order of abundance, 

Oculina diffusa and Phyllangia americana.

− Total density of hermatypic/zooxanthellate corals peaked on the oil and gas platforms in the 

north-central GOM in Transect III, south of Terrebonne Bay, Louisiana. There was an 

extension of higher densities to the north, towards shore, in this region. Slight peaks in 

density were also noted around the FGB, south of Lake Sabine-Port Arthur, Texas, in 

Transect II. Densities dropped to low levels outside of these regions. In no case were corals 

observed within ~50km of shore.

− Densities of M. decactis, a brooder, peaked on platforms in the north central GOM at the 

edge of the continental shelf, south of Terrebonne Bay, Louisiana, in Transect III. Densities 

of this coral were primarily driving patterns of overall hermatypic coral density.

− Densities of D. strigosa, a broadcaster, the second dominant hermatypic in the platform 

coral community, peaked on platforms around the FGB, generally mimicking the pattern 

observed in coral species richness, with an extension of higher densities northward towards 

Port Arthur-Lake Sabine (Transect II).

− Densities of the third dominant hermatypic coral on the platforms–M. cavernosa, a 

broadcaster–was bimodal, with its highest peak around the FGB (Transect II) and a second 

somewhat lower peak on east side of the Mississippi River mouth off Mobile, Alabama in 

Transect IV.

− Density of all corals, including hermatypic/zooxanthellate and ahermatypic/azooxanthellate 

types, was four orders of magnitude higher than hermatypes alone and peaked in the central 

and eastern regions of the northern GOM. These peaks of tens of thousands of colonies 

occurred south of Terrebonne Bay, Louisiana and Mobile, Alabama (Transects III and IV).

− The density pattern of total corals in the northern GOM was driven entirely by the density of 

ahermatypic/azooxanthellate corals, which far surpassed the densities of hermatypic corals.

− The pattern of total ahermatypic/azooxanthellate corals in the northern GOM was driven 

entirely by the densities of T. coccinea, whose densities were in the tens of thousands per 

1,000 m2

.

− Species richness of ahermatypic corals was highest in the western region of the northern 

GOM, south of Matagorda Island, Texas, in Transect I. It peaked in each of the transect 
44

areas, in mid-shelf and inshore areas. In general, ahermatypic coral richness decreased from 

west to east. The peak in Transect I indicated that the FGB were most likely not the source 

of ahermatypic corals in the northern Gulf and that they most likely are derived from the 

southern GOM off Mexico or the Caribbean.

5.2 GENETIC AFFINITIES OF CORALS IN THE NORTHERN GULF OF MEXICO

− Using STRUCTURE, it was determined that genetic affinity between M. decactis

populations on the platforms was highest south of Port Arthur-Lake Sabine, Texas in 

Transect II and decreased with distance to the east. A slight increase in genetic affinity in the 

eastern region indicated some isolation between the populations on either side of the 

Mississippi River (Transects III and IV). A point-drop in genetic affinity at the shelf edge 

south of Terrebonne Bay, Louisiana indicated a population differing from all others in the 

northern GOM, most likely derived from elsewhere.

− Using STRUCTURE analyses, genetic affinity between populations of T. coccinea was also

found to be highest in the west off Matagorda Island, Texas (Transect I) and generally 

decrease to the east. A precipitous drop in genetic affinity off of Mobile, Alabama indicated 

a dramatic difference in populations between that area and the others to the west of the 

Mississippi River mouth, indicating that this hydrographic feature represents a formidable 

barrier to coral larval dispersal in this region.

− STRUCTURE also identified a point drop in genetic affinity offshore from Terrebonne Bay, 

Louisiana similar but more pronounced than that observed in M. decactis also indicates that a 

population very different from the others in the northern Gulf exists here. Either of these 

populations could have been seeded by the Loop Current derived from the Caribbean 

Current, or a jet current originating from the southern GOM.

− In a geographically-focused, high-resolution study of M. decactis populations, AFLPOP 

analyses revealed that the populations off of Terrebonne Bay, Louisiana and those off of 

Mobile, Alabama were almost 100% distinct from each other, showing almost no crosspopulation recognition. Home population recognition was extremely high but cross-site 

recognition was extremely low. These results indicated that larval dispersal across the river 

mouth and even between populations on platforms within a transect on either side of the river 

was highly limited.

− This same trend of no population recognition across the river mouth was even stronger in 

Tuabastraea coccinea. Self-allocation to home sites, however, was highly variable, and 

between-platform population recognition was higher than in M. decactis. This indicated that 

T. coccinea has higher larval dispersal and recruitment capabilities and gene flow than M. 

decactis. The fact that both species are brooders makes them directly comparable.

− The Mississippi River plume appears to be forming a barrier for coral larvae, inhibiting gene 

flow from either side of the plume to the other.
45

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