OCS Study 

BOEM 2015-005

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

New Invasive Species Colonizing

Energy Platforms in the Northern

Gulf of Mexico
OCS Study 

BOEM 2015-005

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

New Invasive Species Colonizing 

Energy Platforms in the Northern 

Gulf of Mexico

Authors

Melissa Genazzio

Paul W. Sammarco

James Sinclair

Yuasung Fiona Tung

January 2015

Prepared under BOEM Cooperative Agreement

by

M10AC20006

Louisiana State University

Coastal Marine Institute

Baton Rouge, Louisiana 70803
DISCLAIMER

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

(BOEM) 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 2015-005. 

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. 

US 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. New Invasive Marine Species Colonizing Energy Platforms in the 

Northern Gulf of Mexico: Verification, and Examination of Spread. US Dept. of the Interior, 

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

Study BOEM 2015-005. 72 pp. 

ABOUT THE COVER

Cover photo by Scott A. Porter. ©2013 by S.A. Porter. All rights reserved. Used with 

permission. 

iii
CONTRIBUTING AUTHORS

• Melissa Genazzio: Louisiana Universities Marine Consortium (LUMCON), 

Chauvin, Louisiana

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

Chauvin, Louisiana

• James Sinclair: Environmental Enforcement Division, Bureau of Safety and 

Environmental Enforcement, US Department of the Interior, New Orleans, 

Louisiana

• Yuasung Fiona Tung: Louisiana Universities Marine Consortium 

(LUMCON), Chauvin, Louisiana

ACKNOWLEDGEMENTS

We express our thanks to the many people who assisted us in this study: at LUMCON, C. 

Sevin, T. Widgeon, and M. Wike (ROV pilot); at ARACAR, J. Blitz and A. Campbell (ROV 

pilots); on the M/V Fling, B. Allen, K. Bush, K. Dies, M. McReynold, B. Oldham, M. Spurgeon, 

and J. Tyler; at National Aeronautics and Space Administration (NASA) and the US Air Force,

D. Perrenod; at LUMCON, D. Beltz (statistical analyses); Others, M. Gaskill. We also extend 

our thanks to the US Department of the Interior Bureau of Ocean Energy Management (BOEM), 

formerly the Minerals Management Service, (Grant #M10AC20006) through the Louisiana State 

University Coastal Marine Institute (CMI) program for their financial support of the project. 

Thanks to L. Rouse and S. Welsh who directed that program. 

iv
CONTENTS

LIST OF FIGURES........................................................................................................VII

LIST OF TABLES ..........................................................................................................IX

EXECUTIVE SUMMARY ................................................................................................ 1

1. INTRODUCTION........................................................................................................ 3

1.1. POPULATION EXPANSION ................................................................................................. 3

1.1.1. Species invasions in the marine environment ........................................... 3

1.1.2. Coral species invasions in the Atlantic....................................................... 3

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

1.2. DEPTH DISTRIBUTION....................................................................................................... 5

1.2.1. Invasion of Tubastraea spp. into the western Atlantic Ocean .................. 5

1.2.2. Depth distribution.......................................................................................... 6

1.2.3. Objectives ...................................................................................................... 6

1.3. COMPETITION FOR SPACE................................................................................................ 6

1.3.1. Invasive species, the marine environment, and Tubastraea micranthus 6

1.3.2. The importance of competition for space ................................................... 7

1.3.3. The importance of differences between Indo-Pacific and Atlantic 

species: comparative geological and evolutionary histories ................... 8

1.3.4. Objectives ...................................................................................................... 8

2. MATERIALS AND METHODS ................................................................................. 10

2.1. POPULATION EXPANSION ...............................................................................................10

2.1.1. Study sites and surveys .............................................................................10

2.1.2. Image processing........................................................................................10

2.2. DEPTH DISTRIBUTION.....................................................................................................12

2.2.1. Study sites and survey techniques ...........................................................12

2.2.2. Image processing........................................................................................12

v 
2.3. COMPETITION FOR SPACE..............................................................................................13

2.3.1. Image processing and assessment of competitive state ........................13

2.3.2. Data handling and analysis ........................................................................13

3. RESULTS................................................................................................................. 14

3.1. POPULATION EXPANSION ...............................................................................................14

3.1.1. Tubastraea micranthus ...............................................................................14

3.1.2. Tubastraea coccinea ...................................................................................21

3.2. DEPTH DISTRIBUTION.....................................................................................................21

3.3. COMPETITION FOR SPACE..............................................................................................33

3.3.1. Tubastraea micranthus ...............................................................................33

3.3.2. Tubastraea coccinea ...................................................................................38

4. DISCUSSION........................................................................................................... 48

4.1. POPULATION EXPANSION ...............................................................................................48

4.2. DEPTH DISTRIBUTION.....................................................................................................49

4.2.1. General discussion .....................................................................................49

4.2.2. Implications of depth distribution..............................................................51

4.3. COMPETITION FOR SPACE..............................................................................................51

LITERATURE CITED.................................................................................................... 54

vi 
LIST OF FIGURES

Figure 1. Map of the north-central Gulf of Mexico, showing locations of the 14 offshore 

energy platforms studied............................................................................................. 5

Fig. 2 Density of T. micranthus on 14 offshore oil/gas platforms in the northern Gulf of 

Mexico. Densities shown in no./10 m2 with 95% confidence limits. ...................... 15

Figure 3. Geographic distribution of the density of T. micranthus in the northern Gulf of 

Mexico, south of the Mississippi River mouth......................................................... 17

Figure 4. Mean colony size of T. micranthus in the northern Gulf of Mexico, south of the 

Mississippi River mouth. .......................................................................................... 18

Figure 5. Geographic distribution of the mean colony size of T. micranthus in the northern 

Gulf of Mexico, south of the Mississippi River mouth. ........................................... 20

Figure 6. Size-frequency distribution of colonies of T. micranthus on three platforms in 

the northern Gulf of Mexico, near the Mississippi River mouth.............................. 22

Figure 7. Density of T. coccinea in the northern Gulf of Mexico on 14 platforms off the 

mouth of the Mississippi River................................................................................. 23

Figure 8. Density of T. coccinea in the northern Gulf of Mexico on 14 platforms off the 

mouth of the Mississippi River................................................................................. 25

Figure 9. Depth distribution of T. micranthus in the northern Gulf of Mexico. ...................... 26

Figure 10. Depth distribution of T. coccinea in the northern Gulf of Mexico........................... 28

Figure 11. Geographic distribution of average depth of distribution for T. micranthus in the 

northern Gulf of Mexico........................................................................................... 29

Figure 12. Geographic distribution of average depth of distribution for T. coccinea in the 

northern Gulf of Mexico........................................................................................... 30

Figure 13. Depth distribution of T. micranthus observed on individual platforms in the 

northern Gulf of Mexico........................................................................................... 31

Figure 14. Depth distribution of T. micranthus observed on individual platforms in the 

northern Gulf of Mexico........................................................................................... 32

Figure 15. Depth distribution of T. coccinea observed on individual platforms in the 

northern Gulf of Mexico. Data based on colony density in no./10 m2. .................... 34

Figure 16. Depth distribution of T. coccinea observed on individual platforms in the 

northern Gulf of Mexico. Data based on percent-cover. .......................................... 35

Figure 17. Depth distribution of T. coccinea observed on individual platforms in the 

northern Gulf of Mexico. Data based on density in no./10 m2. ................................ 36

Figure 18. (A) Percent of wins in competition for space between T. micranthus and all other 

sessile epibiota, data pooled. Presented by platform. (B) Geospatial 

representation of the distribution of competitive wins in the study area.................. 37

vii
Figure 19. (A) Percent of successes in competition for space between T. coccinea and all 

other sessile epibiota, data pooled. Presented by platform. (B) Geospatial 

representation of the distribution of competitive wins in the study area.................. 39

Figure 20. Percent of successes in competition for space specifically between T. micranthus

compared with T. coccinea, from the former’s perspective as the target 

organism. Presented by platform. Mean plus 95% confidence limits shown........... 40

Figure 21. (A) Percent of wins in competition for space between T. coccinea compared with 

the encrusting sponge Xestospongia sp. (with the commensal Parazoanthus 

catenularis).). (B) Geospatial representation of the distribution of competitive 

wins in the study area. .............................................................................................. 41

Figure 22. (A) Percent of successes in competition for space between T. coccinea and the 

light grey encrusting sponge Dictyonella finicularis. (B) Geospatial 

representation of the distribution of competitive wins in the study area.................. 42

Figure 23. (A) Percent of wins in competition for space between T. coccinea and and the 

white encrusting sponge Haliclona vansoesti. Presented by platform. (B) 

Geospatial representation of the distribution of competitive wins in the study 

area............................................................................................................................ 43

Figure 24. (A) Percent of wins in competition for space between T. coccinea and the black 

encrusting sponge Xestopongia carbonaria. Presented by platform. (B) 

Geospatial representation of the distribution of competitive wins in the study 

area............................................................................................................................ 45

Figure 25. (A) Percent of wins in competition for space between T. coccinea and the brown 

encrusting sponge Mycale carmigropila. Presented by platform. (B) Geospatial 

representation of the distribution of competitive wins in the study area.................. 46

Figure 26. (A) Percent of wins in competition for space between T. coccinea and the red 

encrusting sponge Phorbas amaranthus. Presented by platform. (B) Geospatial 

representation of the distribution of competitive wins in the study area.................. 47

viii
LIST OF TABLES

Table 1. List of 14 platforms in the northern Gulf of Mexico, near the mouth of the 

Mississippi River, video-surveyed by ROV for the ahermatypic invasive IndoPacific corals T. micranthus and T. coccinea. .......................................................... 11

Table 2. Summary of results of a posteriori multiple comparisons of means tests performed 

on mean colony densities of T. micranthus on 14 energyplatforms in the northern 

Gulf of Mexico. ........................................................................................................ 16

Table 3. Summary of results of a posteriori multiple comparisons of means tests performed 

on average colony sizes of T. micranthus on 14 energy platforms in the northern 

Gulf of Mexico. ........................................................................................................ 19

Table 4. Summary of results of a posteriori multiple comparisons of means tests performed 

on mean colony densities of T. coccinea on 14 energy platforms in the northern 

Gulf of Mexico. ........................................................................................................ 24

ix
EXECUTIVE SUMMARY

POPULATION EXPANSION

A new Indo-Pacific scleractinian coral has invaded the northern Gulf of Mexico (GOM):

Tubastraea micranthus. Initially, it was observed on one energy platform (GI-93C) near the 

Mississippi River. We determined whether its populations were spreading throughout the region 

and whether there was evidence of rapid population expansion. We also compared population 

density data with that of Tubastraea coccinea, a congeneric species that had successfully invaded 

the western Atlantic Ocean 70 years earlier. Fourteen energy platforms were assessed down to 

138 m depth (max.) by a remotely operated vehicle (ROV) using digital video. Colony densities 

in no./m2 were determined for both species, and colony size for T. micranthus. Data were 

analyzed by platform and also with respect to geographic distribution. T. micranthus densities 

were highest on GI-93C and on GI-116A, southwest (SW) of the mouth of the Mississippi River, 

and were significantly higher than those on most other platforms. Densities declined radially 

from there. Mean colony size was highest on MC-311A, at the head of the Mississippi Canyon, 

suggesting this to be the epicenter of colonization. This site is characterized by blue water 

instead of the turbid, lower salinity water of other sites. This suggests that T. micranthus may 

grow best under blue-water conditions. Size frequency distributions of colonies for T. micranthus

were skewed towards the bin size of 1–200 cm2

, sometimes reaching >90% of the population, 

which indicates that populations are in an explosive growth phase. T. coccinea densities were 

high (range: ~50–300/m2

). Its populations were also centered SW of the Mississippi River. 

