OCS Study 

MMS 95-0021 

Northern Gulf of Mexico 

Chemosynthetic Ecosystems Study 

Final Report 

Volume I : Executive Summary 

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TEXAS 

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MISSISSIPPI ALABAMA GEORGIA 

U.S. Department of the Interior 

Minerals Management Service 

Gulf of Mexico OCS Region 
OCS Study 

MMS 95-0021 

Northern Gulf of Mexico 

Chemosynthetic Ecosystems Study 

Final Report 

Volume I : Executive Summary 

Editors 

Ian R. MacDonald 

William W. Schroeder 

and 

James M . Brooks 

Prepared under MMS Contract 

14-35-0001-30555 

by 

Geochemical and Environmental Research Group 

Texas A&M University 

Texas A&M Research Foundation 

College Station, Texas 

Published by 

U.S . Department of the Interior 

Minerals Management Service New Orleans 

Gulf of Mexico OCS Region May 1996 
DISCLAIMER 

This report was prepared under contract between the Minerals Management 

Service (MMS) and Geochemical and Environmental Research Group . This report 

has been technically reviewed by the MMS and approved for publication. Approval 

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

Service, nor does mention of trade names or commercial products constitute 

endorsement or recommendation for use. It is, however, exempt from review and 

compliance with MMS editorial standards. 

REPORT AVAILABILITY 

Extra copies of the report may be obtained from the Public Information Unit (Mail 

Stop 5034) at the following address: 

U.S. Department of the Interior 

Minerals Management Service 

Gulf of Mexico OCS Region 

Public Information Unit (MS 5034) 

1201 Elmwood Park Boulevard 

New Orleans, Louisiana 70123-2394 

Telephone Number: (504) 736-2519 

1-800-200-GULF 

CITATION 

Suggested Citation: 

MacDonald, I .R., W.W. Schroeder, and J.M. Brooks. 1995 . Chemosynthetic 

Ecosystems Study Final Report, Volume I : Executive Summary. Prepared by 

Geochemical and Environmental Research Group . U.S. Dept. of the Interior, 

Minerals Mgmt. Service, Gulf of Mexico OCS Region, New Orleans, LA. OCS 

Study MMS 95-0021 . 33 pp. 

COVER PHOTOGRAPH 

The foreground photograph shows the submersible Johnson Sea-Link I preparing 

for one of its many dives to study chemosynthetic ecosystems . The map depicts the 

locations of known chemosynthetic ecosystems in the northern Gulf of Mexico . 

iii 
PREFACE 

The Chemosynthetic Ecosystems Study concerns the prominent biological 

communities of tube worms, mussels, and clams that occur at natural hydrocarbon 

seeps on the continental slope and that derive their food supply from chemicals 

associated with the seeps. This is the Executive Summary of the Final Report that 

will be issued for the Study, which is sponsored by the U.S. Department of Interior 

Minerals Management Service (MMS), Gulf of Mexico OCS Region Office (Contract 

14-35-0001-30555) . The Technical Report (Volume II) provides details concerning 

the methods and results obtained during a three year study. Volume II covers the 

geology of hydrocarbon seeps, the community structure, the life history of the major 

fauna and evidence for the persistence of these communities over a 1 - 10 year and 

100 - 1,000 year time scales. Volume II is 338 pages long. An appendix volume 

presents additional data from the study. 

v 
TABLE OF CONTENTS 

1 

2 

3 

4 

5 

6 

7 

8 

9 

Page 

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

Regional Distribution of Hydrocarbon Seepage and Seep Fauna . . . . . . . . . . . . 3 

Near-Surface Geology Hydrocarbon Seeps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

Geochemical Processes in Chemosynthetic Communities . . . . . . . . . . . . . . . . . . . . . . . . 7 

Growth Rates of Symbiont-Containing Fauna at Hydrocarbon Seeps . . . . 9 

Heterotrophic Fauna Associated with Hydrocarbon Seeps . . . . . . . . . . . . . . . . . . . . . . 13 

Temporal Stability of Chemosynthetic Communities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 

Paleontology of Seep Bivalve Communities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 

Vil 
LIST OF FIGURES 

Figure 

Locations where chemosynthetic fauna have been collected or 

photographed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

Page 

ix 
1 Introduction 

Planning for this study was initiated by MMS shortly after the 1984 

discovery of chemosynthetic fauna at natural hydrocarbons seeps on the continental 

slope in the Gulf of Mexico (Kennicutt et al. 1985) . From a management point of 

view, the Study was necessary because of the agency's concern that energy 

exploration and production could harm an important component and previously 

unrecognized component of the slope ecosystem. 

