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 regarding the ecological functioning of these cold seep communities. We distinguish 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 feeding 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 diffuse 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.