Any of a large subclass (Copepoda) of usually minute freshwater and marine crustaceans that form an important element of the zooplankton in the marine environment and in some fresh waters. A common feature uniting all the copepod orders is a single simple eye in the middle of the head, at least in the larval stage. The cephalosome, a shield over the head and some thoracic segments distinguishes the free living forms from parasitic ones. Some species are ectoparasites of marine fishes.