Cold seeps and hydrothermal vents are cognate communities (Van Dover, 2000) with most of the mega fauna found at these sites being closely related (Sibuet and Olu, 1998). The most common organisms at cold seeps are bivalve molluscs and tube worms: Vesicomyid clams live in soft sediments the most common genus found is Calyptogena sp. which can reach lengths of 20cm (Sibuet and Olu, 1998). Calyptogena sp. are very diverse
and are found at almost all cold seep sites preferring areas of relatively low methane, as a result they can tolerate variation in fluid flow and sulphur concentration (Sibuet and Olu, 1998). They also settle where there is an inflow of water into the sediment causing a mixing of subsurface methane and seawater sulphate (Tryon and Brown, 2001). However certain species have bathymetric restrictions, for example Calyptogena phaseoliformis is only found at depths of 3,800m this could be due to limited larval dispersion as there is not strong flow at deep cold seep sites (Sibuet and Olu, 1998). Another common cold seep bivalve is Bathymodiolus mussels, however their global distribution is restricted to the Atlantic and Western Pacific cold- seep areas at depths of 400-2000m (Sibuet and Olu, 1998). These molluscs form dense colonies at areas of high methane fluid flow, high temperature gradients and high carbonate concretions (Sibuet and Olu, 1998) which explains why many colonies are found surrounding brine pools (Van Dover, 2000). Other molluscs found at cold seeps are Solemyidae, Thyasiridae and Lucinidae they are deep burrowers and are found in very small numbers. All these molluscs have gills full of endosymbiotic bacteria either sulphur oxidising bacteria or methanotrophic bacteria (Fisher et al. 2000)(Sibuet and Olu, 1998).
Tube worms form dense colonies anchored into the sediment and stand vertically in the water column they are able to do this as they live in chitinous tubes which they create throughout their life (Bergquist et al. 2003). They feed by collecting sulphide from the water column via diffusion through the plume tissues and also via the sediment by drawing in sulphide from underground pools, after which they send it to an organ called the trophosome which contains symbiotic bacteria which use the sulphide as an energy source (Scott and Fisher, 1995), this is backed up by the fact tube worms have no mouth or digestive system (Van Dover, 2000). Two example of cold seep tube worms are Lamellibrachia luymesi and Seepiophila jonesi (Bergquist et al. 2003).
Other macro and mega fauna that are found are usually suspension or deposit feeders with many being endemic to cold seep environments for example the gastropod Bathynerita naticoides and the crustacean Alvinocaris muricola (Sibuet and Olu, 1998). Methane ice worms (Hesiocaeca methanicola) is also endemic to cold seeps, it is a polychaete worm which grows to 2-4cm in length (Fisher et al. 2000). It lives on the methane hydrates that form under low temperatures and high pressures, feeding on the free living bacteria that also inhabit the methane hydrates, however it does supply the bacteria with oxygen due to the movement of their parapodia which creates a current (Fisher et al. 2000). Most of the detritivores have the same abundance found at cold seep sites as well as outside cold seep
sites meaning they are vagrants, this is also true for carnivores such as Macrourid fish and octopods (Sibuet and Olu, 1998). Other carnivores such as crabs and the gastropod Cataegis meroglypta are colonists meaning there is an increase in abundance around cold seep sites as they feed on the vast mussel beds (Sibuet and Olu, 1998). The species richness found at these sites decreases with depth due to the decrease in current in the deep sea which is needed for larval dispersion (Sibuet and Olu, 1998) however this does not apply for the meiofaunal abundance as this is completely controlled by the abundance of the free living bacteria (Sibuet and Olu, 1998).
Every animal that inhabits the deep sea has to deal with the increased pressure whether they live at a cold seep site or in the abyssal plain. The increased pressure effects the biochemistry of the organism especially at the cell membranes and the enzyme activity (Van Dover, 2000). The increase in pressure starts to compress the molecules in the lipid bilayer of the cell membrane this changes the membrane fluidity and shape, which therefore interferes with the ‘lock and key’ mechanism of the membrane bound enzymes (Van Dover, 2000). To deal with this change adaptations have occurred that control the fluidity in the membrane using homeoviscosity (Van Dover, 2000). As part of this control, the amount of unsaturated fatty acids is increased, unsaturated fatty acids have double carbon bonds making them stronger and therefore less likely to be effected by the extreme pressure (Van Dover, 2000). Enzymes in organisms from the deep sea are also adapted to high pressure; when a volume increase takes place during an enzyme reaction increased pressure usually inhibits it, especially with substrate – enzyme bonding, however studies have shown that enzymes in deep sea organisms constantly function effectively with an increase in pressure (Van Dover, 2000).
As well as pressure cold seep organisms have to deal with large amounts of hydrogen sulphide which is usually fatal in other organisms this is because it binds to cytochrome-c oxidase which is a critical enzyme in the electron transport system (Van Dover, 2000). Therefore it inhibits the aerobic energy supply as well as inhibiting pulmonary function and oxygen transport in the organism (Grieshaber and Volkel, 1998). Some cold seep animals are able to take up sulphide and use it directly as an energy source by oxidising the sulphide within the mitochondria however in limiting oxygen concentrations mitochondrial sulphide oxidation is costly to the animal therefore this process is not common (Grieshaber and Volkel, 1998). Most other cold seep organisms have a symbiotic relationship with anaerobic sulphate reducing bacteria such as Desulfovibrio sp. (Grieshaber and Volkel, 1998). Sulphide rather than hydrogen sulphide is taken up as hydrogen sulphide inhibits enzyme activity, the sulphide that is absorbed binds rapidly to haemoglobin in the bloodstream but does not compete for the oxygen binding site (Van Dover, 2000). Cold seep organisms have large amounts of haemoglobin with a high affinity for sulphide meaning when the organism is exposed to a shortage of sulphur it can still survive (Van Dover, 2000). The sulphide is transported by the bloodstream to the bacterial sites which are slightly acidic causing the haemoglobin to dump the sulphur. Once at these bacterial sites either the trophosome or the gills sulphide oxidising bacteria use the energy from the sulphide to create ATP. Some organisms harbour methanotrophic endosymbiotic bacteria as well as sulphide oxidising bacteria but use methane as their energy source (Sibuet and Olu, 1998). Therefore the bacteria supply the host with their nutritional needs and the host gives the bacteria oxygen, carbon dioxide and sulphide needed by the symbiont for its metabolism (Van Dover, 2000).