The Geochemical and Environmental Research Group (GERG) of Texas A&M 

University was awarded the contract in July 1991. The Study was conducted by ten 

principal investigators and associates under the overall management of GERG as 

follows: 

Principal Investigators* Affiliation Major Responsibility 

Javier Alcala-Herrera TAMU/GERG Chemical Analyses 

James M. Brooks* TAMU/GERG Project Director 

Robert S. Carney* LSU Ecology 

Charles R. Fisher* PSU Physiology 

Norman L. Guinasso, Jr.* TAMU/GERG Data Analysis 

Holgar W. Jannasch* WHOI Microbiology 

Mahlon C. Kennicutt II* TAMU/GERG Geochemistry 

Ian R. MacDonald* TAMU/GERG Ecology/Deputy Proj. Director 

Eric N. Powell* TAMU/OCNG Taphonomy 

William S. Sager* TAMU/OCNG Geology 

William W. Schroeder* UA/DISL Geology/Ecology 

Dan L. Wilkinson TAMU/GERG Field Logistics 

Carl 0. Wirsen WHOI Microbiology 

Gary A. Wolff TAMU/GERG Data Management 

TAMU Texas A&M University; 

GERG Geochemical and Environmental Research Group; 

OCNG Oceanography Department; 

LSU Louisiana State University; 

PSU Pennsylvania State University; 

WHOI Woods Hole Oceanographic Institution ; and 

UA/DISL University of Alabama Dauphin Island Sea Lab . 

Collectively, this group brought to the study a mature program of research 

that included collaboration with investigators from several research universities in 

the Gulf of Mexico region and elsewhere . As a result, the team was able to proceed 
immediately to the fieldwork stage of the program and to build upon an existing 

data base acquired through several previous years of field sampling and analysis . 

The Study was tasked with delineating the abiotic factors that determine 

why communities of chemosynthetic fauna become established and persist at 

hydrocarbon seeps. Assessment of the relative importance of the communities to 

the slope ecosystem was given a high priority. To address the question of possible 

harm due to human activity from primarily energy-related industry, the Study 

examined the growth rates of the major faunal groups, the paleontological record of 

the community, and the incidence and causes of short-term change . 

Because the communities occur at depths of 500 m or greater and tend to be 

small in area, the investigations have relied heavily upon research submarines for 

sample collection and study. Initially, five sites located in water depths of 500 to 

720 m were selected for study on the basis of their faunal composition and 

geographic location . The MMS contracted for 18 dive days with the submarine 

Johnson Sea-Link I and her surface support ship. Additional in kind contributions 

of submarine time with both Alvin and the U.S . Navy NR-1, which were facilitated 

through GERG, have been crucial to the success of the field program. For example, 

use of the Aluin led directly to discovery the deepest known Gulf of Mexico seep 

community in Alaminos Canyon (2220 m depth); and NR-1 surveys have been 

responsible for the initial discovery of many of the study sites. 

Preliminary reports concerning the finding were presented at the MMS 

Information Transfer Meeting on 5 December 1993 . In general, study results show 

that the seep communities are a unique and important component of the slope 

ecosystem of the Gulf of Mexico . Although susceptible to mechanical damage by 

offshore drilling or production activities, the ecosystems are prevalent enough that 

localized disturbances should not effect the viability of these communities . 

2 
2 Regional Distribution of Hydrocarbon Seepage and Seep Fauna 

The vestimentiferan tube worms, mussels (seep mytilids), and the vesicomyid 

and lucinid clams that colonize natural hydrocarbon seeps are able to exploit the 

chemical enrichment that occurs in localized zones when oil and gas arrive at the 

seafloor to varying degrees . Regionally, a map of the known seep communities 

indicates a broad belt of occurrence across the upper to mid-continental slope 

(Figure 1). While the distribution of communities is related to the frequency and 

mode of sampling, the landward limits of seep communities appear to be definable 

and restrict the occurrence of seep fauna to water depths less than about 500 m. 

The physiological and ecological causes of this restriction are not understood. The 

seaward and east-west extents of this distribution are artificially constrained by the 

limits of exploration. The presence of perennial oil slicks, which form over many 

seeps and can be detected by satellite remote sensing, indicate that seeps are 

present across the entire continental slope of the northwestern Gulf of Mexico. 

