Marine life live in the water column of large bodies of water and that cannot swim against a current. They provide a crucial source of food to many large aquatic organisms, such as fish and whales. These organisms include drifting or floating bacteria, archaea, algae, protozoa and animals that inhabit, for example, the pelagic zone of oceans, seas, or bodies of fresh water. Essentially, plankton are defined by their ecological niche rather than any phylogenetic or taxonomic classification. Though many planktonic species are microscopic in size, "plankton" includes organisms covering a wide range of sizes, including large organisms such as jellyfish. The
Marine microorganism a large number of eukaryotic microbial communities have yet to be discovered. At least one microscopic animal group, the parasitic cnidarian Myxozoa, is unicellular in its adult form, and includes marine species. Other marine microanimals are multicellular. Microscopic arthropods are more commonly found inland in freshwater, but there are marine species as well. Microscopic marine crustaceans include some copepods, cladocera and water bears. Some marine nematodes and rotifers are also too small to be seen with the naked eye, as are many loricifera, including the recently discovered anaerobic species that spend their lives in an anoxic environment. Copepods contribute more