MUSSEL SHRIMP

also called seed shrimp, or ostracod (subclass Ostracoda) any of a widely distributed group of crustaceans resembling mussels. Mussel shrimp differ from most crustaceans in having a very short trunk that has lost its external segmentation, or divisions. Nearly all of the body is encased in a hard covering, or carapace. The 2,000 living species include marine, freshwater, and terrestrial forms. Some 10,000 extinct species are known, fossil ostracods being known from Late Cambrian Period (about 523 to 505 million years ago) to recent times. The several extinct species are of particular stratigraphic significance in nonmarine marls, limestones, and shales, often serving as index fossils owing to their abundance, widespread geographic occurrence, and limited vertical range. Certain genera are commonly used as guides in subsurface petroleum exploration, in a manner analogous to that of foraminifera in marine strata. Mussel shrimp are about 1 mm (about 0.04 inch) long. The largest is Gigantocypris agassizi, a Pacific species, which grows to 23 mm. Most mussel shrimp live on or about the sea bottom. Some feed on microorganisms and organic debris; others are predators on small invertebrates; and a few are parasitic. Most prefer shallow water, but some have been collected at depths of 2,000 m (about 6,600 feet).