Collection of oil-laden surface sediments by piston coring also indicates the general 

prevalence of seepage on the Gulf slope. One deep water (2200 m) seep site has 

been explored in the Alaminos Canyon lease area and the thriving chemosynthetic 

community found at that site (Brooks et al . 1990) supports the theory that seeps of 

the lower continental slope have the potential to be colonized. To the east, the 

Mississippi Fan and Mississippi Canyon regions appear to have more limited 

seepage. This is confirmed by the general lack of surface slicks and the lower 

concentrations in sediment hydrocarbons . The Florida Escarpment limits the 

eastward occurrence of known seep communities; however, the first documented 

discovery of tube worms and mytilids in the Gulf of Mexico was at the base of the 

Florida Escarpment, at a depth of 3600 m (Paull et al. 1984). The escarpment 

communities have been linked not to thermogenic hydrocarbons, but to connate 

fluid seepage from the highly porous Florida carbonate platform . The eastern most 

3 
06 

Figure 1 . Map showing the locations of known chemosynthetic communities at hydrocarbon seeps. 

Labels indicate the seven selected as study sties for the Chemosynthetic Ecosystem Study. 
occurrence of hydrocarbon fauna is in the Viosca Knoll area (88° W). The oil 

reserves offshore Mexico and the similarities between the Yucatan and Florida 

Escarpments suggest the potential distribution of seep communities across the 

entire Gulf of Mexico. 

This broad regional distribution belies the localized occurrences of the 

chemosynthetic communities. Because the fauna depend upon nutrients supplied 

by geological processes, they are freed from the restrictions associated with 

increasing water depth imposed upon the photosynthetic food chain. These 

communities often form very dense aggregations in the otherwise food-poor deepsea. However, because their energy source is so specific, seep communities are 

sharply limited by the spatial and temporal characteristics of seepage. Therefore, 

seep communities tend to be dense, but small in area. 

On the northern continental slope, movement of the Louann Salt produces 

both the reservoirs and the migration conduits required for hydrocarbon seepage . 

Seep communities are thus inextricably linked to the underlying salt structures, but 

this dependence is variable on smaller scales. For example, a bathymetric chart of 

the Green Canyon lease area, where many of the most-studied seeps are found, was 

produced from a data set arrayed in a 250 m grid (NOAA 1992), and clearly shows 

several salt structures and slump zones produced by salt movement. However at 

this scale, the occurrence of seep communities do not exhibit an unambiguous 

preference for specific topographic features associated with salt tectonics . This is 

because seepage, although associated with a variety of distinctive geological 

features, typically occupy areas that are only a few tens to a few hundreds of meters 

in width. 

5 
3 Near-Surface Geology Hydrocarbon Seeps 

High resolution geophysical records, submarine observations, and core data 

have been used to describe the geology associated with hydrocarbon seeps on the 

Louisiana continental slope. The slope of the northern Gulf of Mexico is a wellknown site of hydrocarbon seepage and chemosynthetic communities (Brooks et al. 

1986 ; MacDonald et al . 1990) . The study areas include Green Canyon 184/185, 

Green Canyon 234, Garden Banks 386, Garden Banks 424/425 and their 

surrounding areas . The primary objective of the geologic study component was to 

enhance our understanding of how geological processes constrain the location and 

abundance of chemosynthetic communities. 

The primary data base consists of 77 kHz side-scan sonar records, 25 kHz 

seismic reflection profiles, bathymetry data, and video tape photography, which 

were obtained by the research submarine NR-1 in 1990 (GC 184/185, GC 234, and 

GB 386) and 1993 (GB 424/425). Other data available for these areas are push 

cores (GC 184/185, GC 234, and GB 386; Johnson Sea-Link, 1992), piston cores (GC 

234, GB 386 and GB 424/425; R/V Gyre, 1992), 3.5 kHz and 12 kHz echo sounder 

records (GB 386 and GB 424/425; R l V Gyre, 1992), and oil company hazard survey 

data (GC 184/185 and GB 386). Industry data includes 3.5 kHz echo sounder 

records and selected mufti-channel seismic lines. 

Detailed bathymetric maps of each study area were made using NR-1 depth 

data. Slant-range corrected side scan sonar mosaics were constructed for each site 

providing acoustical images of seafloor features . Acoustic reflection characters 

("acoustic facies") seen in 25 kHz subbottom reflection records were mapped. 

Acoustic fades variations were due to changes in the texture and composition of the 

seafloor and sediments. Acoustic fades mapping results was correlated with video 

records from submarines and core samples. 

6 
Acoustic fades mapping results of 25 kHz subbottom reflection records of GC 

184/185 and GC 234 sites show significant differences from those of GB 386 and GB 

4241425. The flat mounds of GB 386 and GB 424/425 are mostly sediment associated 

with solid or porous carbonate rocks. In contrast, GC 184/185 and GC 234 sites 

show more variable features, less hard substrate and denser chemosynthetic 

communities. Chemosynthetic communities are strongly coupled with echo types 

that are interpreted as porous or semi-consolidated carbonate precipitates. 

