Ostracoda (Mussel Shrimps)

(invertebrate zoology) A subclass of the class Crustacea containing small, bivalved aquatic forms; the body is unsegmented and there is no true abdominal region.

(Mussel shrimps)

Phylum: Arthropoda

Subphylum: Crustacea

Class: Maxillopoda

Sublass: Ostracoda

Number of families: 46

Thumbnail description
Generally small crustaceans with reduced body entirely enclosed within an often-calcified bivalved carapace

Evolution and systematics

Ostracods have a long fossil history, being known from the Cambrian, and have undergone extensive radiations, especially since the early Mesozoic. Including fossil forms, the subclass Ostracoda can be divided into six orders, of which three, the Palaeocopida, Myodocopida, and Podocopida, contain recent species. The Palaeocopida contains one modern family, the Punciidae, known from the seas around New Zealand. Myodocopids are predominantly swimmers; one suborder lives in the benthic boundary layer just above the seabed, while the other suborder has radiated into the pelagic zone of the sea. Podocopids are generally smaller than other ostracods and, for the most part, live as epibenthos. Within the Podocopida, there is a tendency for reduction of appendage segments or rami, and from turgor appendages to ones with more exoskeletal integrity and strength. Podocopid carapaces also tend to be more highly ornamented with ridges and spines than those seen in the other orders. The origin of the Ostracoda has long been a mystery. They are unlike all other crustacean groups in their body organization, having retained some larval features such as feeding with antennae as well as with other mouthparts. In addition, the nauplius of ostracods is unique in possessing carapace folds from the earliest stage. The paucity of appendages on the ostracod body has resulted in considerable confusion regarding the homologies of some of the post-cephalic legs. A recent study by Tsukagoshi and Parker (2000) suggests that ostracods have the 5-6-5 or 5-7-4 trunk segmentation pattern of other maxillopodans.

Physical characteristics

Ostracods are most notable in having their entire body enclosed within the carapace folds. As a result, there are many reductions in body segments and appendages. The carapace fold originates at the posterior margin of the larval head shield and extends anteriorly, laterally, and posteriorly to cover the body. Dorsally, the carapace fold is divided by a hinge and flexible cuticle. A system of adductor muscles is used to close the "valves," as the two parts of the carapace fold are called, and hydrostatic pressure applied at the top of the mandible serves to open the valves. The two valves are asymmetrical, with one valve fitting inside the other. Because the valves are often heavily calcified, ostracod valves are common features of the fossil record, have been studied extensively, and a specialized terminology has been developed to describe the fine details. Ostracods have typical crustacean head appendages: antennules, antennae, mandibles, maxillules, and maxillae. In contrast to many other crustacean groups, however, ostracods use their antennae, and sometimes their antennules, for locomotion. As a consequence, these appendages are short and robust. The antennules are uniramous and composed of five to eight segments. The antennae are biramous, with one ramus, the exopod, often reduced in the podocopids. Ostracod mandibles have a coxal gnathobase, with the remainder of the appendage developed into a palp, which is often biramous. Posterior to the mandible is the maxillule, which is quite variable in morphology. The protopod usually has a set of setose endites, and from the protopod arises a short endopod used in manipulating food particles and a dorsally directed setose vibratory plate. It is not known with certainty whether this vibratory plate is an exopod or an epipod. The trunk limbs have had various names applied to them. Posterior to the maxillule is the maxilla, but this limb has usually been called a maxilliped, or first trunk limb, in response to the degree to which the limb looks like the maxillule or the next two pairs of trunk limbs, and is used for feeding or locomotion. If used for feeding, the endopod is shortened and forwardly directed and the vibratory plate is a smaller version of that on the maxillule. If used for locomotion, the maxillae are changed considerably in shape, with the endopod directed posteriorly and the vibratory plate reduced to a simple seta. Of course there are many examples of intermediate forms for this appendage. The next two pairs of trunk limbs are the thoracopods, and they are also quite variable in design. Sometimes there is also variation between right and left limbs as well as sexual dimorphism. In some cases, these limbs are absent.

Myodocopids have transformed the second thoracic limb into a long multiarticulate vermiform appendage capable of extending into the dorsal space between the body and the hinge area of the valves, presumably as a cleaning device. In the most recently evolved podocopids, these trunk limbs look like typical crustacean walking legs and have often been termed pereopods. According to recent research, there are several limbless trunk segments following the two pairs of thoracopods, and the body typically ends in a pair of caudal rami. In most ostracods, the penes of the male are quite large relative to the remainder of the body. In higher podocopids, the penial lobes can be as much as one-third of the body length and are very complicated, highly chitinous structures.

Distribution

Ostracods are found in nearly all marine and freshwater environments, as well as some moist terrestrial habitats.

Habitat

Most ostracod species are benthic, or epibenthic, living on the substrate or on other organisms. One group, the myodocopids, has developed exclusively pelagic species, which can be found throughout the oceanic waters of the world.

Behavior

When benthic ostracods walk, they open the carapace valves, push out the walking legs and antennae, and amble across the substrate with a rocking motion. Swimming in pelagic species involves pushing the terminal section of the antennae out the antennal notches in the carapace and moving the limbs in a rowing motion. Shallow-water ostracods seem to spend most of the day in motion, probably foraging for food particles. The few observations done at night indicate that they usually do not move about at night.