DEPTH DISTRIBUTION

In the marine environment, depth limits accessibility. We assessed the depth distribution of 

two Indo-Pacific invasive coral species–T. micranthus and T. coccinea–on 14 energy platforms 

using ROVs. Pooled data indicated that T. micranthus extends to 138 m and probably deeper and 

T. coccinea is generally limited to < 78 m. Average depths for T. micranthus were shallower 

west of the Mississippi River, in its plume, and deeper towards the east. T. coccinea exhibited a 

similar geographic depth-distribution pattern, but shallower and more subtle. Broad geographic 

patterns were similar, whether using density or percent-cover data as the variable for analysis. 

When considering individual platforms, we found that T. micranthus often reached the bottom 

and sometimes occurred at depths >66 m. T. coccinea were limited to shallow water on all 

platforms. Detailed depth distribution patterns varied significantly between analyses using

density and percent-cover as the variable for analysis; density probably indicated colonization 

rates, and percent-cover colony growth. Depth limitation by T. coccinea may be associated with 

food availability, rather than light, because it is azooxanthellate. 

1
COMPETITION FOR SPACE

Invasion success by an alien species into a new environment depends not only on the rate of 

reproduction, growth rates, mortality rates, or the physical characteristics of the environment; it 

also depends on the ability to successfully compete with native species for resources. For sessile, 

epibenthic marine species, one critical resource is space. We examined the competitive abilities 

of two invasive Indo-Pacific coral species, T. micranthus and T. coccinea, on 14 artificial reefs 

(offshore energy platforms) in the northern Gulf of Mexico, south of the Mississippi River. Both 

species were involved widely in competition for space. Underwater videos were taken using an 

ROV down to 183 m depth on the platforms, and still-capture photos were analyzed from them. 

T. micranthus was competitively superior (<90%) to all competitor species pooled, although 

frequency of competitive wins varied significantly between platforms. Success was highest in the 

NE part of the study area, characterized by blue water, and lowest in the SW area near the river 

plume, indicating that it may be better adapted to blue-water conditions. T. coccinea was also 

competitively superior to all competitors pooled, with little variability between platform 

populations and little spatial variation. The two congeneric coral species, when in competition 

with each other, were equal in their abilities for competitive success, although T. coccinea had a 

slight competitive advantage. T. coccinea’s abilities appeared to peak in the SW part of the study 

area, suggesting that it may be better adapted to surviving in a more turbid, nutrient-rich 

environment. T. coccinea was found to be competitively superior to almost all its competitors, 

including the encrusting sponge Xestospongia sp. (with the commensal Parazoanthus 

catenularis), the light grey encrusting sponge Dictyonella finicularis, the black encrusting 

sponge Xestopongia carbonaria, the brown encrusting sponge Mycale carmigropila, and the red 

encrusting sponge Phorbas amaranthus. Both T. micranthus and T. coccinea are formidable 

competitors for space in the northern Gulf of Mexico. 

2
1. INTRODUCTION

1.1. POPULATION EXPANSION

1.1.1. Species invasions in the marine environment

Species introductions can have major impacts on ecosystems (Roberts and Pullin, 2008). This 

is particularly so in the marine environment because of the ease with which reproductive 

propagules can disperse and colonize nearby habitats once they have established a new 

population (Griffiths, 1991; Johnson and Carlton, 1996; Wonham et al., 2000). Examples include 

marine algae (Chapman et al., 2006), such as Codium fragile, a Japanese cholorophyte 

apparently introduced through the ballast water of ships (Trowbridge, 1998; Pederson, 2000; 

Williams, 2007). This species is now common throughout much of the western Atlantic 

(Chapman, 1999). Another more recent example is the Indo-Pacific volitan lionfish (Pterois

volitans). This species was apparently released into western the waters of Florida ~20 years ago 

(Whitfield et al., 2002; Hamner et al., 2007) and is now distributed in US Atlantic waters off the 

state of New York and south through the Caribbean and South America (Albins and Hixon, 

2011). 

Vectors for transport of invasive marine or freshwater species (Kerr et al., 2005) include the 

ballast water of barges or ships (Chesapeake Bay Commission, 1995; ICES, 2002), the hulls of 

the same (Minchin and Gollasch, 2003), transfer through towing of energy platforms to new sites 

(Hicks and Tunnell, 1993), the accidental release of exotic species from mariculture operations 

(Sapota, 2004), and the deliberate release of exotics by aquarium hobbyists (Weidema, 2000; 

Christmas et al., 2001; Hindar et al., 2006). 

Recently, there has been concern about species occurring that have invaded the Gulf of 

Mexico (GOM; Osman and Shirley, 2007). This includes the Australian scyphozoan Phyllorhiza,

which colonized the region within the past 15 years (Perry and Graham, 2000) and has the ability 

to suppress seasonal zooplankton populations (Graham et al., 2003; Graham and Bayha, 2008) 

important for commercial fisheries. Another is the zebra mussel, Dreissena polymorpha, which 

was originally introduced to the Great Lakes in the mid-1980s and has since spread south 

through North American waterways (Baker et al., 2006; Dextrase and Mandrak, 2006; Ram and 

Pallazola, 2008) all the way to the mouth of the Mississippi River (Anon., 1997; Liffman, 1997). 

1.1.2. Coral species invasions in the Atlantic

There have been very few successful invasions of corals to the Atlantic. Fungia scutaria was 

accidentally introduced to Discovery Bay, Jamaica, W.I. (Judith C. Lang, pers. comm., 1972; 

Paul W. Sammarco, pers. obs., 1973; Lajeunesse et al., 2005). The Indo-Pacific T. coccinea

(Cairns and Zibrowius, 1997) species was first introduced into Puerto Rico in 1943 and by 1948 

had spread to Curacao, Netherlands Antilles (Cairns, 2000). By the late 1990s and mid 2000s, 

the species had spread to Belize and Mexico (Fenner, 1999), Venezuela, northern Gulf of 

Mexico, and the Florida Keys (Fenner, 2001; Fenner and Banks, 2004; Sammarco et al., 2004; 

Shearer, 2008), Brazil (Figueira de Paula and Creed, 2004), Colombia, Panama, the Bahamas, 

3
and throughout the Lesser and Greater Antilles (Cairns, 2000; Humann and Deloach, 2002). T. 

coccinea is also abundant in the Gulf of Mexico on artificial substrata (Sammarco et al. 2004, 

2007a, b, 2012a). It is present on energy platforms in abundances of hundreds of thousands of 

colonies per platform, with average colony densities reaching from 28/m2 to 300/m2. It also 

occurs on deep banks in the northern Gulf of Mexico but in lower abundances (Schmahl, 2003; 

Hickerson et al., 2006; Schmahl and Hickerson, 2006). It may also be found as a cryptic species 

on the coral reefs of the Flower Garden Banks (Fenner and Banks, 2004; Hickerson et al., 2008). 

From 2000 to 2010, Sammarco et al. (2004, 2007a,b, 2008, 2012a) and S.A. Porter conducted 

independent surveys using SCUBA and a remotely operated vehicle (ROV) on the distribution 

and abundance of scleractinian corals on 81 platforms; in both shallow and deep water 

throughout the northern Gulf of Mexico. Surveys stretched from the waters off Corpus Christi, 

Texas to those off Mobile, Alabama. In his surveys, Porter found a new invasive species for the 

Gulf of Mexico, the Indo-Pacific species T. micranthus; (Cairns and Zibrowius 1997) a congener 

of T. coccinea (Sammarco et al., 2010). It was observed on a single platform; GI-93-C 

(28o

32.96’N, 90o40.11’W; Fig. 1). This was near the crossing of two major safety fairways or 

shipping channels southwest of the Port of New Orleans, Louisiana, near the mouth of the 

Mississippi River and Port Fourchon, Louisiana. 

After a population of a new invasive species becomes established, its spread can be rapid and 

broad, greatly confounding any attempt to control or eradicate it (Elton, 2000). Examples include 

the invasion of the fire ant (Solenopsis invicta) into the US (Buhs, 2004). It was accidentally 

introduced into Mobile, Alabama during the 1940s and is now distributed around the margins of 

the 48 continental states in the US from the state of Washington to New Jersey, extending 

approximately 800 km inland. Another example is the nutria (Myocastor coypus), a South 

American coastal herbivore, 20 individuals of which were introduced into Avery Island, 

Louisiana during the 1930s. Its populations are now distributed from Delaware to Texas and 

reach to inland mid-eastern and northwest states. 

T. coccinea exhibited a similar broad range extension since it was introduced in the 1940s. It 

is not yet known whether T. micranthus exhibits the same population growth characteristics as its 

congener. T. coccinea exhibits asexual reproduction using budding, simple colony growth, and 

asexual planula production (Ayre and Resing, 1986; Shearer, 2008) and runner production 

(Pagad, 2007). It also uses sexual reproduction, producing gametes all year-round, even in the 

smallest colonies (2–10 polyps; Glynn et al., 2008a,b). The planular development period is six 

weeks, and the planulae settle and metamorphose within three days. Planular release occurs from 

March through July and it is considered to be highly fecund (Glynn et al., 2008a, b). Its larval 

dispersal capabilities are formidable (Sammarco, 2012b). If the reproductive and dispersal 

capabilities of T. micranthus are similar to those of T. coccinea, the former could reach similarly 

high abundances in the western Atlantic (Sammarco et al., 2004; Shearer, 2008). 

4
Longitude

-91.0 -90.5 -90.0 -89.5 -89.0

Latitude

28.0

28.5

29.0

29.5

ST-81A ST-75-JA

Location of Study Platfo 

 

Terrebonne Bay

Figure 1. Map of the north-central Gulf of Mexico, showing locations of the 14 offshore energy platforms 

studied. 

Platform GI-93-C (triangle) represents the site of first sighting of T. micranthus (Sammarco et 

al., 2010). MC-311A is suggested as the likely epicenter of colonization.

1.1.3. Objectives

The objectives of this part of this study were to quantify the abundance of T. micranthus on 

the initial site of observation (Platform GI-93-C) and to determine possible expansion, including 

degree, direction, and environment of expansion. This was accomplished by conducting surveys 

on GI-93-C and 13 other platforms in the vicinity of GI-93-C, extending from near the surface to 

the bottom of the platforms, 

1.2. DEPTH DISTRIBUTION

1.2.1. Invasion of Tubastraea spp. into the western Atlantic Ocean

T. micranthus (Ehrenberg, 1834; Cairns and Zibrowius, 1997) is an ahermatypic scerlactinian 

coral native to the Indo-Pacific, occurring in the Philippines (Schuhmacher and Zibrowius,

1985), the Maldives (Scheer and Obrist, 1986), the Red Sea (Kleemann, 1992), Australia (Veron 

and Pichon, 1980), and elsewhere. It is a member of the Dendrophyllidae and is azooxanthellate 

(Schuhmacher and Zibrowius, 1985). It was estimated to have colonized the northern GOM 

around 2005 or earlier. It was first reported in 2010 (Sammarco et al., 2010), and has since been 

expanding its populations to other platforms in that region (Sammarco et al., 2014). 

