There are 7500 species of copepods typically distinguished by their tapering shape, median eye and long antennae held at right angles to the body. They include the most abundant members of the zooplankton – the marine genus Calanus and the freshwater Cyclops – and play an important role in aquatic food webs.
This class also includes over 1000 species of ecto- and endo-parasites. Some ectoparasites resemble free-living forms. Others like Penella exoceti embedded in the skin of a whale are so highly modified that they no longer resemble free-living copepods. The Penella themselves are host to the commensal barnacle Conchoderma virgatum.