Epifauna and megafauna

Abundance, composition and characteristics

Community descriptions exist for a wide number of cold seeps (reviewed in Sibuet & Olu 1998, Van Dover 2000, Kojima 2002). At most seeps in the Pacific and Atlantic Oceans, vestimentiferan tube worms (now recognized to be polychaetes), bathymodiolid mussels, and vesicomyid clams (Figure 6C,D) form most of the biomass. As a result, biological research has focused extensively on these groups. Common vestimentiferan genera at seeps include Lamellibrachia, Escarpia and Alaysia.

There are at least 11 species of seep mussels, most in the subfamily Bathymodiolinae, genus Bathymodiolus. Where present, they can often form extensive beds, similar to Mytilus beds on rocky shorelines. Their absence at some seeps off Japan and in the northeastern Pacific is noteworthy but the reasons are not known. Bathymodiolus species may partition the environment by substratum and fluid flow. Of the two species found on the Barbados prism, one species, with both sulphide-and methane-oxidizing symbionts, prefers soft sediment where flow is more diffuse and the other, with only methanotrophic symbionts, occurs on hard, carbonate substratum where fluid flows and methane concentrations are higher (Olu et al. 1996b).

Seep clams are usually members of the family Vesicomyidae, and are the most pervasive of large seep taxa, with a presence at most seeps (Sibuet & Olu 1998, Kojima 2002) (Figure 6D). There are many species in the genera Calyptogena and Vesicomya (Goffredi et al. 2003), and in the Pacific it is not unusual for two or three species to co-occur at seeps (Barry et al. 1997, Kojima 2002). Like the mussels, they can attain high densities (up to 1000 ind m-2 — Japan trenches, Peru) and biomass (10-30 kg m2) (Hashimoto et al. 1989, Olu et al. 1996a) with single fields covering areas up to 7000 m2 (Olu et al. 1996a, 1997). The clams are often aligned linearly along geological structures at the base of steps, in depressions or in cracks (Suess et al. 1998). Calyptogena phaseoliformis (now referred to the genus Ectaegena) in the Aleutian Trench (Suess et al. 1998), Japan Trench (6,180-6,470 m, Fujioka & Murayama 1992) and Ryukyu Trench (5,800 m, Kato et al. 1999) and Calyptogena fossajaponica (6600-6800 m, Kojima et al. 2000b) have the deepest distributions.

The large sizes of the tubeworms (up to 2 m, Bergquist et al. 2003), mussels (up to 36 cm, Van Dover et al. 2003) and clams (up to 18.6 cm, Olu et al. 1996b) at seeps are a result of symbiont-supported chemoautotrophic nutrition. Each of the species hosts either sulphide-oxidizing sym-bionts (Fiala-Medioni et al. 1993), methanotrophic symbionts (Childress et al. 1986) or both (Fisher et al. 1993). They typically have a reduced gut and exhibit little reliance on photosynthetically fixed organic matter raining down from the surface, although the mussels are known to feed.

At some seeps the typical taxa may be absent and thyasirid, solemyid and lucinid bivalves, perviate and monoliferan pogonophoran worms, and trochid or buccinid gastropods may be dominant (Suess et al. 1998, Callender & Powell 2000). Lucinids are reported as dominant at 290-330 m on the Kanesu no Se Bank above the Nankai Trough (Mesolinga soliditesta, Okutani & Hashimoto 1997), and in the eastern Mediterranean Sea (1700 m, Lucinoma kazani n.sp., Salas & Woodside 2002), in the Gulf of Mexico, Green Canyon and Garden Banks (513-754 m, Lucinoma sp., Callender & Powell 2000).

Infaunal thyasirids are dominant at both shallow seeps (North Sea, Dando et al. 1991; Sea of Okhotsk at 750-800 m (Conchocera bisecta), Kuznetsov et al. 1989) and at the deepest chemo-synthetic seep known (7330-7430 m in the Japan Trench (Maorithyas hadalis), Fujikura et al. 1999, Okutani et al. 1999). They have also been reported from Barbados (Olu et al. 1996a), the Gulf of Mexico (MacDonald et al. 1990) and the Laurentian Fan (Mayer et al. 1988). There are fossil thyasirid biofacies in the shallow Gulf of Mexico (Callender & Powell 1997, 2000).

