Ostracods are a class of the Crustacea. They are often called seed shrimps because of their appearance.
Some 65,000 species (13,000 of which are living) have been identified.
Ostracods are small crustaceans, typically around 1 millimetre (0.04 in) in size, but varying from 0.2 millimetres (0.0079 in) to 30 mm (1.2 in) in the case of Gigantocypris.
Their bodies are flattened from side to side and protected by a bivalve-like, chitinous or calcareous valve or "shell". The hinge of the two valves is in the upper (dorsal) region of the body.
Marine ostracods live in the zooplankton. Others live in the benthos, on or in the upper layer of the sea floor.
Many ostracods are also found in fresh water. Terrestrial species of Mesocypris are known from humid forest soils of South Africa, Australia, New Zealand and Tasmania. They have a wide range of diets, and the group includes carnivores, herbivores, scavengers, and filter feeders.
Copepods may not be monophyletic. Ostracod taxa are grouped into a Class based on gross morphology (what they look like). Their DNA sequence analysis in their mitochondria has been examined. The results are not clear.