5
As an invasive coral species, T. micranthus is preceded by a congeneric species: T. coccinea.

The discovery of T. micranthus has raised the question of whether this species may have the 

same capabilities as its congener to spread throughout the western Atlantic. At this point, its 

populations are confined to an area with a radius of ~50 km, just south of the mouth of the 

Mississippi River (Sammarco et al., 2014). 

1.2.2. Depth distribution

The question arises as to whether this species is potentially harmful to native species and can 

disrupt native populations and community structure (Clout and Veitch, 2002a). This is a marine 

invasion, and many marine species possess swimming larvae dispersed by currents and often 

spend varying periods of time in the water column before settling (e.g., Ackerman et al., 1994; 

Osman and Shirley, 2007). This can be confounded by the species having multiple breeding 

seasons per year, or breeding continuously (Sakai et al., 2001), as is suspected to be the case with 

the lionfish (Pterois volitans; Albins and Hixon, 2008; also see Clout and Veitch, 2002b). T. 

coccinea is highly fecund and is known to reproduce year-round (Glynn et al., 2008 a, b). 

In the marine environment, accessibility is partially defined by depth and the new depth 

range of the target organism (Bax et al., 2002). Different depths may be reached by different 

techniques, and these techniques grow rapidly more expensive (and risky) with increasing depth. 

1.2.3. Objectives

In this part of the study, we assessed the depth-distribution of T. micranthus on a set of 

energy platforms in the northern GOM, near the mouth of the Mississippi River. The objectives 

of the study were: 1) to determine the depth distribution on the platform of first observation; 2) 

to determine the same for a number of platforms in the same region; and 3) to compare these 

depth distributions with those of its congener, T. coccinea. This was done with the assistance of 

an ROV. 

1.3. COMPETITION FOR SPACE

1.3.1. Invasive species, the marine environment, and Tubastraea micranthus

Invasive species have long been regarded as problematic for the communities they have 

colonized (Secord, 2003). Data about the impacts of invasive species are disparate and, in some 

cases, equivocal (Gurevitch and Padilla, 2004). In some cases, they have displaced native species 

entirely (Mooney and Cleland, 2001). In others, they have become dominant in the community 

(Belote et al., 2003). In still others, they have adapted to their new environs, becoming an 

integral part of the community without causing local extinction or major changes to the 

community (Preston et al., 2012). 

Over the past 70 years, there have been several invasions of coral species to the western 

Atlantic. A minor one was that of Fungia scutaria (Judith C. Lang, pers. comm., 1972; Paul W. 

Sammarco, pers. obs., 1973; Lajeunesse et al., 2005,), which was accidentally introduced to 

northern Jamaican waters in Discovery Bay. Fungia is a vagile Indo-Pacific fungiid coral and 

6
lives primarily on soft-bottom (Russell-Hunter, 1968). It has not yet been reported to have any 

major effect on the benthic community and appears to have integrated well into the community, 

having invaded what was probably an open niche in its Caribbean soft-bottom environment (see 

Alamaru et al., 2009 for description of unique feeding habits). A second suite of coral species 

has also invaded the western Atlantic, all congeners of the genus Tubastraea. The Indo-Pacific 

species T. coccinea was first recorded in Puerto Rico during the 1940s, most likely on the hull of 

a ship going through the Panama Canal (Cairns, 2000). Its geographic range expansion 

throughout the tropical and sub-tropical western Atlantic required 60–70 yrs. More recently, T. 

tagusensis and T. coccinea were first observed to occur in the Tamoios Ecological Station 

Marine Protected Area (MPA) in Brazilian waters in 2011, with rapidly expanding populations 

(Silva et al., 2011). 

We recently reported that populations of T. micranthus have invaded an area south of the 

Mississippi River mouth (Sammarco et al., 2010), colonizing offshore energy platforms in this 

region. They are also rapidly spreading throughout this region (Sammarco et al., 2014) and can 

occur to depths below the known depth-distribution of T. coccinea (Sammarco et al., 2012c). 

1.3.2. The importance of competition for space

Population growth rates, rate of range expansion, and broad depth distributions are not the 

only characters that define potential success of a biological aquatic invasion (Griffiths, 1991; 

Johnson and Carlton, 1996; Wonham et al., 2000). Nor are rates of colony growth, reproduction, 

or mortality (MacArthur and Wilson, 1967). Certainly, local environmental variables will help 

determine the ability of a new foreign species to survive there including temperature and salinity 

(Dassuncao, 2009); nutrient concentrations (Alino et al., 1992, Engelhardt, 2011); and turbidity, 

sedimentation, and light (Thomsen and McGlathery, 2007); not to mention potential exposure to 

native diseases. Space availability can also be a factor of influence. Many factors contribute to 

the success or failure of an introduction. One of the primary factors influencing invasion success 

is the ability of the invading species to out-compete native species for local resources (Callaway 

and Aschehoug, 2000; Mangla et al., 2011; Mooney and Cleland, 2001). In the case of a benthic 

or demersal species, and particularly for sessile, epibenthic species, this means competition for 

space (Stachowicz et al., 2002; Sorte et al., 2010; McIntosh et al. 2006), which is critical to 

survival. If a species cannot compete successfully for space, it will not succeed in its invasion, 

despite high potential colony growth rates, reproduction rates, larval dispersal rates, etc. 

(MacArthur and Wilson, 1967; MacArthur, 1972). 

Sessile epibenthic fauna use many mechanisms to compete for space. One is the chemical 

inhibition of one organism by another, or allelopathy, which has been demonstrated in numerous 

terrestrial plants (Rice, 1984; Qasem and Foy, 2001), Indo-Pacific alcyonacean soft corals on the 

Great Barrier Reef (Sammarco et al., 1983; Coll et al., 1990, 1992; Sammarco and Coll, 1992; 

Maida et al., 2001; Fleury et al., 2004), and between corals and algae (e.g., Ritson-Williams et 

al., 2013). Another is direct overgrowth of one species over another (Jackson and Buss, 1975;

Jackson, 1977), as in Millepora alcicornis overgrowing Gorgonia ventalina (Wahle, 1980). 

Another is biological disturbance, for example, using currents to divert food away from another 

neighboring organism, as occurs between bryozoans, sponges, and other organisms (Buss, 1979; 

Best and Thorpe, 1986). One of the most effective means that marine organisms have of 

7
expanding their territory, however, is extracoelenteric digestion (Lang, 1971, 1973; Sheppard

1979) or the development of sweeper tentacles (Richardson et al. 1979). This is a mechanism 

used by some scleractinian corals to defend and acquire space when dealing with other sessile 

epibenthic fauna, particularly other scleractinian corals. Mesenterial filaments are extruded from 

the gut of the coral, and extend some distance to touch a neighbor. Batteries of nematocysts 

housed in the filaments are discharged on the neighbor, resulting in death of that portion of the 

colony, leaving space for colony growth. 

1.3.3. The importance of differences between Indo-Pacific and Atlantic species: 

comparative geological and evolutionary histories

The geographic origin of an invading species may also have an influence on the probability 

of success of invasion. This is particularly true when considering different oceans of origin, i.e.,

the Atlantic compared with the Pacific. Evidence suggests that marine species from the western 

Indo-Pacific are better adapted with respect to predator defense than their Atlantic counterparts 

(Vermeij, 1978). This is a function of the geological and evolutionary history and the differential 

extent to which the two regions have remained undisturbed (Stanley 1979, 1981, 1984, 1985, 

1986; Sammarco and Coll, 1992). Palaentological data suggest that the western Pacific is an 

older, more stable, and less disturbed system, compared to the Atlantic (Kuhlmann, 1985). The 

Atlantic has experienced several major extinctions that the Pacific has not: one in the early 

Miocene, and one more recently in the late Pliocene and early Pleistocene, at the onset of the last 

glaciation. These extinctions were due primarily to major cooling events. Each represented a 

genetic bottleneck for the species living at those times and mass extinctions. This in turn resulted 

in a lower species diversity in the Atlantic, with species having broader niches and fewer specific 

mechanisms to deal with their prey and their neighbors in competition. Co-evolution of predator 

and prey, and among competitors, has not evolved to the level required for survival in the 

Pacific. 

Phylogenetic radiation in the Indo-Pacific species was permitted to continue at a more 

constant rate than in the Atlantic, in the absence of these perturbations. Species diversity grew, in 

congruence with the stability-time hypothesis (Klopfer, 1959; Klopfer and MacArthur, 1960, 

1961; Sanders, 1969; Slobodkin and Sanders, 1969). These new abundant species had narrower 

and narrower niches, becoming more effective, efficient, and aggressive with respect to 

competition and predation. When one of these Indo-Pacific species meets a species from the 

evolutionarily younger Caribbean fauna, provided that environmental conditions are agreeable to 

the invading species, the native species has a much lower chance of surviving the encounter. 

Thus, comparing competitive abilities is important for determining the ability of a new species to 

establish itself in a new environment and successfully expand its populations. 

1.3.4. Objectives

Here, we have assessed the competitive abilities of the Indo-Pacific T. micranthus against 

other sessile, epibenthic Caribbean fauna occurring on the pilings of offshore oil and gas 

platforms in the northern Gulf of Mexico. We assessed the frequency of success and wins and 

losses in competition for space in this new invasive species with respect to other species it 

encounters during colony growth. For comparative purposes, we also did this for T. coccinea. 

8
1.4 Duration

Because of field trip delays caused by poor weather conditions, this study ran for almost 

fouryears, including data collection, data analysis, graphic analysis, and writing. The data 

ultimately collected were voluminous, requiring us to break the research up into three subprojects, each of which received attention in data collection, processing, analysis, graphing, and

interpretation. Various reports were prepared during the course of the study and submitted to 

BOEM.

9
2. MATERIALS AND METHODS

2.1. POPULATION EXPANSION

2.1.1. Study sites and surveys

The study sites were Platform GI-93-C and 13 other platforms surrounding it within a 20 km 

radius (Fig. 1; Table 1). Specific platforms were chosen in consultation with BOEM. Surveys 

were performed using the M/V Fling (33 m, Gulf Diving, Inc., Freeport, Texas) and the R/V 

Acadiana (18 m, LUMCON, Cocodrie, Louisiana). The field portion of the study was conducted 

over two years, utilizing 12 days of ship-time. We spent approximately two-thirds of one day 

surveying each platform using an ROV. 

We used LUMCON’s Deep Ocean Engineering Phantom S2 ROV, which has 333 m of 

umbilical capable of surveying down to 170 m depth. We employed the techniques previously 

used successfully in earlier similar surveys (Sammarco et al., 2010, 2012a). ARACAR’s 

SeaBotix LBV-300 and BOEM’s similar ROV were also used as back-ups when the primary 

ROV required maintenance. All units were fitted with vertical and horizontal propulsion units, 

site-to-surface color video units, a topside monitor, lights, laser beams providing a spatial scale 

reference, and a sample retrieval unit (fixed grab). 

