Ostracod

Ostracod swimming motions (in real time)

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. Most species have completely or partly lost their trunk segmentation, and there are no boundaries between the thorax and abdomen, and it has therefore been impossible to tell if the first pair of limbs after the maxillae belongs to the head or the thorax. With a few exception, like platycopids which have an 11-segmented trunk, the abdomen in ostracods has no visible segments.[17]

The head is the largest part of the body, and bears four pairs of appendages. Two pairs of well-developed antennae are used to swim through the water. In addition, there are a pair of mandibles and a pair of maxillae. The thorax has three primary pairs of appendages. The first of these has different functions in different groups. It can be used for feeding (Cypridoidea) or for walking (Cytheroidea), and in some species it has evolved into a male clasping organ. The second pair is mainly used for locomotion, and the third is used for walking or cleaning, but can also be reduced or absent. Both the second and third pair are absent in suborder Cladocopina.[18] In the Myodocopina the third pair is a multisegmented cleaning organ that resembles a worm. Their external genitals seem to originate from the fusion of three to five appendages. The two "rami", or projections, from the tip of the tail point downward and slightly forward from the rear of the shell.[19][20][21]^: 40

All ostracods have a pair of "ventilatory appendages" that is beating rhythmically, which creates a water current between the body and the inner surface of the carapace. Podocopa, the largest subclass, have no gills, heart or circulatory system, so the gas exchange take place all over the surface. The other subclass of ostracods, the Myodocopa, do have a heart, and the family Cylindroleberididae also have 6-8 lamellar gills. Certain other larger members of Myodocopa, even if they don't have gills, have a circulatory system where hemolymph sinuses absorbs oxygen through special areas on the inner wall of the carapace.[22][23] Nitrogenous waste is excreted through glands on the maxillae, antennae, or both.[19]

The primary sense of ostracods is likely touch, as they have several sensitive hairs on their bodies and appendages. Compound eyes are only found in Myodocopina within the Myodocopa.[24] The order Halocyprida in the same subclass is eyeless.[25] Podocopid ostracods have just a naupliar eye consisting of two lateral ocelli and a single ventral ocellus, but the ventral one is absent in some species.[19][26][27] Platycopida was assumed to be completely eyeless, but two species, Keijcyoidea infralittoralis and Cytherella sordida, have been found to both possess a nauplius eye too.[28]

Male ostracods have two penises, corresponding to two genital openings (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 to join the males. Some species are partially or wholly parthenogenetic.[19] Superfamily Darwinuloidea was assumed to have reproduced asexually for the last 200 million years, but rare males have since been discovered in one of the species.[47]

In the subclass Myodocopa, all members of the order Myodocopida have brood care, releasing their offspring as first instars, allowing a pelagic lifestyle. In the order Halocyprida the eggs are released directly into the sea, except for a single genus with brood care. In the subclass Podocopa, brood care is only found in Darwinulocopina and some Cytherocopina in the order Podocopida. In the remaining Podocopa it is common to glue the eggs to a firm surface, like vegetation or the substratum. These eggs are often resting eggs, and remain dormant during desiccation and extreme temperatures, only hatching when exposed to more favorable conditions.[48][49] Species adapted to vernal pools can reach sexual maturity in just 30 days after hatching.[50] There is no larval stage or metamorphosis (direct development). Instead they hatch from the egg as juveniles with the bivalved carapace and at least three functional limbs. As the juvenile grows through a series of molts they acquire more limbs and develop further the already existing ones.[51] They reach sexual maturity in the final instar and then never molts again. The number of instars they go through before adulthood varies. In Podocopa it is eight or nine (but family Entocytheridae and suborder Bairdiocopina has only seven),[52] the Halocyprida goes through six or seven, and Myodocopida only four to six. They are able to produce several offspring many times as adults (iteroparity).[53][54][55]

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 Cytherocopina in the cuspidariid clams in detecting ostracods with cilia protruding from inhalant structures, thence drawing the ostracod prey in by a violent suction action.[56] Predation from higher animals also occurs; for example, amphibians such as the rough-skinned newt prey upon certain ostracods.[57]

Some ostracods, such as Vargula hilgendorfii, have a light organ in which they produce luminescent chemicals.[58] These ostracods are called "blue sand" or "blue tears" and glow blue in the dark. Their bioluminescent properties made them valuable to the Japanese during World War II, when the Japanese army collected large amounts from the ocean to use as a convenient light for reading maps and other papers at night. The light from these ostracods, called umihotaru in Japanese, was sufficient to read by but not bright enough to give away troops' position to enemies.[59] Bioluminescence has evolved twice in ostracods; once in Cypridinidae, and once in Halocyprididae.[60] In bioluminescent Halocyprididae a green light is produced within carapace glands, and in Cypridinidae a blue light is produced and extruded from the upper lip.[61][62] Most species use the light as predation defense, but the male of at least 75 known species of the Cypridinidae, restricted to the Caribbean, use pulses of light to attract females. This bioluminiscent courtship display has only evolved once in ostracods, in a cypridinid group named Luxorina that originated at least 151 million years ago.[63][64] Ostracods with bioluminescent courtship show higher rates of speciation than those who simply use light as protection against predators.[65]