Temporal range: Cambrian–Recent
|Subclasses and Orders|
Ostracods are a class of the Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 70,000 species (only 13,000 of which are extant) have been identified, grouped into several orders. They are small crustaceans, typically around 1 mm (0.039 in) in size, but varying from 0.2 to 30 mm (0.0079 to 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. Ostracods are grouped together based on gross morphology, but the group may not be monophyletic; their molecular phylogeny remains ambiguous.
Ecologically, marine ostracods can be part of the zooplankton or (most commonly) are part of the benthos, living on or inside the upper layer of the sea floor. Many ostracods, especially the Podocopida, are also found in fresh water, and 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.
As of 2008, around 2000 species and 200 genera of nonmarine ostracods are found. However, a large portion of diversity is still undescribed, indicated by undocumented diversity hotspots of temporary habitats in Africa and Australia. Of the known specific and generic diversity of nonmarine ostracods, half (1000 species, 100 genera) belongs to one family (of 13 families), Cyprididae. Many Cyprididae occur in temporary water bodies and have drought-resistant eggs, mixed/parthenogenetic reproduction, and the ability to swim. These biological attributes preadapt them to form successful radiations in these habitats.
Ostracods are "by far the most common arthropods in the fossil record" with fossils being found from the Cambrian to the present. An outline microfaunal zonal scheme based on both Foraminifera and Ostracoda was compiled by M. B. Hart. Freshwater ostracods have even been found in Baltic amber of Eocene age, having presumably been washed onto trees during floods.
Ostracods have been particularly useful for the biozonation of marine strata on a local or regional scale, and they are invaluable indicators of paleoenvironments because of their widespread occurrence, small size, easily preservable, generally moulted, calcified bivalve carapaces; the valves are a commonly found microfossil.
The body of an ostracod is encased by two valves, superficially resembling the shell of a clam. A distinction is made between the valve (hard parts) and the body with its appendages (soft parts).
Soft parts 
The body consists of a head and thorax, separated by a slight constriction. Unlike many other crustaceans, the body is not clearly divided into segments. The abdomen is regressed or absent, whereas the adult gonads are relatively large.
The head is the largest part of the body, and bears most of the appendages. Two pairs of well-developed antennae are used to swim through the water. In addition, there is a pair of mandibles and two pairs of maxillae. The thorax typically has two pairs of appendages, but these are reduced to a single pair, or entirely absent, in many species. The two "rami", or projections, from the tip of the tail, point downwards and slightly forward from the rear of the shell.
Ostracods typically have no gills, instead taking in oxygen through branchial plates on the body surface. Most ostracods have no heart or circulatory system, and blood simply circulates between the valves of the shell. Nitrogenous waste is excreted through glands on the maxillae, antennae, or both.
The primary sense of ostracods is likely touch, as they have several sensitive hairs on their bodies and appendages. However, they do possess a single naupliar eye, and, in some cases, a pair of compound eyes, as well.
Palaeoclimatic reconstruction 
Male ostracods have two penes, corresponding to two genital openings, or gonopores on the female. The individual sperm are often large, and are coiled up within the testis prior to mating; in some cases, the uncoiled sperm can be up to six times the length of the male ostracod itself. Mating typically occurs during swarming, with large numbers of females swimming the join the males. Some species are partially or wholly parthenogenetic.
In most ostracods, eggs are either laid directly into the water as plankton, or are attached to vegetation or the substratum. However, in some species, the eggs are brooded inside the shell, giving them a greater degree of protection. The eggs hatch into nauplius larvae, which already have a hard shell.
A variety of fauna prey upon ostracods in both aquatic and terrestrial environments. An example of predation in the marine environment is the action of certain Cuspidariidae in detecting ostracods with cilia protruding from inhalant structures, thence drawing the ostracod prey in by a violent suction action. Predation from higher animals also occurs; for example, amphibians such as the Rough-skinned Newt prey upon certain ostracods.
Some ostracods have a light organ in which they produce luminescent chemicals. Most use the light as predation defense, while some use the light for mating (only in the Caribbean). These ostracods are called "blue sand" or "blue tears" and glow blue in the dark at night.
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|Wikispecies has information related to: Ostracoda|
- Ostracoda fact sheet, Guide to the Marine Zooplankton of South-eastern Australia]
- Key to the two subclasses
- International Research Group on Ostracoda
- Ostracoda Fact Sheet
- Huge sperm of ancient crustaceans