2.1.2. Image processing

Imagery was processed using a DellTM PrecisionTM 340 and T3400 desktop computers with a 

Pentium® 4 processor and a DellTM PrecisionTM M4300 Workstation fitted with a duo-core 

processor and Microsoft® video imaging software. Image analysis software included Nero® 7.0, 

VideoLAN®, and Microsoft® Windows® Media Player®, capable of zoom and still-image 

capture. Images were analyzed at each 3 m interval within a video transect. 

Data were collected for both T. micranthus and T. coccinea for comparative purposes. In the 

case of T. coccinea, population densities were so high (up to hundreds per still image) that counts 

were estimated visually using a log5 code system (0=1, 1=5, 2=25, 3=125, etc.), similar to that 

used in the field by Williams (1982) and Halford et al. (2004) for reef fish counts. Two laser dots 

of known inter-dot distance within the video field of view were used to standardize for both coral 

density (no. coral colonies per unit area) and coral colony size. A transparent 10 x 10 2.54 cm 

grid was placed over the computer screen to assist sampling and taking measurements. Mean 

colony densities were calculated for each platform along with standard deviations and 95% 

confidence limits. 

Colony size was measured for all T. micranthus colonies. Colonies were assumed to be 

elliptical in shape, and measurements were made of the major and minor axes, as was done by 

Sammarco (1980). Estimated area was calculated as A = π x r1 x r2, where r1 and r2 are the major 

and minor radii, respectively for a two-dimensional ellipse. Mean colony size was calculated for 

each platform along with standard deviations and 95% confidence limits. 

10
2.1.3. Data Analysis and Graphics

All quantitative data were logged in Microsoft® Excel® files and stored on the primary 

workstation. Data were backed-up on a 250G Western Digital® external hard-drive, updated 

daily as well as on the LUMCON computer network, which is backed-up regularly. 

Table 1. List of 14 platforms in the northern Gulf of Mexico, near the mouth of the Mississippi River, videosurveyed by ROV for the ahermatypic invasive Indo-Pacific corals T. micranthus and T. coccinea. 

Platform name, owner, and latitude and longitude of geographic location provided.

Platform

Number Code Owner Latitude Longitude

1 GI-90A-1 Apache Corp. 28.575144 -90.072429

2 GI-90A-2 Apache Corp. 28.575144 -90.072429

3 GI-93C Apache Corp. 28.548886 -90.068677

4 GI-115A Walter Oil & Gas Corporation 28.3076123 -90.0219665

5 GI-116A Apache Corp. 28.30928306 -90.07054334

6 MC-109A Stone Energy Corporation 28.86467752 -88.93079054

7 MC-311A Apache Corp. 28.642636 -89.794241

8 SP-87D Apache Corp. 28.72001853 -89.43078669

9 SP-89B Apache Corp. 28.680464 -89.387596

10 ST-75-JA(B) Stone Energy Corporation 28.76955709 -90.74085664

11 ST-81A Stone Energy Corporation 28.78656092 -90.42747823

12 ST-185A Black Elk Energy Offshore Operations, LLC 28.495501 -90.203098

13 ST-185B Black Elk Energy Offshore Operations, LLC 28.47493 -90.235942

14 ST-206A Apache Corp. 28.45372522 -90.38341283

Coral density data were analyzed by parametric tests. Analyses included ANOVA and a 

posteriori Multiple Comparison Tests between Means–T-K, GT-2, and T’ tests. Basic statistics 

(mean, s.d., n, range, g1 – skewness, and g2 – kurtosis) were calculated for colony size frequency 

distributions. Analyses were performed using BiomStat© 3.2 and 3.3 (Rohlf and Slice, 1996). In 

those cases where data were not normally distributed, they were transformed by square root of 

(Y+ 0.5) for assist in normalizing the data, allowing parametric statistics to be performed on 

them (Sokal and Rohlf, 1981). 

Two-dimensional graphics were performed using SigmaPlotTM 10.0. Some data are presented 

within a geographic context in three dimensions, and these were constructed using SURFER®

8.0 (Golden Software). Data consisted of latitudes, longitudes, and the variate in question. 

Averages were determined by kriging, a geostatisical gridding method designed for use with 

irregularly spaced data, using a smoothing interpolator. Additional details may be found in 

Golden Software® (2002). 

11
2.2. DEPTH DISTRIBUTION

2.2.1. Study sites and survey techniques

Platform GI-93-C served as a control platform of the study, because it was the location of 

first sighting and possessed the highest abundances of T. micranthus (Sammarco et al. 2010). 

Fourteen additional platforms surrounding it within a 20 km radius were also surveyed for T. 

micranthus (Fig. 1; Table 1; Sammarco et al., 2014). Some of these possessed this species, and a 

subset of these yielded population data sufficient in sample size to analyze for depth distribution; 

they were thus used in this study (Table 1). 

Surveys were run from the surface to the bottom of the platforms, to a maximum of 138 m 

depth. For safety purposes, the down-current side of the platform was always surveyed. This was 

done in order to keep the ROV and its umbilical outside of the structure, avoiding having the 

umbilical drawn inside the jacket of the platform and possibly snagging or becoming entangled 

there. (It is unlikely that there would be any difference in coral settlement on upcurrent vs. 

downcurrent sides of the platforms, because most settlement would have been influenced by 

microcurrents associated with pilings on either side. Obstruction of the far-field flow was 

minimal.) Vertical pilings were surveyed along with two sets of horizontal struts, usually at 

depths of 13–17 m, and 21–27 m. Two to four vertical pilings were surveyed and processed for 

each platform, depending upon the number of pilings available for survey and wind and sea 

conditions. 

The camera of the ROV was always oriented at a horizontal angle to the substrate. That is, 

the images being captured were always on vertical surfaces, whether a vertical, horizontal, or 

diagonal strut was being surveyed. This eliminated the possibility of any changes in competitive 

advantage from target organisms being differentially oriented on different parts of the platform 

jacket. 

2.2.2. Image processing

Within each video transect, images were frozen and analyzed at 2 m intervals. On vertical 

pilings, one quadrat was analyzed for every 2 m of depth per transect. Up to four replicates per 

depth (derived from up to four pilings) per platform were captured. These were analyzed for the 

presence of both Tubastraea species. Counts of these species were taken for each quadrat to 

provide colony density data (Sammarco, 2012a), standardized to no./10 m2

. The total for each 

taxa was tallied for each quadrat. Horizontal struts afforded many more quadrats per depth, of 

course, than vertical pilings; therefore, horizontal strut area was estimated and standardized for, 

before calculating densities of either species. 

So that we could compare depth distributions, data were collected for both T. micranthus and 

T. coccinea. Total colony density and percent-cover data were collected. Mean colony density

and percent-cover were calculated for each two-meter depth bin on each platform. 

Basic statistics (g1–skewness, and g2–kurtosis; see Sokal and Rohlf, 1981) were calculated 

for colony size frequency distributions. In addition, distributions were tested against normal and 

Poisson distributions using Lilliefors Tests and Goodness of Fit Tests, respectively. 

12
2.3. COMPETITION FOR SPACE

All platforms surveyed for T. micranthus occurred within a 25 km radius south of the mouth 

of the Mississippi River (Figure 1; Table 1; see Sammarco et al., 2014). Some of these possessed 

this species. A subset of that group yielded population data sufficient in size to analyze T. 

micranthus with respect to competition for space. All 14 platforms were used for competition 

analysis in T. coccinea. On each platform, two to four vertical pilings were surveyed as well as 

two sets of horizontal struts, usually at depths of 13–17 m, and 21–27 m. The number of vertical 

pilings surveyed was dependent upon the number of pilings available for survey and wind and 

sea conditions. 

2.3.1. Image processing and assessment of competitive state

In order to make comparisons regarding the capabilities of these two congeneric invasive 

species, data were collected for both T. micranthus and T. coccinea. Because T. coccinea

population densities were extraordinarily high, competitive interaction data were collected only 

from up to five colonies per quadrat, selected haphazardly. 

Evidence of either intra- or inter-specific competition involving corals and other sessile 

epibiota was recorded. The techniques used here were similar to those used by Sammarco (1980, 

1982) and Sammarco and Carleton (1982). Sessile epibenthic organisms occurring within 8 mm 

of the target coral were assessed for interaction. An observation of overgrowth was assumed to 

be evidence of competition for space, as has been used in previous related studies (Jackson and 

Buss, 1975; Jackson, 1977). Competitive encounters were judged from the target coral’s 

perspective. The competition variable was defined simply as the number of competitive wins 

over the total number of interactions within that taxonomic group. 

2.3.2. Data handling and analysis

The test used to determine whether there were significant successful wins in a set of 

interactions (where the win frequency is significantly greater than 50%) was Fisher’s Exact Test. 

Differences in competitive success between populations on the respective platforms were 

examined using a Goodness of Fit Test using the G-statistic, against a 1:1 expected ratio of win 

to loss (Sokal and Rohlf, 1981). Data were plotted as bar diagrams using SigmaPlot 10.0. 

13
3. RESULTS

3.1. POPULATION EXPANSION

3.1.1. Tubastraea micranthus

Data derived from analysis of ROV videos revealed that T. micranthus had indeed spread to 

surrounding platforms. Of 14 platforms surveyed, this species was found on nine (Fig. 2). 

Densities varied by platform and was highest on the platform of initial sighting, GI-93-C, 

averaging approximately 15/m2

. ANOVAs and subsequent a posteriori tests revealed that 

densities on GI-93C and GI-116A were significantly higher than all other platforms Table 2). 

Details about inter-platform comparisons may be found in Table 2. T. micranthus did not occur 

on ST-185A&B, GI-94B, ST-81A, or ST-75JA(B). 

When density data were placed into a geographic context, it could be seen that the peak 

density occurred to the southwest of the mouth of the Mississippi River (Fig. 3), next to two 

major safety fairways servicing the Port of New Orleans and the Port Fourchon (Sammarco et 

al., 2010). A second somewhat smaller peak in density could be seen south of GI-93-C. In 

general, densities fell off in all directions in near proximity to these points, with a minor peak 

west-southwest of the Mississippi River mouth. There was a moderate rise in densities to the east 

of the Mississippi River. 

Patterns of average colony size for T. micranthus did not follow that of average density. The 

maximum average colony size was found on Platform MC-311-A (Fig. 4). It was significantly 

higher than on all other platforms (Table 3), suggesting that this may have been the site of 

original colonization. The reason for this is that these colonies may have had the longest time of 

opportunity for growth compared with the other platforms. The next largest average T. 

micranthus colony size was found on MC-109A, and it was significantly higher than the average 

colony size on SP-87D, GI-116A, and GI-93C. Average colony size on almost all other 

platforms was equivalent (not significantly different), except GI-115A versus ST-206A, and GI93C compared with SP-87D. When these differences are placed into a geographical context, the 

three-dimensional representation of average colony size demonstrates that colony size not only 

peaks at MC-311-A, but it also drops off evenly from that point in all directions, with no 

secondary peaks in that region (Fig. 5). 

14
Figure 2. Density of T. micranthus on 14 offshore oil/gas platforms in the northern Gulf of Mexico. 

Densities shown in no./10 m2 with 95% confidence limits. 

