Cold seep benthic communities in the Japan subduction zones: spatial organization, trophic strategies and evidence for temporal evolution

Submersible exploration of the Japan subduction zones has revealed benthic communities dominated by clams (Calyptogena spp.) around sed~ment porewater seeps at depths from 3850 to 6000 m.Photographic and video records were used to produce microcartographlc reconstructions of 3 contrasting cold seep sites.Spatial and abundance relations of megafauna lead us to propose several hypotheses regard~ng the ecological functioning of these cold seep communities.W e dlst~nguish between the likely direct use of reducing substances in venting fluids by Calyptogena-bacteria symbioses and indirect usage by accompanying abyssal species not known to harbour symbionts.Microdistribution and known or presumed feedmg modes of the accompanying species indicate 2 different sources of organic matter enrichment within cold seep sites: organic debris originating from bivalve colonies, and enrichment of extensive areas of surrounding surface sediments where weaker, more hffuse porewater seepage may support chemosynthetic production by free-living bacteria.The occurrence of colony-like groupings of empty Calyptogena valves indicates that cold seeps are ephemeral, with groups of living clams, mixed living and dead clams, and empty dissolving shells marlung the course of a cycle of porewater venting and offering clues to the recent history of cold seep activity.