Figure 7 (A) carbonate slabs with aggregations of the urchin Allocentrotus fragilis. Hydrate Ridge, Oregon, 590 m; urchin diameter ~5 cm; (B) aggregations of moribund gastropods (Neptunia sp.) with egg cases, gastropod length ~8 cm. Also in the picture are hagfish and the asteroid Rathbunaster californicus. April 2001, Eel River margin, 500 m.

Figure 7 (A) carbonate slabs with aggregations of the urchin Allocentrotus fragilis. Hydrate Ridge, Oregon, 590 m; urchin diameter ~5 cm; (B) aggregations of moribund gastropods (Neptunia sp.) with egg cases, gastropod length ~8 cm. Also in the picture are hagfish and the asteroid Rathbunaster californicus. April 2001, Eel River margin, 500 m.

Pogonophorans form dense fields at seeps on the Hakon Mosby Mud Volcano (Sclerolinum, Oligobrachia, Pimenov et al. 1999), in the Gulf of Alaska (Spirobrachia, Suess et al. 1998, Levin & Michener 2002) and occasionally in the Gulf of Mexico (3234 m, R.S. Carney, personal communication). Other seep epifauna include bresiliid shrimp, cladorhizid and hymedesmid sponges (Olu et al. 1997), serpulids, pennatulids and caprellid amphipods (Olu et al. 1996b) and galatheid crabs (though these may be vagrants sensu Carney 1994). Shrimp (family Bresilidae) are much less common at seeps than vents and have been documented only at seeps in the Gulf of Mexico, Florida, Barbados and Blake Ridge. Sponges with methanotrophic bacterial symbionts are abundant on Barbados mud volcanoes where they occur in bushes up to 2 m in diameter (Olu et al. 1997). Gas hydrate mounds in the Gulf of Mexico provide a specialized substratum for the ice worm Hesiocaeca methanicola, which burrows into the deposits.

Occasionally non-seep species will exhibit enhanced densities in the vicinity of deep-water seeps. Aggregations of holothurians (Scotoplanes, Peniagone) and large tubiculous polychaetes were documented by Sibuet et al. (1988) at the Japan Trench and Kashima Seamount seep sites. Holothurians aggregate on the flanks of hydrate and tar mounds in the Gulf of Mexico (MacDonald et al. 2003, 2004). At upper-slope depths off Oregon and California dense aggregations of sea urchins (Figure 7A), buccinid gastropods (Figure 7B), cnidarians (Figure 3) and asteroids occur on or near seeps (Levin, unpublished data).

Zonation, distribution and geochemistry

Concentric (circular) zonation of fauna has been noted by Sahling et al. (2002) at Hydrate Ridge in Oregon, by Barry et al. (1997) and Rathburn et al. (2003) in Monterey Bay and by Olu et al. (1997) at mud volcanoes near the Barbados accretionary prism. Central areas with methane-rich fluid mud or strong flows are devoid of fauna or covered by bacterial mats. These areas are surrounded by different species of clams. At 'Extrovert Cliffs' in Monterey Bay (960 m water depth), 2-m diameter seep patches consisted of a dark gray bacterial mat encircled by a yellow bacterial mat, which was surrounded by Calyptogena clams (Figure 8, Rathburn et al. 2003). Barry et al. (1997) document different sulphide preferences in different Calyptogena species from this region. Similar concentric structures were observed at Hydrate Ridge (770 m) on the Oregon margin, where mounds several metres in diameter contain mats of sulphur bacteria surrounded by two

Figure 8 Seep 'ring' consisting of bacterial mats in the core (~45 cm) and a concentric ring of vesicomyid clams (1 m diameter) Extrovert Cliffs, Monterey Bay, 960 m. (Photo copyright 2000, Monterey Bay Research Aquarium.)

species of vesicomyid clams (Calyptogena pacifica, C. kilmeri), which were encircled by the solemyid Acharax. The biological zones coincided with changes in the porewater hydrogen sulphide and alkalinity and in oxygen penetration (Sahling et al. 2002). The lowest oxygen penetration and highest sulphide concentrations were associated with bacterial mats; greater oxygen penetration and lower sulphide levels were associated with clam beds (Rathburn et al. 2003, Levin et al. 2003). Off Peru, the spatial distribution of Calyptogena clam beds was strongly linked to features such as joints, scars and screes related to slope instabilities, which are likely to conduct or expose sulphide (Olu et al. 1996a).