In overview, most densities are highly significantly different from each other (p < 0.001, one-way 

ANOVA). Data transformed by square-root of (Y + 0.5) for purposes of normalization (Sokal and Rohlf, 

1981). See Table 2 for details of pair-wise inter-platform comparisons and text for detailed discussion.

15
Table 2. Summary of results of a posteriori multiple comparisons of means tests performed on mean 

colony densities of T. micranthus on 14 energyplatforms in the northern Gulf of Mexico. 

T’, T-K, and GT-2 tests were used. Results of pairwise comparisons shown. Platforms are shown in order 

of density, high to low. An asterisk denotes a significant difference between coral densities on two given 

platforms. 

Platform 

Name

GI93CGI116AMC109AST206ASP87DMC311AG115ASP89BGI90AST185AST185BGI94BST81AST75JA(B)

GI-93C * * * * * * * * * * * 

GI-116A * * * * 

MC-109A 

ST-206A 

SP-87D 

MC-311A 

G-115A 

SP-89B

GI-90A

ST-185A

ST-185B

GI-94B

ST-81A

ST75JA(B)

16
Figure 3. Geographic distribution of the density of T. micranthus in the northern Gulf of Mexico, south of the Mississippi 

River mouth. 

Location of platforms shown by platform numbers. Platform codes of numbers shown in Table 1. Note the primary peak 

(at GI-93C), the secondary peak (at GI-116A).

Density No./m

2

T. micranthus Density, N. Gulf of Mexico

17
Figure 4. Mean colony size of T. micranthus in the northern Gulf of Mexico, south of the Mississippi River 

mouth.

Note that the largest average colony sizes are found on MC-311A, a site which occurs in blue water 

in Mississippi Canyon, unlike many of the other sites. Significant difference between colony sizes on 

different platforms (p < 0.001, one-way ANOVA; see Table 2 for detailed comparisons). Data transformed 

by square root (Y + 0.5) for normalization purposes.

Tubastrea micranthus 

Mean Colony Size by Platform

Mean Colony Size (cm2

)

y

GI-93-C

MC-109A

ST-206A

ST-185-B

GI-94-B

GI-116-A

MC-311-A

GI-115-A

ST-185-A

SP-87D

SP-89B

GI-90-A

ST-81A

ST-75JA(B)

Platform

_ 10

_ 100

_ 500

_ 1000

_ 1500

_ 2500

18
Table 3. Summary of results of a posteriori multiple comparisons of means tests performed on average 

colony sizes of T. micranthus on 14 energy platforms in the northern Gulf of Mexico. 

T’, T-K, and GT-2 tests were used. Results of pairwise comparisons shown. Platforms are shown in 

order of average colony size, high to low. An asterisk denotes a significant difference between coral 

colony sizes on two given platforms. 

Platform 

Name

MC-311A MC-109A ST-206A SP89-B SP-87D GI-116A GI-93C GI-90A GI-115A

MC-311A * * * * * * * * 

MC-109A * * * 

ST-206A * 

SP89-B 

SP-87D * 

GI-116A 

GI-93C 

GI-90A

GI-115A

19
Figure 5. Geographic distribution of the mean colony size of T. micranthus in the northern Gulf of Mexico, south of the Mississippi River mouth. 

Note the primary peak (at MC-311A, within Mississippi Canyon) and how average colony size decreases radially from that point, indicating that 

this site may be the initial site of colonization and may also possess the best environmental conditions for growth for this species.

Colony 

Size in

cm

2

Tubastraea micranthus

Mean Colony Size, All Platforms

20
Size-frequency diagrams were constructed for T. micranthus colonies on each of the 

platforms. In the case of Platform MC-109-A, it could be seen that a large proportion of the 

colonies - ~60% - were between one and 100 cm2 in area (max. diameter = ~11 cm) in area (Fig. 

6). Frequencies of larger average colony sizes fell off rapidly after this. This pattern of a 

logarithmic decrease in frequency of average colony sizes, heavily biased towards the smallest 

size frequency, was mimicked on all other platforms where T. micranthus occurred, as 

exemplified by GI-93C and GI-116A (Fig. 6). 

3.1.2. Tubastraea coccinea

T. coccinea invaded the Caribbean approximately 70 years ago (Cairns, 2000). Its range 

extension has been broad, and its abundances have reached high levels, particularly more 

recently on artificial reefs such as the platforms in the Gulf of Mexico. 

Densities of T. coccinea were much higher than those of T. micranthus, with the highest 

densities in this survey reaching about 300 colonies/m2

, 20-fold higher than that of the new 

invasive species: T. micranthus (Fig. 7). The platforms had significantly different densities. 

Platform ST-185B exhibited the highest concentrations of T. coccinea, which were equivalent to 

those on GI-116A, but higher than all other platforms (Table 4). Densities on GI-116A were 

higher than almost all other platforms. Densities on ST-206A were approximately equivalent to 

all other platforms except the above two plus MC-311A and ST-185A. 

The geographic distribution of T. coccinea colony density in the study region is similar in 

pattern to that of T. micranthus (Fig. 8). The major peaks in both species occur southwest of the 

mouth of the Mississippi River, and densities decrease radially in all directions from there. 

3.2. DEPTH DISTRIBUTION

Results from data based on density compared with percent-cover were tested against each 

other and found to be highly significantly different from each other. This was despite apparent 

visual similarities in the depth distributions. Thus, results are shown for both of these variables. 

When depth distribution data based on coral density from all platforms were pooled, it 

became evident that T. micranthus occurred at all depths surveyed, from near the surface to the 

deepest encountered in this study: 138 m (Fig. 9a). This was also evident if one measured 

abundance by percent-cover (Fig. 9b). The overall pattern of depth distribution was bi-modal, 

with a peak in shallow water (12-18 m) and another set of peaks in deep-water (108–138 m), and 

the pooled depth distribution data based on density and percent-cover of T. micranthus yielded 

visually similar (although significantly different) patterns (Fig. 9b). 

21
Figure 6. Size-frequency distribution of colonies of T. micranthus on three platforms in the northern Gulf 

of Mexico, near the Mississippi River mouth.

MC-109A (top), GI-93C (bottom left), and GI-116A (bottom right). Examplary of distributions found on 

all platforms. Note the over-representation of smaller-sized colonies, indicating explosive population 

growth with low doubling times. Platform MC-109A: Mean = 198.6 cm2, s.d. = 281.14, ni = 47, g1 = 1.92, 

g2 = 3.36. Platform GI-116A: Mean = 34.7 cm2, s.d. = 45.60, ni = 24, g1 = 2.74, g2 = 8.23. Platform GI93C: Mean = 33.5 cm2, s.d. = 94.18, ni = 472, g1 = 7.19, g2 = 63.11. 

22
Figure 7. Density of T. coccinea in the northern Gulf of Mexico on 14 platforms off the mouth of the 

Mississippi River. 

Densities shown in no./m2 with 95% confidence limits. Densities are highly significantly different from 

each other (p < 0.001, one-way ANOVA). Data transformed via square-root of (Y + 0.5) for purposes of 

normalization (Sokal and Rohlf, 1981). See Table 4 for details of inter-platform comparisons. 

23
Table 4. Summary of results of a posteriori multiple comparisons of means tests performed on mean 

colony densities of T. coccinea on 14 energy platforms in the northern Gulf of Mexico. 

T’, T-K, and GT-2 tests were used. Results of pairwise comparisons shown. Platforms are shown in 

order of density, high to low. An asterisk denotes a significant difference between coral densities on two 

given platforms. 

ST185BGI116AMC311AST185AGI94BST206AGI115AMC109ASP87DGI93CSP89BGI90AST81AST75JA(B)

ST-185B * * * * * * * * * * * * 

GI-116A * * * * * * * * * * 

MC-311A * * * * 

ST-185A * * 

GI-94B * 

ST-206A 

GI-115A 

MC-109A 

SP-87D 

GI-93C

SP-89B 

GI-90A 

ST-81A

ST75JA(B)

24
Figure 8. Density of T. coccinea in the northern Gulf of Mexico on 14 platforms off the mouth of the Mississippi River. 

Densities shown in no./m2 with 95% confidence limits. Densities are highly significantly different from each other (p < 0.001, one-way ANOVA). 

Data transformed via square-root of (Y + 0.5) for purposes of normalization (Sokal and Rohlf, 1981). See Table 4 for details of inter-platform 

comparisons. 

Density

No./m

2

T. coccinea

Mean Density, All Platforms

1

3

4

5 6 89 7

1

1

1 1

1

3

25
Figure 9. Depth distribution of T. micranthus in the northern Gulf of Mexico. 

Data pooled from three production platforms: GI-93C, GI-116A, and MC-109A. (A) Distribution based 

on colony density in no./10 m2 and provided in frequency (%). Significantly different than a normal 

distribution (p < 0.001, Lilliefors test) and Poisson distribution (p < 0.001, Goodness of Fit test). g1 = 

8.583, g2 = 94.761. (B) Same, but data derived from percent-cover instead of colony density. Significantly 

different than a normal distribution (p < 0.001, Lilliefors test) and Poisson distribution (p < 0.001, 

Goodness of Fit test). g1 = 9.597, g2 = 113.657. Significant difference between T. micranthus

distributions measured by density vs. percent-cover (p < 0.001, R x C Goodness of Fit test). 

26
T. coccinea occurred in much higher numbers than T. micranthus on all platforms, and, for 

this reason, afforded higher sample sizes and more sites for analysis. When all coral density data 

were pooled for T. coccinea over all platforms, this species exhibited quite a different pattern in

depth distribution than T. micranthus, whether measured by colony density or percent-cover

(Fig. 10). Overall, effective maximum depth of T. coccinea on all of the study sites was 72–78 

m, although a small number of colonies were observed at 90–96 m depth at one site (Fig. 10). 

The general distribution was unimodal, with peak average colony density occurring at 30–36 m 

depth when measured by colony density and 18–24 m when measured by percent-cover. It 

should be noted that T. coccinea percent-cover reached levels of ~60% on this platform, clearly 

dominating the benthic community. 

When one considers the overall geographic pattern of this distribution, it is clear that the 

shallowest average depths of distribution based upon colony density are found west of the 

Mississippi River mouth (Fig. 11a). The deeper distributions are southeast of the Mississippi 

River mouth, at the head of the Mississippi River Canyon. The pattern of average depths with 

respect to geographic position using percent-cover as the variable is almost identical to that 

calculated using coral density (Fig. 11b). 

When one examines the geographic pattern of these average depths based on average colony 

density in T. coccinea, it varies greatly from that of T. micranthus. First, it is relatively uniform 

across the study area (Fig. 12a). Second, on the average (as might be expected from the previous 

analysis), it is much shallower than T. micranthus. There is a general trend for average depth of 

T. coccinea to be shallower in the west than in the east, in a manner similar to that of T. 

micranthus; but, by comparison, this trend is much more subtle. The trends, once again, are 

almost identical between patterns generated through colony density and percent-cover (Fig. 12b). 