Push cores collected from the submarine Johnson Sea-Link and piston cores 

collected from R/V Gyre were analyzed for water content, bulk density, porosity and 

carbonate percentage with depth. The analysis results and visual description of the 

cores were used to interpret the acoustic fades at each study site . Several piston 

cores collected from the GB 424/425 site exhibited violent gas expansion inside the 

cores when brought on deck. This may have been caused by the rapid decomposition 

of gas hydrates. 

Active mud slides and sediment flows on flanks of the mound in the GB 

424/425 site were observed. Following these features upslope, active mud vents 

discharging gas, oil and mud at the perimeter of the mound top were observed. 

Sediments entrained by this discharge have probably contributed to the formation 

of the mound. Gas and droplets of oil were observed to be escaping from the vents . 

A temperature probe, lowered immediately beneath the surface of the fluid mud 

within the vent, recorded a temperature of 20°C, about 12°C warmer than the 

surrounding seawater. 

4 Geochemical Processes in Chemosynthetic Communities 

Chemosynthetic communities are closely linked to a number of geochemical 

phenomena or processes . Directly or indirectly, these processes are primarily the 

result of oil and gas seepage. Historically, the recognition of oil seepage, gas-charged 

7 
sediments (seismic wipeout zones), and hydrates lead to the discovery of seep 

communities. This fundamental coupling of biochemical processes and the chemical 

environment is at best incompletely understood. A common feature at both cold 

seeps and hot vents is the co-occurrence of anoxic and oxic conditions that allow for 

the storage of reduced compounds in close proximity to oxygenated waters. This 

dynamic system can only be maintained through an active supply of reduced 

compounds . The supply of these reduced compounds can result from seepage or 

venting (a deep source) and/or bacterial processes (aerobic and anaerobic) . The 

seepage of oil and gas provides an unconventional source of carbon and possibly 

some portion of the hydrogen sulfide needed for the community's energy and 

nutrition requirements. Oil and gas are readily available to hydrocarbon oxidizing 

bacteria that naturally occur in the marine environment. Microbial processes form 

the basis of the ecosystem and provide the mechanism to transform chemical energy 

into biochemical energy, mediated by free-living as well as endosymbiotic bacteria. 

All locations studied have a number of common features including oil 

seepage; gas charged sediments; hydrates; authigenic carbonate; active hydrocarbon 

oxidation; chemosynthetic biomass (not only in the symbiotic organisms but also in 

background fauna); elevated EOM, TOC, and salinity; and biodegraded oil. While it 

is still not clear which of the features are essential, it is clear that all of these 

attributes are closely related. A supplementary source of carbon is provided by oil 

and gas seepage . The oxidation of hydrocarbons contributes to maintaining anoxic 

conditions . Most, if not all, carbon and energy transformations are mediated by 

bacteria. Sulfur, an important element, and hydrate formation decomposition are 

intimately associated with the continuous supply of natural gas. It is also clear that 

pore waters are the main repository of reduced compounds and bottom waters 

supply oxygen. Steep gradients are present in almost all chemical properties and 

areal (including vertical) patchiness and heterogeneity are extreme . 

8 
The progressive overprinting of precipitated carbonates, biologically degraded 

hydrocarbons, gas hydrates, and active seepage or venting that can occur within a 

given portion of a community makes it difficult to determine what critical, or 

essential, geochemical requirements are needed for proliferation and maintenance 

of communities. However, the presence of oil and gas seepage is a fundamental 

process in creating the chemical environment that fosters these unusual 

accumulations of biomass in the deep ocean . 

Careful sampling of the local environment associated with individual 

biological components suggests that there are predictable geochemical associations. 

It appears that Beggiatoa mats and other bacteria play a role in starting a process 

of seafloor modification in areas of long term thermogenic seepage. Tube worms, in 

contrast, appear to occupy environments that have been modified by thermogenic 

seepage effects over a long span of time. Tube worms appear to need a hard 

substratum as a point of attachment. Authigenic carbonate rock provides such a 

substratum in an otherwise unfavorable, mud-dominated environment. A long span 

of time is required to precipitate enough carbonate rock to serve large populations of 

tube worms. Seep mussels are methanotrophic and the potential sources for 

methane at seeps are quite diverse . Methane may have biogenic or thermogenic 

origins. It can be obtained in mytilid micro-habitats as a dissolved fraction of 

hypersaline brines or dissolved in sea water as a result of active gas venting. 

Formation and dissociation of gas hydrates further complicates the picture by 

alternately sequestering or releasing methane. 

