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The crustacean class Ostracoda occurs in marine, freshwater, and wet-terrestrial habitats worldwide. Roughly 13,000 extant species in four orders have been described, but 4x species more are known only as fossils. As such ostracods are important index fossils. Of the four extant orders, only the Order Podocopida has freshwater species, around 2000 in 200 genera globally. In North America, there are 298 freshwater ostracods (57 genera, 11 families) (Martens et al., 2008).

Ostracods, also known as mussel or seed shrimp, derive their common names from their bivalved shells and small size (mm) (and the fact that the name “clam shrimp” is already taken). The two lateral valves (of calcium carbonate and chitin) are articulated by a dorsal ligament and closed by a single adductor muscle. Segmentation is not clearly evident in the body plan, and the typical crustacean appendages and tagmata are highly modified. There are only three pairs of thoracic segments, and the abdomen is reduced to a pair of caudal furcae. In general, the 1st and 2nd antennae are used for swimming, although ostracods generally remain close to the substrate. The other head appendages bear branchial plates that beat to generate a water current for gas exchange and selective filter-feeding. Ostracods lack compound eyes. Among freshwater species, asexual reproduction is common, and zygotes/eggs may serve as resting stages during harsh environmental conditions. Development is indirect, with a nauplius larval stage.

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Bibliography

• Martens, K. & S. Savatenalinton. 2011. A subjective checklist of the Recent, free-living, non-marine Ostracoda (Crustacea). Zootaxa 2855: 1-79. [Available online]
• Smith, A.J. & L.D. Delorme. 2010. Ostracoda. [in] J.H. Thorp & A.P. Covich (eds.). Ecology and Classification of North American Freshwater Invertebrates, 3rd edition: 725-771.
• Covich, A.P., J.H. Thorp & D.C. Rogers. 2010. Introduction to the Subphylum Crustacea. [in] J.H. Thorp & A.P. Covich (eds.). Ecology and Classification of North American Freshwater Invertebrates, 3rd edition: 695-723.
• Martens, K., I. Schön, C. Meisch, & D.J. Horne. 2008. Global diversity of ostracods (Ostracoda, Crustacea) in freshwater. Hydrobiologia 595: 185-193. [Available online]
• Smith, D.G. 2001. Ostracoda (seed shrimps). Pennak's Freshwater Invertebrates of the United States, 4th edition: 521-544.
• Underhill, J.C., W.D. Schmid & D.E. Gilbertson. 1982. Taxonomic Keys to the Common Animals of the North Central States, exclusive of the parasitic worms, terrestrial insects, and birds, 4th edition. 205 pp.
• Kitchell, J.A. & D.L. Clark. 1979. Distribution, ecology, and taxonomy of Recent freshwater Ostracoda of Lake Mendota, Wisconsin. Publication of the Natural History Council, University of Wisconsin-Madison 1. 24 pp. [Available online]
• Tressler, W.L. 1959. Ostracoda. [in] W.T. Edmondson (ed.). Ward & Whipple Fresh-Water Biology, 2nd edition: 657-734.
• Tressler, W.L. 1947. A check list of the known species of North American freshwater Ostracoda. American Midland Naturalist 38: 698-707. [Available online]

Web Pages

• Encyclopedia of Life
• Fossil and Living Ostracoda of the World
• Identification and Ecology of Australian Freshwater Invertebrates
• Integrated Taxonomic Information System
• Kempf Database Ostracoda
• Ostracode Diversity Project
• Tree of Life
• WikiSpecies
• Wikipedia
• World Ostracoda Database
• World Register of Marine Species