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Ectosymbioses
The EPR Pompeii worm Alvinella pompejana represented in Figure 3, has
specialized structures on its dorsal surface that are colonised by a diverse community of
bacteria, including large filamentous forms visible to the naked eye. While described as
the most highly evolved epibiotic association among all marine invertebrates, the
functional role of the Pompeii worm’s bacteria remains uncertain. They may serve as a
source of nutrition for the worm or in detoxifying the microenvironment within the
worm’s tube 11. The worm does have a functional digestive system and exhibits depositfeeding
behaviour. Since at least some of the epibiotic bacteria are know to be
chemosynthetic, and are probably sulphide oxidizers, they could both provide a
supplementary food source for A. pompejana and act as a barrier against the diffusion of
hydrogen sulphide into the worm’s tissues.
Free-living microbial growth
Microorganisms can also grow abundantly on animal and mineral surfaces that
are exposed to hydrothermal fluids 12,13. Filamentous bacteria often produce dense
aggregations that are visible to the naked eye as fluffy tufts and mats. These surfacegrowing
bacteria are a potential food source for grazing and deposit-feeding animals
such as snails, limpets and worms. These same microbial films can be detached from
surfaces by turbulence, and either consumed by filter or suspension feeding animals
around the vent or exported to the surrounding deep sea.
3.2 MAR vent ecosystems
The visually spectacular vestimentiferan tubeworms, the archetypal organisms of
vents in the eastern Pacific, are conspicuously absent from known vent sites on the Mid-
Atlantic Ridge. Reasons for this do not appear to be strictly related to the geographic
separation of the Atlantic and Pacific oceans, since tubeworms are abundant at cold seeps
in the Gulf of Mexico.
Endosymbioses
Only mussel endosymbioses are known from vents on the Mid-Atlantic Ridge.
The mussels are similar to the EPR species although some have been found to harbour
methane-oxidizing bacteria in their gills 14. Other vent mussels from the MAR host a dual
symbiosis, containing both sulphide oxidising and methane oxidising bacteria in their
gills 15. This is a highly unusual condition in multicellular animals.
Ectosymbioses
Swarms of shrimp bearing ectosymbiotic bacteria on their legs and in specialised
branchial (gill) cavities occur on hydrothermally active sulphide deposits on the MAR.
106 International Seabed Authority