5 Growth rates of Symbiont-Containing Fauna at Hydrocarbon Seeps 

The "robustness" of a community of animals is related to the growth rates 

and the population structure of the community: A community of fast-growing young 

animals will recover from physical disturbance much more quickly than a 

9 
community of slow-growing older individuals . In the case of animals which rely on 

chemoautotrophic or methanotrophic symbionts, another piece of information basic 

to understanding the community dynamics is the nature of the animals reliance on 

the reduced chemicals which are the primary substrates of their autotrophic 

symbionts. The primary goals of this portion of the Study was to determine the 

growth rates of the key ecosystem organisms. 

There are three main groups of chemosynthetic metazoans endemic to the 

hydrocarbon seep sites on the Louisiana Slope of the Gulf of Mexico: vesicomyid 

clams, vestimentiferan tube worms, and mussels which are provisionally designated 

"seep mytilids" (Brooks et al . 1987). Each group employs a distinct strategy to 

obtain the chemical substrates required by its symbionts. In situ growth 

experiments were designed based on our knowledge of each group. A set of devices 

were also designed and constructed for this Study to collect small volumes of water 

from specific physiologically relevant points in the animals immediate habitat. 

These samples were then analyzed for the chemical substrates of the symbionts. 

The two species of tube worms present at these sites are in two different 

families (Escarpidae and Lamellibrachidae) but have similar gross anatomy, 

physiology, and lifestyles (Jones 1985). The worms attach to hard substrate when 

they settle and are immobile after metamorphosis to the adult form . The adults 

have no mouth, gut, or anus and apparently depend entirely on their symbionts to 

meet their nutritional needs. The bacterial symbionts require sulfide, oxygen, and 

inorganic carbon for chemoautotrophy (Fisher et al . 1988). Due to its immobility, 

the host tube worm can only live in an environment with a constantly replenished 

supply of nutrients . It is well documented that the hydrothermal vent 

vestimentiferans take up these dissolved gases across their plumes; however, these 

hydrocarbon-seep animals may acquire sulfide by diffusion across the portions of 

10 
their tubes that is buried in sediments containing large concentrations of reduced 

compounds (MacDonald et al . 1989; Fisher 1990). 

Sulfide was undetectable in 8 of 9 samples taken around the plumes of the 

vestimentiferans and in 15 of 17 samples taken between the plumes and the 

sediment. However, over half of the samples of interstitial water 10 cm below the 

sediment-water interface contained detectable sulfide, with concentrations ranging 

from a few micromolar to over one millimolar. These data tend to support the 

hypothesis that seep vestimentiferans can acquire sulfide by diffusion across the 

portion of their tube buried in sediment. 

To determine the growth rates of the vestimentiferans, numbered bands were 

placed around the tubes, close to the anterior ends . The location of the bands was 

documented using video, and their yearly growth determined by subsequent visits 

to video or collect the banded animals The growth rates of both species was 

determined to be extremely slow. The fastest growing individual laid down 2 cm/yr 

of tube material and the average for both species was near 0.5 cm/year . This is in 

stark contrast to their hydrothermal vent tube worms which can grow one hundred 

times faster. Because individuals of Lamellibrachia sp. are between 2 and 3 meters 

in length, it can be concluded that individuals in excess of 200 years old are present 

in many of the large tube worm aggregations. 

The undescribed hydrocarbon seep mussel (seep mytilid Ia) contains methane 

oxidizing symbiotic bacteria in its gills that provide the bulk of the animal's 

nutrition (Childress et al. 1986). Because of the relative ease in handling methane 

(as compared to hydrogen sulfide) and the fact that this mussel can be maintained 

in the laboratory at ambient pressures for extended periods, this species has proven 

amenable to laboratory study, and there is the isolated symbionts (Fisher et al. 

1987; Cary et al. 1988) . Although these bivalves are mobile they occur in dense 

"beds" attached to rocks or each other by byssal threads. In fact the results of a 

11 
transplant experiment conducted as part of this study suggests that appropriate 

physical substrate as well as high levels of methane are necessary for establishment 

of mussel beds. 

Experiments were designed to chemically characterize the animals micro 

habitat, determine the density and population size structure of beds, and measure 

the mussels in situ growth rates . A bed of mussels was marked with a float and 

numerous water samples were taken from precise locations among the mussels. 

Animal collections were then made within a 0.5 m diameter ring that was 

temporarily positioned on the mussel beds. All of the animals collected were 

measured and 150 were individually tagged and returned to the point of collection . 

This procedure was repeated in two subsequent years at several sites. In addition 

to determining growth rates of the marked and recollected animals, the 

physiological condition of a subset of each mussel collection was determined using 

three different indices. 

