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Different shrimp species occupy different habitats and there are at least two distinctive
forms of ectosymbionts. The ectosymbiosis is most prominent in Rimicaris exoculata,
which has an enlarged gill chamber housing a dense flora of bacterial filaments 16. Two
other common MAR shrimp species, Alvinocaris markensis and Chorocaris chacei, also carry
some bacterial filaments on their carapace and appendages but these associations are
much less developed compared to Rimicaris exoculata 16. As in the case of the EPR
alvinellid worms, the nutritional or detoxifying roles of these ectosymbioses remain
uncertain. All shrimp can be observed to actively feed on chimney surfaces, and guts of
collected specimens usually contain abundant mineral particles 16. In addition, the two
larger shrimp species (Alvinocaris markensis and Chorocaris chacei) are listed among the
predators of the smaller Rimicaris exoculata 17.
Free-Living Microbial Growth
As on the EPR, microorganisms grow abundantly on mineral surfaces that are
exposed to hydrothermal fluids 18. Filamentous bacteria often produce dense aggregations
that are visible to the naked eye as fluffy tufts and mats. These bacteria are a potential
food source for grazing and deposit-feeding animals.
4. GENE FLOW ALONG THE GLOBAL RIDGE SYSTEM
From the time of the first discovery of hydrothermal vent communities,
biologists have been asking how vent animals manage to persist in this ephemeral and
spatially discontinuous habitat. How are new vents colonized? Why do we find the same
species at vents hundreds and thousands of km apart? Why do other species have very
restricted distributions? These questions lead to a fundamental point about how genetic
information is transmitted along the global ridge system. Most hydrothermal vent
species colonize new sites by producing larvae that have some swimming ability but, as
for most marine larvae, are primarily transported by ocean currents. If there are barriers
to the movement of larvae between different areas of the ridge crest, their species
compositions will eventually begin to differentiate as a result of natural mutations or
local extinctions. If gene flow is maintained through the exchange of larvae, then the
populations and assemblages of species will continue to resemble each other.
Part of the answer to questions about gene flow along ridges has come from
sampling of vents around the world and comparing species inventories between
geographic locations 19. New techniques in molecular biology are also being applied to
this problem, both to confirm differences between morphological species and to compare
populations of the same species in order to quantify the rate of genetic exchange along
the ridge axis. While published data are few and sampling of the global ridge system is
still very incomplete, several interesting observations and hypotheses have emerged
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