Amphipods are found in almost all aquatic environments, from fresh water to water with twice the salinity of sea water and even in the Challenger Deep, the deepest known point in the ocean. They are almost always an important component of aquatic ecosystems, often acting as mesograzers. Most species in the suborder Gammaridea are epibenthic, although they are often collected in plankton samples. Members of the Hyperiidea are all planktonic and marine. Many are symbionts of gelatinous animals, including salps, medusae, siphonophores, colonial radiolarians and ctenophores, and most hyperiids are associated with gelatinous animals during some part of their life cycle. Some 1,900 species, or 20% of the total amphipod diversity, live in fresh water or other non-marine waters. Notably rich endemic amphipod faunas are found in the ancient Lake Baikal and waters of the Caspian Sea basin.