The results of this work element can be summarized as follows : 1) The 

mussels flourish in sites with very high levels of methane (>1 mM) immediately 

below the bed and high levels (>60 mM) among the animals . 2) At active sites, the 

juveniles grow very quickly (up to 2 cm/yr in shell length) and adults grow at rates 

comparable to healthy intertidal mussels at similar temperatures. 3) Populations at 

sub-optimal conditions can be in good health but grow at very slow rates, suggesting 

that these animals can live a long time (50 to 100 years). 4) Because of the high 

densities of animals in some beds, biomass can be quite high (up to 20 kg/m2) and 

the potential methane oxidation rates of these beds are the highest ever measured 

in any environment on the planet (up to 200 g/m2/day) . 5) Significant differences 

were found in two of the sites in subsequent years (in chemistry, growth rates, and 

physiological condition of the animals) which suggests significant variability in the 

habitat on a one to ten year scale. 

12 
6 Heterotrophic Fauna Associated with Hydrocarbon Seeps 

All chemosynthetic assemblages clustered within the emissions of hydrogen 

sulfide and methane, at both cold seeps and hydrothermal vents, are in dramatic 

contrast to the surrounding deep-sea benthos. High population densities are found 

for both heterotrophic and chemoautotrophic species. This suggests that the 

heterotrophic components of chemosynthetic communities are not independent of 

the ecology of the surrounding sea floor. The deep-sea is not an ecologically 

monotonous desert, but is characterized by bathymetric and latitudinal gradients of 

biomass, species diversity, and species composition (Carney et al. 1983 ; Gage and 

Tyler 1990 ; Rex et al . 1993). IF these factors influence vent and seep ecology, then 

consistent geographic and bathymetric patterns should emerge as the number of 

regions studied increases. The goal of this work element was to study the 

interaction between seep fauna and background heterotrophic fauna . The origin 

and characteristics of the heterotrophic fauna is an important aspect of this 

interaction. 

There is an abundant heterotrophic fauna exploiting both the food source and 

habitat of the upper slope seeps. This fauna is numerically dominated by gastropod 

mollusks (summarized in Waren and Bouchet 1993). In approximate sequence of 

dominance, the most common species present are Bathynerita naticoidea, Provana 

sculpts, Cataegis meroglypta, and Buccina canatae. Less common gastropods 

include Cancellaria rosewateri, Cantrainia macleani, Gymnobela extensor, 

Hyalorisia gales, Gaza fisheri, a chiton Ichnochiton sp ., and a rare undescribed 

limpet. All except G. fisheri have been collected directly from mussel mats. A Timid 

bivalve, Acesta bullisi is found attached by byssal fibers to the rocks, or enclosing 

the anterior opening of tube worms. 

13 
Crustacea living in closest association with mussel clumps and worm bushes 

are a common galathetiid, Munidopisis sp . ; a much rarer galatheiid Eumunida 

pitta . ; and the shrimp Alvinocaris stactophila . Large mobile crustaceans are 

conspicuous in seep areas, but less commonly in direct contact with tube worms and 

mussels, include the decapods Paralomis cubensis, Rochina crasser, Chaceon 

quinquedens, Chaceon fenneri, and the giant isopod Bathynomus giganteus . 

Echinoderms are represented by a common predatory sea-star, Sclerasterias sp; a 

filter feeding sea-star of the family Brisingidae, and sea urchin Echinus sp. More 

than 10 species of fish have been sighted during dives, but only five seem to show a 

consistent affinity for the seep communities: Eptatretus sp . (hagfish), 

Synaphobranchus sp . (cutthroat eels), Urophycis cirratus (Gulf hake), 

Hoplostetlzus sp ., and Chaunax pictus (pancake batfish) . 

The principle description of the fauna of the West Florida Escarpment has 

been given by Hecker (1985) . The associated heterotrophs are dominated by 

gastropods . An undescribed trochid is very abundant. A limpet has been named and 

described (McClean 1990). A neogastropod was reported . The crustacea collected 

consist of a species of Munidopsis and a bresilliid shrimp . Unidentified ophiuroids 

and a vermiform holothuroid identified from photographs as Chiridota were 

present. The associated heterotrophic fauna of the 2000 m deep Alaminos Canyon 

shares some elements in common with shallower seeps but also has distinctive 

components . The gastropod fauna is markedly reduced. Archaeogastropod limpets 

predominate to the virtual exclusion of all other forms . A single crushed specimen 

of a buccinid differing slightly from B. canatae was collected . Bresiliid shrimp 

apparently of the same species as common shallower species were present, but 

reached larger size . Large (1 cm) amphipods were abundant. Galatheiid crabs were 

observed, but only a single specimen was recovered . It was not the Munidoposis sp. 

found shallower. Large decapod crustaceans appeared to be absent, and only a 

14 
single fish was collected, an eel pout. Authigenic carbonate rocks in the seeps had 

scattered specimens of a verrusamorph barnacle and small ophiuroids . 

