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The Ostracoda (ostracods) are a group of tiny bivalved (i.e. with a hinged two-part shell) crustaceans that includes both freshwater and marine forms. They are abundant worldwide in virtually all types of aquatic environments and are known to depths of 7,000 m in the sea. Some are commensal on other crustaceans or on echinoderms. Ostracods are mostly between 0.1 and 2.0 mm, but a few are larger (Gigantocypris reaches 32 mm!). Body segmentation is reduced in ostracods relative to most other crustaceans. There are around 13,000 described extant species of ostracods and the group has a rich fossil record going back to the Ordovician (Brusca and Brusca 2003).
Most ostracods are benthic crawlers or burrowers, but many are planktonic suspension-feeders and a few are terrestrial in moist habitats. One species (Sheina orri) is known to be parasitic on fish gills.
(Brusca and Brusca 2003)
Some ostracod species are sexual (producing young from fertilized eggs) and others are asexual (with their offspring developing from unfertlized eggs). Eggs typically are deposited and develop outside the body, but some (especially marine) species have "brood pouches" in which the eggs develop and from which newly hatched nauplii are released. Delorme (2001) provides a detailed account of ostracod biology, with a focus on the freshwater fauna of North America.