The copepods include around 12,000 described species of mostly small aquatic crustaceans. Most are between 0.5 mm and 10 mm in length, but some free-living forms exceeed 1.5 cm and some parasitic forms may reach 25 cm. Copepods are extremely abundant in the ocean (from the surface to 5000 meters deep, including at least one species described from a hydrothermal vent), in freshwater, in estuarine habitats, and in interstitial habitats (i.e, between sand grains). The majority of known free-living copepods belong to the Calanoida, Harpacticoida, or Cyclopoida, although these groups also include some parasitic species. Calanoids have greatly elongated antennules. Most calanoids are planktonic and as a group they are very important primary consumers in both freshwater and marine food webs. Harpacticoids have a more worm-like shape, with (in contrast to both calanoids and cyclopoids) the posterior segments not much narrower than the anterior ones. Both the antennules and antennae are quite short in harparcticoids; cyclopoids have moderately long antennae, although never as long as the antennules of calanoids. The antennae are uniramous (i.e., unbranched) in cyclopoids, but biramous in calanoids and harpacticoids. Most harpacticoids are benthic (bottom-dwelling) and occur in a wide range of aquatic environments; at least a few freshwater and marine species are known to form cysts. Cyclopoids are found in both marine and freshwater habitats and most are planktonic. (Brusca and Brusca 2003)
The non-parasitic copepods move by crawling or swimming, using some or all of the thoracic limbs. Many of the planktonic forms have dense setae on their appendages, making them resistant to sinking. Calanoids are mainly planktonic feeders. Although benthic harpacticoids are often reported to be detritus feeders, many feed mainly on microorganisms living on the surface of detritus or sediment particles (e.g., diatoms, bacteria, and protists). (Brusca and Briusca 2003; Margulis and Chapman 2010)
Of the seven remaining orders, the Mormonilloida are planktonic; the Misophrioida are known from deep sea epibenthic habitats as well as anchialine caves in both the Atlantic and Pacific; and the Monstrilloida are planktonic as adults, but as larvae are endoparasites of gastropod mollusks, polychaete annelid worms, and occasionally echinoderms. Members of the orders Poecilostomatoida and Siphonostomatoida are exclusively parasitic. Siphonostomatoids are ectoparasites or endoparasites of various invertebrates as well as marine and freshwater fishes. They are often very tiny and may exhibit a reduction or loss of body segmentation. Poecilostomatoids parasitize invertebrates and marine fishes and may also show a reduced number of body segments. The Platycopioida are benthic copepods known mainly from marine caves. The Gelyelloida are known only from European groundwaters. (Brusca and Brusca 2003; Margulis and Chapman 2010)