It is to be expected the associated heterotrophic species at cold seeps, like 

those at hydrothermal systems (Tunnicliffe 1991), reflect mixed sources; some may 

be truly endemic to chemosynthetic communities ; some may be colonists from the 

surrounding benthos which have established resident seep populations, while still 

others are vagrants making temporary use of the seep site without developing 

resident populations . 

The Gulf of Mexico sites afford a special opportunity to examine the following 

question: How many of the seep associated heterotrophs are endemic, colonizing, or 

vagrant? The Gulf has been extensively sampled and the bathymetric distribution 

of the larger normal (megafauna) benthos well documented (Pequegnat 1983; 

Galloway and Pequegnat 1988). Therefore, it is possible in many cases, to 

determine which of the species collected at seeps are colonists, and which are 

vagrants. 

Several observations are readily apparent. First, the upper slope and 

Alaminos Canyon heterotrophic fauna are very dissimilar and may share only the 

shrimp Alvinocaris stactophila in common. If other elements are shared, their 

relative abundance at the two sites must be so dramatically different as to have 

escaped detection at the level of sampling performed. Second, endemism is low at 

the species level, comprising five endemics out of 39 species . At the level of genus, 

only the two coiled gastropods Bathyneritia naticoidea and Prouanna scuplta, 

possibly the Alaminos canyon limpet, and the shrimp Aluinocaris stactophila are 

endemic. Third, the colonizers and vagrants comprise about 10% of the surrounding 

species richness, 23 species in seeps at the upper slope sites and 251 species in the 

background fauna. Fourth, the distinction between vagrant and colonizer seems to 

be related to size and mobility. Colonizers tend to be smaller, slower, and 

15 
subdominant in the background. Vagrants tend to be larger, highly mobile, and 

may include dominant elements from the background. 

The low endemism of the Gulf of Mexico seep sites, especially the upper slope, 

and the high degree of colonization by fauna from the surrounding sea floor is 

markedly different from the hydrothermal vent systems where 95% of the species 

are endemic (Tunnicliffe 1991 ; 1992). This difference may be attributable to the fact 

that seep sites lie within the biodiversity gradients of the continental margins, 

while vents tend to be located away from the continents . The abundance, diversity, 

and nature of the species on continental slopes afford a much higher opportunity for 

colonization and exploitation by vagrants. 

7 Temporal Stability of Chemosynthetic Communities 

Life spans of the Gulf chemosynthetic fauna, where they can be determined 

by growth experiments, are on the order of tens to possibly hundreds of years. 

Short-term change in their communities should therefore result from variation in 

fluid flow or other seep-related processes. Successful settlement events should be 

relatively rare and the effects of predation relatively limited. Comparison of seep 

communities photographed at intervals of up to three years reveals little direct 

evidence for change in either community composition or density. This is in marked 

contrast with the chemosynthetic communities at the Galapagos hydrothermal vent, 

where entire aggregations of tube worms were replaced with mussels over a period 

of less than five years (Hessler et al. 1988). Indirect evidence, on the other hand 

points to change and various rates. Most rapid change will undoubtedly result from 

catastrophic events such as blow-outs and slumping. Upended carbonate slabs and 

sharp sided pockmarks are clear indicators that such blowouts do occur. Over a 

circa-annual time scale, variation in water temperature can destabilize near-surface 

hydrates and greatly increase local seepage (MacDonald et al. 1993). Finally, re16 
channelization of fluid-flow may choke off fluid flow in a highly localized area; 

evidence for this can be seen in the defunct mussel beds that are commonly found in 

the vicinity of living beds . In general, however, the incidence of short term change 

seems to be sufficiently low that detection is unlikely in the short-term. 

8 Paleontology of Seep Bivalve Communities 

Six distinct biofacies have been recognized from petroleum seep assemblages; 

dominated respectively by mytilid mussels, vesicomyid clams, Lucinoma clams, 

thyasirid clams, vestimentiferan tube worms, and the normal slope fauna. A long 

term history of these seep assemblages is not well known. This component was 

designed to examine the long term record of seep assemblages by comparing guild 

and tier structure, paleoenergetics, and changes in community composition of the 

six biofacies at a number of petroleum seep sites . 

