Cladocera is an order of small crustaceans commonly called water fleas. Around 620 species have been recognised so far, with many more undescribed.
They are everywhere in inland aquatic habitats, but rare in the oceans. Most are 0.2–6.0 mm (0.01–0.24 in) long, with a down-turned head, and a carapace covering the thorax and abdomen. There is a single median compound eye. They swim with jerking flea-like movements.
Most species show cyclical parthenogenesis, where asexual reproduction is occasionally supplemented by sexual reproduction. The sexual reproduction produces resting eggs that allow the species to survive harsh conditions and disperse to distant habitats.