Examining patterns of depth distribution in T. micranthus on individual platforms based on 

coral density provides additional insight into the overall patterns. On GI-93C, the coral extended 

in distribution all the way to the bottom: 66 m (Fig. 13a). It also exhibited a clear peak average 

density in relatively shallow water at 18-24 m depth. By comparison, on MC-109A, this species 

not only extended all the way to the bottom of the platform at 138 m depth, it was not observed 

until a depth of 66–72 m (Fig. 13b). Where it occurred on this platform, the distribution was 

relatively uniform. On GI-116A, T. micranthus’ depth distribution was limited to mid-depths of 

30–48 m (Fig. 13c). GI-116A lies in close proximity to GI-93C, to the south. 

When one considers the same platforms as above, but using percent-cover instead of colony 

density data for T. micranthus, once again, similar patterns are revealed. With respect to GI-93C, 

average percent-cover peaks at 18–24 m depth, but extends down to 54–60 m (Fig. 14a). On 

MC-109A, the range of occurrence was once again 66–138 m depth, although percent-cover 

peaked at 102–108 m depth (Fig. 14b). On GI-116-A, the pattern of average percent-cover 

closely mimicked that generated by density data, concentrating at mid-depths, ranging from 30–

48 m depth (Fig. 14c), but being absent from both very shallow and very deep waters. 

27
Figure 10. Depth distribution of T. coccinea in the northern Gulf of Mexico. 

Data pooled from six energy production platforms: GI-94B, GI-116A, MC-311A, ST-185A, ST-185B, 

and ST-206A. (A) Distribution based on colony density in no./10 m2 and provided in frequency (%). 

Significantly different than a normal distribution (p < 0.001, Lilliefors test) and Poisson distribution (p < 

0.001, Goodness of Fit test). g1 =4.001, g2 = 33.264. (B) Same, but data are derived from percent-cover 

instead of colony density. Significantly different than a normal distribution (p < 0.001, Lilliefors test) and 

Poisson distribution (p < 0.001, Goodness of Fit test). g1 = 0.553, g2 = -1.123. Significant difference 

between T. coccinea distributions measured through densitycompared with percent-cover (p < 0.001, R x 

C Goodness of Fit test). Significant difference between distributions of T. micranthus compared with T. 

coccinea whether measured by density or percent-cover (p < 0.001, R x C Goodness of Fit Test, in both 

cases). 

28
Figure 11. Geographic distribution of average depth of distribution for T. micranthus in the northern Gulf of Mexico.

(A) Data based on colony density in no./10 m2. (B) Data based on percent-cover.

29
Figure 12. Geographic distribution of average depth of distribution for T. coccinea in the northern Gulf of Mexico. 

(A) Data based on colony density in no./10 m2. (B) Data based on percent-cover. 

30
Figure 13. Depth distribution of T. micranthus observed on individual platforms in the northern Gulf of 

Mexico. 

Data based on colony density in no./10 m2. Heavy horizontal line represents depth of bottom. (A) T. 

micranthus depth distribution on Platform GI-93C. n = 125. Significantly different than a normal 

distribution (p < 0.001, Lilliefors test) and Poisson distribution (p < 0.001, Goodness of Fit test). g1 = 

4.000, g2 = 20.646. (B) T. micranthus depth distribution on MC-109A. Significantly different than a normal 

distribution (p < 0.001, Lilliefors test) and Poisson distribution (p < 0.001, Goodness of Fit test). g1 = 

2.358, g2 = 5.095. (C) Depth distribution of T. micranthus on GI-116A. Significantly different than a normal 

distribution (p < 0.001, Lilliefors test) and Poisson distribution (p < 0.001, Goodness of Fit test). g1 = 

4.659, g2 = 23.771. 

31
Figure 14. Depth distribution of T. micranthus observed on individual platforms in the northern Gulf of 

Mexico. 

Data based on percent-cover. Heavy horizontal line represents depth of bottom. (A) T. micranthus

depth distribution on Platform GI-93C. n = 125. Significantly different than a normal distribution (p < 

0.001, Lilliefors test) and Poisson distribution (p < 0.001, Goodness of Fit test). g1 = 3.973, g2 = 16.836. 

(B) T. micranthus depth distribution on MC-109A. Significantly different than a normal distribution (p <

0.001, Lilliefors test) and Poisson distribution (p < 0.001, Goodness of Fit test). g1 = 2.310, g2 = 5.311. (C) 

Depth distribution of T. micranthus on GI-116A. Significantly different than a normal distribution (p < 

0.001, Lilliefors test) and Poisson distribution (p < 0.001, Goodness of Fit test). g1 = 4.403, g2 = 19.336. 

32
T. coccinea’s depth distribution patterns on individual platforms were variable, but always 

remained within the shallower depths. On Platform GI-116A, T. coccinea exhibited its highest 

average densities at relatively shallow depths: 12–18 m (Fig. 15a). Another peak set of densities 

was observed at 36–42 m. This platform occurred in 78 m of water, and T. coccinea’s deepest 

colonies occurred at 60–66 m. Platform ST-185B occurred in waters only 48 m deep, and T. 

coccinea’s depth distribution extended all the way to the bottom there (Fig. 15b). Its peak colony 

densities were found, however, to occur at 30–36 m depth. On nearby Platform ST-185A, the 

pattern of depth distribution and maximum depth of occurrence was similar in range and peak 

depth to ST-185B (Fig. 15c). 

The pattern of depth distribution of T. coccinea on Platform GI-116A based on percent-cover 

was broken. It was tri-modal in character, with percent-cover peaking at 12–18 m depth, 30–36 

m, and 60–66 m (Fig. 16a). In addition, colonies were observed down to the maximum depth of 

the platform at 72–28 m. On Platforms ST-185A&B, the patterns of depth distribution were 

similar with respect to peak percent cover (30–76 m), but the depth of this region was only 48–

54 m (Figs. 16b&c). T. coccinea again extended all the way to the bottom. 

On Platform ST-206A, an overall trend very similar to that observed on ST-185A&B was 

observed, except peak colony densities occurred at 18–24 m depth instead of 30–36 m (Fig. 17a). 

The pattern was similar on GI-94B, with a higher peak in colony density at 24–30 m depth (Fig. 

17b). The pattern on Platform MC-311A was somewhat different. First, the maximum depth of 

the platform was 120 m. The maximum depth of colonization and growth in T. coccinea was 

66–72 m (Fig. 17c). This underlines the fact that T. coccinea was distributed in relatively shallow 

water. Second, its depth distribution pattern was unimodal, almost following a normal 

distribution, with higher colony densities occurring over a wide depth range, covering 12–54 m. 

3.3. COMPETITION FOR SPACE

3.3.1. Tubastraea micranthus

From the ROV videos, it became evident that, on average, T. micranthus was a significantly 

better competitor for space than its associates (Fig. 18a). Its positive competitive abilities, 

however, varied significantly between populations. Competitive success varied from ~50% on 

GI-90A to ~90% on SP-87-D. On no platform, however, did T. micranthus’ competitive success 

fall below 50%. This meant that, overall, it was winning in competition against the other sessile 

epibenthic fauna it encountered and that it could slowly dominate the community. With respect 

to its geographic distribution throughout the study area, the highest average win frequency 

occurred in the NE portion of the study area off Venice, Louisiana, and the lowest win frequency 

occurred in the SW, off Grand Isle, Louisiana (Fig. 18b). 

33
Figure 15. Depth distribution of T. coccinea observed on individual platforms in the northern Gulf of 

Mexico. Data based on colony density in no./10 m2.

Heavy horizontal line represents depth of bottom. (A) T. micranthus depth distribution on Platform ST185B. n = 20. Significantly different than a normal distribution (p < 0.001, Lilliefors test) but not 

significantly different from a Poisson distribution (p >0.05, Goodness of Fit test). g1 = 3.639, g2 = 14.634. 

(B) T. micranthus depth distribution on MC-109A. Significantly different than a normal distribution (p < 

0.001, Lilliefors test) and Poisson distribution (p < 0.001, Goodness of Fit test). g1 = 2.358, g2 = 5.095. (C)

Depth distribution of T. micranthus on GI-116A.Significantly different than a normal distribution (p < 0.001, 

Lilliefors test) and Poisson distribution (p < 0.001, Goodness of Fit test). g1 = 4.659, g2 = 23.771. 

34
Figure 16. Depth distribution of T. coccinea observed on individual platforms in the northern Gulf of 

Mexico. Data based on percent-cover. 

Heavy horizontal line represents depth of bottom. (A) T. coccinea depth distribution on Platform GI116A. n = 44. Not significantly different than a normal distribution (p > 0.05, Lilliefors test) but significantly 

different from a Poisson distribution (p > 0.001, Goodness of Fit test). g1 = 0.539, g2 = -0.460. (B) T. 

micranthus depth distribution on ST-185B. Significantly different than a normal distribution (p < 0.01, 

Lilliefors test) but not from a Poisson distribution (p > 0.05, Goodness of Fit test). g1 = 0.489, g2 = -1.296. 

(C) Depth distribution of T. micranthus on ST-185A. Significantly different than a normal distribution (p < 

0.001, Lilliefors test) and Poisson distribution (p < 0.001, Goodness of Fit test). g1 = 0.827, g2 = -0.750. 

35
Figure 17. Depth distribution of T. coccinea observed on individual platforms in the northern Gulf of 

Mexico. Data based on density in no./10 m2.

Heavy horizontal line represents depth of bottom. (A) T. coccinea depth distribution on Platform ST206A. n = 50. Significantly different than a normal distribution (p < 0.05, Lilliefors test) and a Poisson 

distribution (p < 0.01, Goodness of Fit test). g1 = 0.591, g2 = -0.254. (B) T. micranthus depth distribution 

on MC-311A. Significantly different than a normal distribution (p < 0.001, Lilliefors test) and from a 

Poisson distribution (p < 0.001, Goodness of Fit test). g1 = 1.092, g2 = 0.521. (C) Depth distribution of T. 

micranthus on GI-94B. Significantly different than a normal distribution (p < 0.001, Lilliefors test) and a 

Poisson distribution (p < 0.001, Goodness of Fit test). g1 = 2.837, g2 = 14.899. 

36
Figure 18. (A) Percent of wins in competition for space between T. micranthus and all other sessile 

epibiota, data pooled. Presented by platform. (B) Geospatial representation of the distribution 

of competitive wins in the study area.

(A) Percent competitive success plus 95% confidence limits shown. Tested for significant variation 

from 1:1 ratio of wins to losses. Competitive successes significantly higher than neutral outcome or losses 

(p < 0.01, Fisher’s Exact Test). Also, significant difference between frequency of competitive success 

between platforms (p < 0.001, Goodness of Fit Test, G-statistic). 

37
3.3.2. Tubastraea coccinea

Because T. coccinea was much more abundant and widely distributed on the platforms than 

T. micranthus, it became possible to assess it for its competitive ability against various 

epibenthic organisms. Due to low sample size, this was not possible in T. micranthus. 

Competitive success in T. coccinea against all other sessile epibiota was highly significantly 

positive at an average of ~55% wins. There was significant variation between platforms (Fig. 

19a), but that variability was actually quite low in magnitude, indicating a uniform ability of this 

species to compete for space. This uniformity was reflected in a geospatial analysis of the 

competitive abilities of this species (Fig. 19b). 

When competition for space was examined specifically between T. micranthus against T. 

coccinea, from the perspective of T. micranthus, overall, the two species were found to be

approximately equivalent in competitive abilities (Fig. 20). This response did not appear to vary 

between platform populations. The win frequency averaged ~50% on all platforms where both 

species were found actively competing with each other for space. 

