Evolutionary relationships of deep-sea hydrothermal vent and cold-water seep organisms

Robert Vrijenhoek
Rutgers University

Monday, November 16,1998
3:00 p.m.—Pacific Forum

Dense biological communities found at deep-sea hydrothermal vents and cold-water sulfide/hydrocarbon seeps share numerous faunal similarities. Biologists have speculated broadly about the ecological and evolutionary connections between vent and seep organisms, but incomplete knowledge of their genealogical relationships hampers a comprehensive biogeographic analysis.

I review recent molecular studies aimed at addressing questions about the relationships and dispersal abilities of these unusual endemic faunas. First, I address the origin of vent taxa. Are they derived from seep ancestors, or vice versa? Studies of clams, mussels, and vestimentiferan tubeworms support the hypothesis that the seep taxa are ancestral to most vent-endemic species. Second, I ask whether these deep-sea organisms are ancient. Did they escape the major global extinction events that devastated biological diversity in the photic zone? Molecular systematic analyses reveal that vent- and seep-endemic clams, mussels, tubeworms, and shrimp all constitute recent (probably Cenozoic) radiations from shallow-water ancestors. So, who occupied these habitats in the Mesozoic and Paleozoic? Third, I ask whether these deep-sea organisms evolved in parallel with their endosymbiotic bacteria. The answer depends on the mode of endosymbiont transmission (vertical versus horizontal), which differs among several mollusks and tubeworms organisms. Finally, I examine various modes of colonization and dispersal in vent and seep organisms and address the effects on retention of genetic diversity and maintenance of cohesion within species.

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