The Diplostraca or Cladocera, commonly known as water fleas, is a superorder of small, mostly freshwater crustaceans, most of which feed on microscopic chunks of organic matter, though some forms are predatory.

[2][3][4][5][6] The oldest fossils of diplostracans date to the Jurassic, though their modern morphology suggests that they originated substantially earlier, during the Paleozoic.

[7] Most are 0.2–6.0 mm (0.01–0.24 in) long, with a down-turned head with a single median compound eye, and a carapace covering the apparently unsegmented thorax and abdomen.

[9] The thorax bears five or six pairs of lobed, leaf-like appendages, each with numerous hairs or setae.

These ephippial eggs can be transported over land by wind, and hatch when they reach favourable conditions, allowing many species to have very wide – even cosmopolitan – distributions.

[12] Diplostraca are nested within the clam shrimp, being most closely related to the order Cyclestherida, the only living genus of which is Cyclestheria.

[14] According to the World Registry of Marine Species, Cladocera is a synonym of the superorder Diplostraca, which is included in the class Branchiopoda.

The following families are recognised:[2] Superorder Diplostraca Gerstaecker, 1866 (=Cladocera) The word "Cladocera" derives via Neo-Latin from the Ancient Greek κλάδος (kládos, "branch") and κέρας (kéras, "horn").