Feeding ecology and diet

Ostrocods were long thought to be filter feeders, but recent evidence shows that they are quite capable of grazing on diatoms and other fractions of marine detritus. Most ostracods are apparently detritus feeders, with a few being capable of scavenging or predation.

Reproductive biology

Ostracods mate by putting the opened ventral side of the valves together, or the male may mount the female dorsally from behind and insert the large penes into the open valves of the female. Eggs may be laid freely in the environment or, as in some podocopids, are incubated inside the valves, usually above the abdomen. The first larval stage that hatches is a nauplius, with the carapace fold covering the body and appendages. At this stage, the animal can walk or swim by means of the antennae and mandibles. At each successive molt, new legs or leg primordia are usually added. Adult stages are reached in 5–8 molts, depending on order and family. Lifespan is usually a year or less, but little is known about this for deep-sea-dwelling species.

Conservation status

Most ostracod species occur in sufficient numbers and are not threatened, and none are listed by the IUCN. A few, however, such as the entocytherids, are symbionts of freshwater crayfish and isopods, and so may become rare as their hosts disappear.

Significance to humans

The fossilized valves of ostracods have long been used by paleontologists as indicators of past habitat and climate conditions. Because no species of ostracod extends over a long geological time period, they are useful indicators of ages of deposits.

Species accounts

Resources

Benson, R. H., et al. Treatise on Invertebrate Paleontology, Part Q, Arthropoda 3. Lawrence, KS: Geological Society of America and University of Kansas Press, 1961.

Hartmann, G., and M.-C. Guillaume. 1996. Classe des Ostracodes. Traité de Zoologie, VII. Crustaces, Fasc. 2. Paris: Masson et Cie, 1996.

Ganning, B. "On the Ecology of Heterocypris salinus, H. incongruens and Cypridopsis aculeate (Crustacea: Ostracoda) from Baltic Brackish-water Rockpools." Marine Biology 8 (1971): 271–279.

Vannier, J., and K. Abe. "Functional Morphology and Behavior of Vargula hilgendorfii (Ostracoda: Myodocopida) from Japan, and Discussion of Its Crustacean Ectoparasites: Preliminary Results from Video Recordings." Journal of Crustacean Biology 13 (1993): 51–76.

Vannier, J., K. Abe, and K. Ikuta. "Feeding in Myodocopid Ostracods: Functional Morphology and Laboratory Observations from Videos." Marine Biology 132 (1998): 391–408.

[Article by: Les Watling, PhD]

A major taxon of the Crustacea containing small bivalved animals 0.004–1.4 in. (0.1–33 mm) long, with most between 0.04 and 0.08 in. (1 and 2 mm). They inhabit aquatic environments in nearly all parts of the world. Semiterrestrial species have been described from moss and leaf-litter habitats in Africa, Madagascar, Australia, and New Zealand, and from vegetable debris of marine origin in the Kuril Archipelago. Of the more than 2000 species extant, none is truly parasitic and most are free-living. However, a few fresh-water and marine forms live commensally on other animals. Most ostracodes are scavengers, some are herbivorous, and a few are predacious carnivores. Exceptional biological features are known; there are myodocopine ostracodes that produce bioluminescence, and some species of podocopines form a secretion from spinning glands to enable them to climb polished surfaces.

Researchers disagree on the hierarchical classification of the Ostracoda. It is recognized as a distinct class by some and a subclass within the Maxillopoda by others. Consequently, the subdivisions within the Ostracoda may be ranked as subclasses, superorders, or orders. Six major subdivisions are currently recognized: Bradoriida, Phosphatocopida, Leperditicopida, Paleocopa, Myodocopa, and Podocopa. The first four taxa are extinct. The Myodocopa are further subdivided into the Myodocopida and Halocyprida, and the Podocopa into Platycopida and Podocopida. All fresh-water Ostracoda belong to the Podocopida. See also Maxillopoda; Podocopa.

Knowledge of the morphology of the Ostracoda is based primarily on extensive study of many species. The two valves, sufficient to enclose the rest of the animal, are joined dorsally along a hinge, that may vary from a simple juncture to a complex series of teeth and sockets. From the dorsal part of the carapace, the elongate body is suspended as a pliable sac, with lateral flaps of hypodermis extending between the lamellae of the valves. It is unsegmented, but reinforced by chitinous processes for rigidity in the vicinity of the appendages. There is no true abdominal region.

Ostracodes of the Podocopida and Myodocopida possess seven pairs of segmented appendages: antennules, antennae, mandibles, maxillules, and three pairs of thoracic legs. The body terminates posteroventrally in a pair of furcae or caudal processes. The Platycopida have two pairs of thoracic legs, and the Halocyprida frequently only one pair. Appendages have muscles within them and are connected by musculature to the valves and to a centrally located chitinous structure, the endoskeleton. They are usually specialized for particular functions, such as swimming, walking, food gathering, mastication, and cleaning the interior of the carapace.

Ostracodes have separate sexes, although many species are parthenogenetic and lack males. Most ostracodes lay their eggs on the substrate or on vegetation, but some transfer them to the posterior space within the carapace, where they hatch and the young brood is retained for a time.