Several taxa emerged as common competitors of T. coccinea. The first taxonomic group 

observed to be an important competitor for space was the encrusting sponge Xestospongia sp. 

(with the commensal Parazoanthus catenularis). T. coccinea was significantly competitively 

superior to this sponge on all platforms, with an overall success rate of ~75%, ranging from 68-

100% (Fig. 21a). Populations on each of the platforms followed this pattern, except for one, 

exhibiting no significant advantage. A geospatial analysis of the distribution pattern of T. 

coccinea’s competitive win frequencies indicates that it peaks in the SW part of the study area, 

with a single platform in the same vicinity also showing the lowest win rate (Fig. 21b). 

The light grey encrusting sponge Dictyonella finicularis was also an abundant competitor for 

space with T. coccinea. The coral was significantly competitively superior to the sponge overall. 

Competitive success, however, was highly variable with respect to this organism, and 

significantly different between platforms. Competitive win frequencies varied between 45% and 

100% between platform populations (Fig. 22a). The geospatial analysis revealed that the area of 

greatest competitive success was in the E, with the lowest competitive successes in the SW (Fig. 

22b). 

Even greater variability was observed in competition between T. coccinea and the white 

encrusting sponge Haliclona vansoesti, also common on these platforms. On average, T. 

coccinea did not have any significant competitive edge over this organism, with a mean success 

rate of 60% (Fig. 23a). However, the level of competitive success varied significantly between 

platforms, ranging from 30% to 100%; that is, there were significant levels of competitive 

success on individual platforms, with high variances between platforms. A geospatial analysis 

revealed that competitive ability peaked in the NE and SW, and was lowest in the W (Fig. 23b). 

38
Figure 19. (A) Percent of successes in competition for space between T. coccinea and all other sessile 

epibiota, data pooled. Presented by platform. (B) Geospatial representation of the distribution 

of competitive wins in the study area.

(A) Mean plus 95% confidence limits shown. Tested for significant variation from 1:1 ratio of wins to 

losses. Competitive successes significantly higher than draws or losses (p < 0.001, Fisher’s Exact Test). 

Also, significant difference between frequency of competitive success between platforms (p < 0.001, 

Goodness of Fit Test, G-statistic). 

39
Figure 20. Percent of successes in competition for space specifically between T. micranthus compared 

with T. coccinea, from the former’s perspective as the target organism. Presented by platform. 

Mean plus 95% confidence limits shown. 

40
Figure 21. (A) Percent of wins in competition for space between T. coccinea compared with the encrusting 

sponge Xestospongia sp. (with the commensal Parazoanthus catenularis).). Presented by 

platform. (B) Geospatial representation of the distribution of competitive wins in the study area.

(A) Mean plus 95% confidence limits shown. Tested for significant variation from 1:1 ratio of wins to 

losses. Competitive successes significantly higher than draws or losses (p < 0.001, Fisher’s Exact Test). 

Also, significant difference between frequency of competitive success between platforms (p < 0.001, 

Goodness of Fit Test, G-statistic). 

41
Figure 22. (A) Percent of successes in competition for space between T. coccinea and the light grey 

encrusting sponge Dictyonella finicularis. Presented by platform. (B) Geospatial representation 

of the distribution of competitive wins in the study area. 

(A) Mean plus 95% confidence limits shown. Tested for significant variation from 1:1 ratio of wins to 

losses. Competitive successes significantly higher than draws or losses (p < 0.001, Fisher’s Exact 

Test).Also, significant difference between frequency of competitive success between platforms (p < 0.001, 

Goodness of Fit Test, G-statistic). 

42
Figure 23. (A) Percent of wins in competition for space between T. coccinea and and the white encrusting 

sponge Haliclona vansoesti. Presented by platform. (B) Geospatial representation of the 

distribution of competitive wins in the study area. 

(A) Mean plus 95% confidence limits shown. No significant difference between observed frequency of 

competitive success or loss, and an expected 1:1 ratio of the two (p > 0.05, Fisher’s Exact 

Test). Significant difference between frequency of competitive success between platforms, 

however (p < 0.001, Goodness of Fit Test, G-statistic). 

43
Another abundant competitor for space was the black encrusting sponge Xestopongia 

carbonaria, which occurred as numerous small colonies at all depths. T. coccinea was a 

formidable and significant competitor for space against this organism, averaging ~70% wins 

(Fig. 24a). This competitive advantage varied greatly and significantly between platform 

populations, however, ranging from 50–100% between platforms. A spatial analysis of 

competitive abilities indicated a peak in the SW portion of the study area (Fig. 24b). 

The brown encrusting sponge Mycale carmigropila represented another abundant competitor 

for space with T. coccinea, and was generally larger in colony size than the Xestopongia 

carbonaria. Again, T. coccinea was on average significantly competitively superior to Mycale 

carmigropila, at a frequency of 60% (Fig. 25a). Competitive success varied significantly 

between platforms, ranging from 50–83%. T. coccinea was most competitively successful 

against this organism in the SW of the study area (Fig. 25b). 

The red encrusting sponge Phorbas amaranthus was another primary competitor of T. 

coccinea. The coral was significantly successful in competition for space against this sponge, 

again averaging about 60% competitive wins (Fig. 26a). Success varied significantly between 

platforms, ranging from 45–100%; success was particularly strong on several platforms. The 

geospatial distribution of the coral’s competitive abilities clearly peaked in the eastern portion of 

the study area (Fig. 26b). 

44
Figure 24. (A) Percent of wins in competition for space between T. coccinea and the black encrusting 

sponge Xestopongia carbonaria. Presented by platform. (B) Geospatial representation of the 

distribution of competitive wins in the study area. 

(A) Mean plus 95% confidence limits shown. Tested for significant variation from 1:1 ratio of wins to 

losses. Competitive successes significantly higher than draws or losses (p < 0.001, Fisher’s Exact Test). 

Also, significant difference between frequency of competitive success between platforms (p < 0.001, 

Goodness of Fit Test, G-statistic). 

45
Figure 25. (A) Percent of wins in competition for space between T. coccinea and the brown encrusting 

sponge Mycale carmigropila. Presented by platform. (B) Geospatial representation of the 

distribution of competitive wins in the study area. 

(A) Mean plus 95% confidence limits shown. Tested for significant variation from 1:1 ratio of wins to 

losses. Competitive successes significantly higher than draws or losses (p < 0.001, Fisher’s Exact 

Test).Also, significant difference between frequency of competitive success between platforms (p < 0.001, 

Goodness of Fit Test, G-statistic). 

46
Figure 26. (A) Percent of wins in competition for space between T. coccinea and the red encrusting 

sponge Phorbas amaranthus. Presented by platform. (B) Geospatial representation of the 

distribution of competitive wins in the study area. 

Mean plus 95% confidence limits shown. Tested for significant variation from 1:1 ratio of wins to 

losses. Competitive successes significantly higher than draws or losses (p < 0.01, Fisher’s Exact Test). 

Also, significant difference between frequency of competitive success between platforms (p < 0.001, 

Goodness of Fit Test, G-statistic). 

47
4. DISCUSSION

4.1. POPULATION EXPANSION

Data about the mean colony size of the Tubastraea micranthus colonies on these platforms 

have important implications for potential time and place of colonization by this species. They 

also have implications for definition of niche specificity and the population dynamics of this 

species. First, colony size did not peak at GI-93-C where the highest colony densities occurred; it 

peaked on the MC-311-A platform, andwas significantly higher than all other survey platforms, 

and with average colony size clearly falling off in all directions from there. It is possible that

these large colonies were afforded a longer period of time for growth, due to an earlier 

colonization time, and represent the oldest among the platform communities. 

In addition, the environment of MC-311A is different from that of GI-93C. GI-93C occurs on 

the continental shelf in 64 m depth of water, and it periodically receives water from the 

Mississippi River plume as it meanders back and forth in this region. The plume is, of course, 

characterized by high turbidity, a high sediment load, high nutrients, and low salinity. On the 

other hand, MC-311A occurs beyond the edge of the continental shelf, at the head of Mississippi 

Canyon. It is more frequently characterized by blue water (low turbidity, low sediment load, low 

nutrients, and a more stable stenohaline environment; Rabalais et al., 1996; Weisberg and He, 

2003; Green et al., 2006). Thus, one might hypothesize that T. micranthus grows better in a bluewater environment than a coastal one, subjected to typical coastal environmental variability. On 

the other hand, Tubastraea coccinea’s highest densities were found on ST-185B and GI-116A. 

Both of these sites have environments similar to GI-93-C, and are regularly subjected to water 

from the Mississippi River plume. 

This observation raises an interesting point regarding potential impacts of Tubastraea

micranthus compared with T. coccinea. T. coccinea invaded the Caribbean in the 1940s (Cairns, 

2000; Humann and DeLoach, 2002; Fenner and Banks, 2004). Since that time, it has spread as 

far south as Brazil (Figueira de Paula and Creed, 2004) and as far north as the Flower Garden 

Banks (Fenner, 1999, 2001; Fenner and Banks, 2004), the Florida Keys (Shearer, 2008), and 

platforms in the northern Gulf of Mexico (Sammarco et al., 2012a). During this period, it has 

become evident that populations of this species are able to nearly monopolize artificial hardbottom substrata such as offshore platforms. In no case, however, have there been reports of this 

species dominating natural coral reef environments, despite the fact that it has been reported to 

occur on these reefs (Sammarco, 2012b; Hickerson et al., 2006). 

We propose that the reason for this deficit of colonies on natural reefs is that T. coccinea

cannot compete well for space with the natural sessile epibenthic fauna and flora found on a 

coral reef. On the other hand, there may be naturally occurring predators there that suppress their 

populations when they occur fully exposed. On these natural reefs, the colonies observed tend to 

be found in their natural numbers and habitat, which is cryptic and in low numbers, with other 

ahermatypic corals. The concern here is that the natural environment for T. micranthus in the 

Indo-Pacific is on the upper surfaces of reef substratum, fully exposed (Schuhmacher, 1984). If 

and when this species encounters a natural coral reef, it is possible that it may be successful at 

outcompeting naturally occurring sessile epibenthic fauna and flora for space. Although some 

48
studies have been done on the secondary metabolites of T. coccinea and T. tagusensis (Lages et 

al., 2010, 2012), to date, none have been done on T. micranthus. Its toxicity and degree of 

palatability to predators, which could potentially control its populations, are currently unknown. 

The size-frequency distributions of T. micranthus are indicative of a population in an 

explosive growth phase (McNaughton and Wolf, 1979). In the case of MC-109-A, almost 60% 

of the colonies are between 1 and 100 cm2 in area while the largest size colony was 1,200 cm2

. In 

addition, the size-frequency distributions were highly consistent from platform to platform; thus, 

this explosive aspect of population growth is occurring across all newly colonized platforms. The 

shape of this size-frequency distribution is similar to that of a “wide-based pyramid,” described 

by human demographers to be indicative of human populations with a high growth rate and low 

doubling times, e.g., India, China, and Indonesia (Miller, 2000). 

