The Ostracoda is a sub-class of relatively small crustaceans which occur in most aquatic habitats. They range in size from 0.3mm long in some interstitial forms to the large oceanic myodocopids of the genus Gigantocypris which attain lengths of up to 30mm. The oceanic planktonic ostracods all belong to the order Myodocopida.
Ostracods around Great Britain
The majority in the oceanic waters around Britain belong to the family Halocyprididae. The family Cypridinidae is also represented by the genera Gigantocypris and Macrocypridina. In continental shelf seas, particularly inshore in shallow water, other myodocopid genera will be encountered in the plankton, particularly at night when they migrate up off the bottom into the water column. These species are poorly known in British waters. The best source of information on them is Kornicker, 1987.
The oceans cover 71% of the Earth's surface to an average depth of 3800m. Shelf seas around the continental margins, whose depths are limited to 200m or less, contribute only 5% to this total. In the North Atlantic, planktonic ostracods are rare in these inshore waters, but they occur almost everywhere in the deeper regions. At our temperate latitudes, they are often missing from the surface 100m, but otherwise they are usually the second-most abundant group of macroplanktonic animals, second only to the copepods. Thus some of the dominant species of planktonic ostracods must be amongst the most abundant invertebrates in the world and yet they remain almost totally unknown, even amongst oceanic biologists.
Fossils of Planktonic Ostracods
Planktonic ostracods, unlike their benthic counterparts, have little if any calcification of the carapace, so they are very poorly represented in the fossil record. Even so, fossils of planktonic ostracods have been found in rocks of the Cambrian Era, so they have been around in the World's oceans for as long as the benthic species.