Autochthonous seep assemblages (those that develop in place) are 

characterized by a unique tier and guild structure, size-frequency composition, and 

animal density that together identify the paleoenergetic's structure of these 

communities and distinguish them from other autochthonous assemblages of the 

shelf and slope . All seep assemblages were dominated by primary consumers, 

whereas the non-seep assemblage was dominated by carnivores . Carnivore 

dominance seems to be typical of shelf (or euhaline) death assemblages. Seep 

assemblages, in contrast, retain the theoretically-expected rarity of predaceous 

forms in fossil assemblages. The infaunal tier was well represented in most 

petroleum seep assemblages because a large fraction of the shelled biota were 

infauna or semi-infauna . Dominance by shelled infauna is unusual . Local 

enrichment of food resources and the dominance of shelled primary consumers 

explain the guild and tier structure of seep assemblages . Nearby autochthonous 

assemblages relying on planktonic production are substantially different in each of 

17 
these criteria. The non-seep autochthonous assemblages were dominated by 

epifauna and semi-infauna as is typical for continental shelf and slope assemblages. 

A hindcasting paleoenergetics model was used to estimate the energy demand 

of both seep and non-seep assemblages. The two important products of the 

hindcasting model are paleoproduction and paleoingestion . Paleoproduction is 

equal to the individual's biomass-at-death, while paleoingestion is the amount of 

energy consumed by the organism over its life span, which is the assimilated energy 

divided by the assimilation efficiency. Hindcasting of energy demand based on the 

observed biomass as preserved and an estimate of sedimentation rate shows that 

energy demand by the community exceeds the supply from plantkonic rain in seep 

communities, a crucial verification of the adequacy of the hindcasting model . 

Bathymodiolus sp., Lucinoma atlantis, and Thyasira oleophila shells were 

radiocarbon dated from the seep sites at GC 234, GC 184, and GB 425, respectively. 

A calibration curve of amino acid dating was used to increase the number of lucinids 

and thyasirids that could be aged. Piston cores from the "Mussel Beach" area of 

GC 234 recovered two buried mussel beds. The first bed, centered at about 95 

centimeters depth, contained shells estimated to be between 2000 and 3000 years 

old. Mussels from the deeper bed, at about 195 centimeters, were estimated to be 

between 3000 and 4000 years old. Thus, the two mussel beds were approximately 

1000 years apart in age . Both of these beds contained individuals in life position, 

and some of the mussels still had byssal thread attachments, indicating the 

importance of catastrophic burial in the preservation of mussel beds. The lucinids 

from GC 184, at 68 centimeters depth, were calculated to be over 3500 years old, 

while the GB 425 thyasirids at 40 centimeters depth were 500 to 1000 years old. 

The lucinids from a depth of 10 centimeters at GC 272 were found to be modern in 

age, and no deeper individuals were encountered. 

18 
Change in community composition rarely occurred over the length of the 

cores (about 200 centimeters) taken from the mussel, lucinid, and thyasirid beds. 

Even at GC 234, an alternative seep fauna did not occupy the site between the 

times of mussel bed burial. In the other cases (thyasirid and lucinid biofacies), sites 

were occupied more or less continuously, but with varying infaunal densities. Thus 

individual seep communities are unique, persistent and resilient over long time 

scales. A transition from non-seep to seep biofacies was never evident in any of the 

cores; accordingly the ages previously mentioned represent minimal ages of the seep 

sites studied. 

Finally, a computer program was designed to reconstruct the original 

abundance and size-frequency distribution from a collection of bivalve fragments. 

The need for this shell reconstruction program comes from the fact that many seep 

bivalves are obtained in fragmented condition due to taphonomic processes 

(processes which alter the fossil record). A primary attribute of the program design 

was the use of simple readily measured characters (shell length, width, thickness, 

weight) allowing rapid analysis of shell fragments . The program was tested using a 

mussel, two lucinids, and an arc shell . In each case, the whole shells were 

measured and then broken into a minimum of three to four pieces, and then re 

measured. Numerical abundance was slightly overestimated and size-frequency 

skewed slightly towards smaller size classes . In terms of paleoproduction and 

paleoingestion, these errors proved of little consequence . Accordingly, this modeling 

approach shows promise as a way to reconstruct the original size-frequency and 

abundance of shells from their fragments permitting their use in this type of 

paleoecological analysis. 

19 
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Hecker, B. 1985. Fauna from a cold sulfur seep in the Gulf of Mexico, comparisons 

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22 
Tunnicliffe, V. 1991 . The biology of hydrothermal vents : ecology and evolution. 

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90 . 

23 
~N,ENT OF Ty The Department of the Interior Mission ~PQ ~'y~ 

a p As the Nation ' s principal conservation agency, the Department of the Interior has responsibility for 

most of our nationally owned public lands and natural resources. This includes fostering sound use 

of our land and water resources; protecting our fish, wildlife, and biological diversity ; preserving the 

_. a9 

q 9 

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res p onsibilities are to mana g e the mineral resources located on the Nation's Outer Continental Shelf 

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~c~ a 

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