MacDonald et al. (2003) note that vestimentiferan tube worms in the Gulf of Mexico are abundant at upper slope depths (<1000 m) and at the base of the slope (>2500 m) but not in the middle (1000-2000 m). They propose that gas hydrates fuel the shallow systems but are more stable with less flux of hydrocarbons at mid depths, and that the deepest communities are fueled by another source unrelated to gas hydrates.

Epifauna as sources of habitat heterogeneity

Seep tubeworms, mussels and clams typically serve as 'ecosystem engineers' that generate extensive habitat complexity both above and below ground. Their tubes, shells and byssus threads support a myriad of smaller taxa (Carney 1994, Bergquist et al. 2003, Turnipseed et al. 2003). There are epizoonts on shells and tubes, and byssus-thread associates. Common among these are gastropods in the families Neolepetopsidae, Provannidae and Pyropeltidae, actinians, dorvilleid and scale polychaetes. Each of the large dominant seep species also supports specialized commensal taxa including nautiliniellid (Miura & Laubier 1990) and phyllodocid polychaetes (E. Hourdes, personal observation) as well as bivalves (Acesta sp., C. Young, in preparation). Sponge and serpulid thickets (worms 20 cm long, thickets of 20-30 ind m2) also introduce habitat complexity at seeps (Olu et al. 1996b, 1997) but their associated faunas have not been studied.

Vestimentiferans support a rich community of associated invertebrates above and below the sediment surface (Bergquist et al. 2003). In the Gulf of Mexico, Lamellibrachia cf luymesi and Seepiophila jonesi form hemispherical 'bushes' that are several metres high and wide. A collection of seven of these bushes yielded 66 species of which 18 are considered to be endemic (Bergquist et al. 2003) and five (four bivalves and a sponge) appear to harbor symbionts. The most abundant taxa within Gulf of Mexico tubeworm aggregations are gastropods (Bathynerita, Provanna), shrimp (Alvinocaris), mussels (Bathymodiolus), crabs (Munidopsis), nemerteans, polychaetes (Harmothoe, sabellids), amphipods (Orchomene and Stephonyx sp.) and sipunculans (Phalascosoma). Densities of many taxa increase with habitat complexity, measured as tubeworm density, but decline with age of the tubeworm aggregation. Increasing patch age leads to a decline in primary producers (symbiont-bearing taxa) and increasing importance of secondary and higher predators, as well as non-endemic species. Species richness also increases with patch size, tube surface area and vesti-mentiferan biomass.

Successional changes corresponding to aggregation composition and age may be driven by environmental factors, especially sulphide. Order of magnitude declines in biomass and density of associated fauna in older aggregations may reflect indirect effects of diminishing sulphide production (Bergquist et al. 2003). Similar results have been obtained for tubeworm associates at hydrothermal vents on the Juan de Fuca Ridge. Tubeworm aggregation complexity and successional stage (driven by venting) had a strong influence on the numbers of species and composition (Tsurumi & Tunnicliffe 2003). It appears that species richness of tubeworm aggregations is lower at vents than seeps (only 37 taxa were found among 350,000 specimens), with gastropods and polychaetes dominant (Tsurumi & Tunnicliffe 2003).

Diversity of mussel bed associates has been assessed quantitatively in the Gulf of Mexico and on the Blake Ridge at depths of 2500-3600 m (Turnipseed et al. 2003). These habitats shared only four species. Blake Ridge mussel beds contain numerous chirodotid holothurians, deposit-feeding sipunculans and alvinocarid shrimp (similar to Alvinocaris muricola). Smaller taxa included chaetopterid, maldanid and capitellid polychaetes, as well as nematodes (Van Dover et al. 2003). Large predators are galatheid crabs, octopus, fishes and anemones. Comparison of mussel-bed fauna at the Gulf of Mexico and Blake Ridge seep sites to those of four hydrothermal vents revealed species richness nearly 2 times greater at seeps than vents (Turnipseed et al. 2003).

Beds of vesicomyid clams are a feature of many seeps throughout the oceans. Typically the clams nestle within the upper few centimetres of sediments and the associated clam bed fauna is more of a sediment community than is the case for vestimentiferan and mussel bed assemblages, which may occur on carbonate or biogenic substrata (Van Dover et al. 2003). Although clam aggregations exist at most seeps, there has been limited quantitative sampling of associated fauna. Influence of seep clams on associated infauna is discussed later in the section on macrofauna.

Was this article helpful?

0 0

Post a comment