(mussel-shrimps, seed-shrimps)

Ostracods are minute to very small crustaceans completely enclosed in a bivalved carapace. They may superficially resemble Conchostraca (clam-shrimps) although are generally smaller. Ostracods do not have a multi-segmented trunk with foliose gills as do conchostracans, but instead a reduced body of no more than four segments and with only three pairs of leg-like appendages. In total, these crustacea show two pairs of antennae, two pairs of mouthpart appendages, three pairs of thoracic appendages, and a pair of furcal claws. Both the first and second antennae (antennules, antennae) are well developed, readily visible and protrude beyond the carapace.

Three superfamilies are known from Australian inland waters. These are the Cytheroidea, Darwinuloidea, and Cypridoidea. Identification of adults to superfamily is relatively simple. Relevant features are the structure of the trunk appendages (legs 1-3), segmentation of the antennules, and shape of the carapace.

Identification beyond superfamily can be difficult for other than adult females (specimens in which eggs can be seen through the carapace). For this reason, and also because Ostracoda often are smaller than the minimum size of animal dealt with in this key, the key runs only to superfamily level. The families are described in taxon notes.