More than 200 vent fields have been documented since the late 1970's (Kelly et al, 2005). Hydrothermal vent sites are far from uniform. Factors affecting the structure of vent communities include vent field size, habitat stability, water depth, venting fluid temperature, habitat diversity, ecological succession stage, and larval exchange barriers (Tsurumi & Tunnicliffe, 2003). Compared to other deep-sea communities, hydrothermal vents show much higher productivity but much lower species diversity (Van Dover, 2000). Within individual communities, the distributions of organisms are affected by the environmental gradient created by the mixing of vent fluids with ambient sea water. Animals and microorganisms with greater tolerances for high temperatures and vent chemistry can be found at locations in closest proximity to vents, while organisms with lower tolerances are spaced farther away. Mobile animals not specialized for vent environments may make brief excursions into vent communities to grab a quick bite.
The first vent fields discovered were located in the Pacific Ocean. By the mid 1980's two deep-sea hydrothermal sites were identified along the Mid Atlantic Ridge, and three additional sites were discovered between 1993-1997 (Desbruyeres, 2003). In 2000 the Kiarei vent site was discovered in the Indian Ocean (Hashimoto, 2001). Because many of the longest known and most extensively studied sites are located in the Pacific Ocean, these tend to be the best-known communities. The most well studied sites of hydrothermal venting in the Pacific include those of the Galapagos Rift, the East Pacific Rise, and Juan de Fuca.
Factors associated with geographic location and the degree of isolation from other venting sites exerts a substantial influence on the organisms most likely to be found at a given hydrothermal vent. In the Pacific Ocean, similar communities are found at the Galapagos Rift and the East Pacific Rise, but the biogeography of Juan de Fuca is considerably different. As the community composition of the first two sites has already been discussed, here I will focus on the vent fields found around the Juan de Fuca Ridge. The majority of these fields are colonized by a single species of tube worm, Ridgea piscesae, which colonizes in closely associated groups of interwoven tubes, creating 'tubeworm bushes.' The bushy forms then serve as substrate for other invertebrates (Tsurumi & Tunnicliffe, 2001; Tsurumi & Tunnicliffe, 2003). The number of organisms and species colonizing an individual bush depends on the size and complexity of the aggregation. Although dozens of species can be found associated with these tubeworm clumps, only a few account for most of the biomass.
The Atlantic hydrothermal communities are substantially different from those found in the Pacific. Instead of being dominated by tube worms, the communities at deep water sites along the Mid Atlantic Ridge have swarms of shrimps that cluster around vent chimneys. Shallower vent sites are generally dominated by mussel beds. Studies of trophic structure in Atlantic vent systems show short food chains, with free-living and symbiotic bacteria composing the base. Mussels host symbiotic bacteria, while shrimp feed on free-living bacteria and small invertebrates feed on bacteria or detritus. Crabs can be found at intermediate and top levels of the food chain along with fish (Colaco et al, 2002).
An interdisciplinary team of scientists studied the biogeography of Kairei and Edmond vent fields in the Indian Ocean in 2001 (Van Dover, 2001). Shrimp swarm the centrally located black smokers in the Kairei Field, while anemones become more dominant in peripheral locations. Other common organisms include gastropods, crabs, flatworms, polychaetes and barnacles. The community found at Edmond Field is similar to that of Kairei, although with a lower degree of diversity. Thirty-six invertebrate taxa found in the Indian Ocean sites overlap with taxa previously known only to live in the Pacific. Only one exclusively Atlantic species was found, the shrimp Rimicaris exoculata. Some taxa can be found at vent sites in all three oceans, but molecular comparisons also confirm the closer genetic association between Pacific and Indian Ocean taxa. However, Indian Ocean communities are different enough to constitute a separate biogeographic province from either the Atlantic or the Pacific.
How organisms move from one site to another is important in regulating the composition of vent communities, both on biogeographic and local spatial scales. Many benthic invertebrates, such as tubeworms, polychaetes, and bivalve mollusks that populate hydrothermal vents have non-motile adult forms. Consequently, behavioral and transport processes effecting mobile larval stages strongly influence the distribution of these organisms. Some larvae must remain within their natal habitats to maintain local populations, but new sites cannot be colonized if some organisms are not either passively or actively transported to new locations (Metaxas, 2004). Larvae of R. pachyptila, known to rapidly colonize new vent sites, have the metabolic capacity to stay active for over a month without feeding and are thought to take advantage of ocean currents that flow along the East Pacific Rise (Marsh et al, 2001). The questions associated with organismal dispersal are fascinating, but can be difficult to study, due to remote locations, complex physical processes, and large quantities of very small organisms with high mortality rates. The Larvae At Ridge Vents (LARVAE) project participated in field expeditions in the late 1990's in order to study such questions (Mullineaux, 1998). Still, there is much to be learned.
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