Cold-seep ecosystems are biological communities sustained by the seepage from the ocean subsurface of cold fluid containing chemicals (especially methane but also sulfide). Since their discovery in 1984, cold-seep ecosystems have been described from various oceans and seas. More than 200 seep species have been described so far, and many of them (approximately one-third) are symbiont-bearing species. In methane-rich seep environments, symbioses are developed between gutless bivalve mollusks, roundworms, and tubeworms, which may host in their soft tissues (gills, for example) dense communities of methane-consuming and sulfide-oxidizing bacteria. By a microbial process known as chemosynthesis, these endosymbiotic bacteria use the chemical energy derived from the oxidation of sulfide and methane to synthesize organic compounds that serve as food for their mollusk and worm hosts. In marine sediments of a methane seep, sulfide-oxidizing activity is also performed by free-living bacteria that may develop as thick mats composed of giant filaments. This microbial biomass may directly serve as nourishment for many organisms, in contrast to the symbiotic relationships in the gutless and/or mouthless animals.