We believe that we have confirmed that T. micranthus has successfully invaded the Gulf of 

Mexico and is exhibiting signs of producing rapidly expanding populations in the region. Its 

congener, T. coccinea, has already demonstrated formidable capabilities in this area. Preliminary 

data on depth distribution and competitive abilities (see below) give cause for further concern 

about the invasion of T. micranthus. We believe that T. micranthus has the ability for extensive 

geographic expansion in the Caribbean. If introduced populations are left unchecked, the newly 

introduced populations will become well integrated into the original community, creating a new 

community structure and stable equilibrium defining a new set of ecological interactions between 

species (Mooney and Cleland, 2001; Krushelnycky and Gillespie, 2008). 

4.2. DEPTH DISTRIBUTION

4.2.1. General discussion

Perhaps one of the first and most important results to emerge from this aspect of this study is 

the finding that T. micranthus has the ability to readily reach a depth of 138 m in its new 

environment. This is more than three times deeper than its reported depth range in its native 

Indo-Pacific habitat from 40 m (Anon., 2013) to 50 m (Cairns, 2013) in depth. These surveys 

were most likely performed by SCUBA, with its inherent depth limitations. In addition, this 

species appears to survive and grow equally well in both shallow and deep water, but with an 

apparent preference for deeper water. T. coccinea, on the other hand, appeared to be restricted to 

shallower depths. It primarily ranged from 1–78 m depth. Almost all of the colonies were found 

in shallower depths. Thus, T. coccinea seems to have a narrower and shallower depth range than 

T. micranthus. With respect to T. micranthus, 138 m was only the depth of the deepest platform 

surveyed here. It is likely that T. micranthus is capable of colonizing substrate at deeper depths. 

In addition, the geographic patterns of depth distribution for the two species varied greatly 

from each other. First, T. micranthus exhibited its shallower depth distributions west of the study 

area, in the primary plume of the Mississippi River. Its deepest distributions occurred at the head 

of the Mississippi Canyon. This region does receive meanders of the Mississippi River plume, 

but generally receives blue water from the Gulf of Mexico (Sturges and Lugo-Fernandez, 2005). 

T. coccinea, on the other hand, occurred in much shallower waters overall throughout the study 

region, but it also tended to display a shallower average depth distribution in the west, like T. 

49
micranthus. This implies that T. micranthus may be limited in its depth distribution by 

hyposalinity, turbidity, sedimentation, or other river-associated discharge characteristics. It is 

unlikely that either species is limited by light, because both are azooxanthellate (Creed, 2006; 

Cairns, 2013). Temperature probably does not play a role in limiting depth in T. micranthus,

because a temperature differential may be expected to occur at these depths. 

One anomaly that emerged in both species was that the shallowest depth of distribution for T. 

micranthus occurred in the vicinity of GI-93C, while the deepest depth for T. coccinea was 

found in the same region. It is unlikely that T. micranthus colonized deeper habitats because of 

lack of space availability in shallower water due to dominance by T. coccinea; it was found in 

shallow water elsewhere. Coral larvae are known to be able to sense pressure (Stake and 

Sammarco, 2003). It could be that the larvae of these species simply have different preferences 

for depth ranges for settlement. 

Analyses of platforms GI-93C and MC-109A confirmed that T. micranthus will colonize 

substrate available to the deepest depth available, at least to a depth of 138 m, which was the 

maximum depth encountered in this study. MC-109A occurs within Mississippi Canyon and 

receives blue water from the Gulf of Mexico. GI-93C, on the other hand, occurs in the western 

plume of the Mississippi River and receives turbid, sediment-laden, lower salinity water 

regularly as the plume meanders regularly over the site (Sturges and Lugo-Fernandez, 2005). 

(GI-116A’s restriction of T. micranthus to intermediate depths, despite the availability of 

substratum in deeper waters, may be a small sample size effect.) 

An analysis of T. coccinea populations on six platforms covering both shallow and deep 

sites, indicated that its depth distribution is clearly more limited than that of T. micranthus. T.

coccinea occurred down to the maximum depth of the platforms while in shallow water, but it 

did not approach the maximum depth of platforms in deeper water, as indicated by the 

distribution found on MC-311A. The details of these depth distributions once again illustrate that 

T. micranthus occupies a wider spatial niche than T. coccinea and can accommodate a broader 

set of environmental conditions, including temperature. 

The fact that percent-cover data track the coral density data so closely in generating 

geographic patterns indicates that, for these two species, either variable may be used in 

quantitative calculations to estimate broad-scale patterns. On the other hand, there are significant 

differences in specific depth distribution patterns generated by these two variables. This is 

because density data provide an estimate of colony counts and recruitment, while percent-cover 

provides an estimate of asexual budding, colony growth, and success (or lack of such) in 

competition for space. The object of the sampling should be considered and matched with the 

technique before choosing one variable or the other as the basis for estimating depth distribution. 

50
4.2.2. Implications of depth distribution

Whenever the eradication of a new invasive species is considered, population accessibility is one 

of the major factors that must be taken into consideration when assessing the potential efficacy 

of any such attempts (Clout and Veitch, 2002b). Even if the primary spatial distribution of an 

organism is accessible to eradication efforts, if there is a reasonable portion of that population 

which is not accessible to control or mitigation, the population could possibly still rebound. It is 

possible to eradicate a population, particularly if small, from all parts of its new domain, but the 

probability of success decreases with decreasing accessibility and time. In addition, in the marine 

environment, the cost of access increases greatly with increased depth. 

A second major factor influencing attempts to control or eradicate an invasive marine species 

is multiple invasions through time. Such would thwart even the most effective eradication, for 

the exercise would have to be repeated through time to keep the population under control 

(Fonseca, 2001). 

There has been debate as to whether invasion by T. coccinea has actually caused any damage 

or loss of native species in the western Atlantic. Unfortunately, no data are available on its 

impact, except for extensive cover and potential monopolization of benthic space; thus, this 

question remains open. However, preliminary evidence, both field and laboratory, suggests that 

T, micranthus is highly aggressive and capable of outcompeting local sessile epifauna for space 

(Sammarco et al., 2012d,e; Hennessey and Sammarco, 2014). 

4.3. COMPETITION FOR SPACE

Two major points may be drawn from the fact that T. micranthus is a better competitor for 

space than the various organisms it encounters on these offshore platforms. First, there is a high 

probability of success of invasion as this species overgrows and kills its sessile epibenthic 

neighbors (Vila and Weiner, 2004; Mangla et al., 2011). It should be recalled that the average 

competitive success frequencies against all species pooled, although variable, do not fall below 

50%. Second, this success is variable between platforms. This suggests that we are observing 

these interactions very early in the successional development phase of these newly disturbed, 

disequilibrial benthic communities. A stable equilibrium, as defined by extended competitive 

interactions in the community, has not yet been achieved (MacArthur and Wilson, 1967, 

MacArthur, 1972). That is, natural selection has not yet eliminated the weaker T. micranthus 

colonies or the weaker members of the competing species. This process would appear still to be 

in progress at this point in time. 

We have evidence to suggest that T. micranthus does not grow as well under conditions of 

high turbidity and high nutrients as T. coccinea. The results of the current study support this 

finding; T. micranthus did not compete well here under these turbid conditions. The lowest 

competitive success frequencies were observed in the SW part of the study region, in the plume 

of the Mississippi River. Also consistent with the above analytical results is the fact that when 

competition for space is considered from the perspective of T. coccinea, this coral species 

51
performs best in competition in the SW, in the plume of the Mississippi River, and poorly in the 

NE, at the head of Mississippi Canyon, under blue-water conditions. 

The fact that competitive abilities between T. micranthus and T. coccinea were almost 

exactly the same when in competition with each other is an indicator that T. micranthus may 

have a capacity for invasion similar to its predecessor. Thus far, there is no evidence indicating 

that one of these coral species will out-compete the other for space. Indeed, it would appear that 

they can be expected to coexist in their new environment. 

When one considers T. coccinea and its competitive win frequency over all sessile epibiota 

on these platforms, we see some important differences arise between it and T. micranthus.The 

first concerns the average frequency of competitive success, which is 55% and significantly 

positive. This indicates that T. coccinea is still an important competitor in the western Atlantic 

and is most likely still slowly expanding its range there. Encroachment by T. coccinea on space

held by other sessile epibiota is most likely slow and difficult; these older communities may well 

have reached an equilibrium. The T. coccinea populations would now appear to be adapted to 

their new environment. They have been integrated into the community and are coexisting with 

their neighbors. Competition for space has stabilized, permitting coexistence. The second point is 

the consistency of the competitive win frequency in this species. There is very low variability in 

the competitive win frequency between platforms. This reinforces the concept that the older coral 

populations have reached some level of equilibrium with respect to competitive efficacy in these 

communities. 

Both congeneric coral species have numerous sessile epibenthic species with which they 

must interact and attempt to procure space required for living, growth, and reproduction. T. 

coccinea has clearly been very successful at this in the Caribbean, as is evident from its current 

geographic distribution. This study confirms that this competitive success is helping to maintain 

these populations in their new environments. The quantitative data regarding competition for 

space within each of the species observed is clear. For example, T. coccinea is a strong 

competitor for space with the encrusting sponge Xestospongia sp. (with the commensal 

Parazoanthus catenularis). Both competitors possess stinging nematocysts with which they are 

most likely capable of defending themselves, but T. coccinea is the stronger competitor. 

T. coccinea’s competitive abilities were also strong with respect to the light grey encrusting 

sponge Dictyonella finicularis and the white encrusting sponge Haliclona vansoesti, both of 

which were encountered frequently. The high frequency of competitive success against these two 

organisms indicates that this sponge will most likely become less represented in the benthic 

community, at least on these artificial reefs, as time progresses. The high variability in the 

observed responses by various platform populations indicates that these competitive interactions 

have not yet come to equilibrium. In addition, it appears that Dictyonella finicularis has a higher 

probability of out-competing this invasive coral under conditions of high turbidity and nutrient 

concentrations. The opposite was true for the Haliclona vansoesti, which had less of a 

competitive advantage under those conditions. Competitive abilities in T. coccinea are speciesspecific (Sammarco et al., 1985). 

52
T. coccinea’s competitive abilities appeared to be nearly equal to those of the black 

encrusting sponge Xestopongia carbonaria, the brown encrusting sponge Mycale carmigropila, 

and the red encrusting sponge Phorbas amaranthus. Thus, there are numerous species which 

appear to have survived competition for space with T. coccinea and will probably survive this 

coral’s continuing expansion throughout the northern Gulf of Mexico and the Caribbean 

In conclusion, it would appear that the scleractinian coral T. micranthus is as a formidable 

new invasive species for the northern Gulf of Mexico, as its predecessor, T. coccinea, was and 

continues to be. T. micranthus probably invaded during the mid-2000s, and its populations have 

already expanded from a single platform (Sammarco et al., 2014) to 9 out of 14 platforms in the 

region south of the Mississippi River mouth in less than a decade . It has also been shown that, 

unlike T. coccinea, it prefers deeper depths (Sammarco et al., 2012c). From the above 

documentation of T. micranthus’ competitive abilities against native fauna and flora in the 

northern Gulf of Mexico, it would appear that its competitive abilities are comparable to T. 

coccinea. It has the potential for being another highly successful invasive species for the Gulf of 

Mexico and the Caribbean. We now have a deeper understanding of the magnitude of the threat 

this species may be to native fauna and flora. 

53
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