Copepod

Copepods (/ˈkoʊpɪpɒd/; meaning "oar-feet") are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic (drifting in sea waters), some are benthic (living on the ocean floor), and some continental species may live in limnoterrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests, bogs, springs, ephemeral ponds, and puddles, damp moss, or water-filled recesses (phytotelmata) of plants such as bromeliads and pitcher plants. Many live underground in marine and freshwater caves, sinkholes, or stream beds. Copepods are sometimes used as biodiversity indicators.

Classification and diversity

Copepods form a subclass belonging to the subphylum Crustacea (crustaceans); they are divided into ten orders. Some 13,000 species of copepods are known, and 2,800 of them live in fresh water.

Characteristics

Copepods vary considerably, but can typically be 1 to 2 mm (0.04 to 0.08 in) long, with a teardrop-shaped body and large antennae. Although like other crustaceans, they have an armoured exoskeleton, they are so small that in most species, this thin armour, and the entire body, is almost totally transparent. Some polar copepods reach 1 cm (0.39 in). Most copepods have a single median compound eye, usually bright red and in the centre of the transparent head; subterranean species may be eyeless. Like other crustaceans, copepods possess two pairs of antennae; the first pair is often long and conspicuous.