Ostracoda

(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

List of Species

No common name

Vargula hilgendorfii

order

Myodocopida

family

Cypridinidae

taxonomy

Vargula hilgendorfii (Muller, 1890), Japan.

other common names

None known.

physical characteristics

Typical myodocopid, with rounded, smooth, unpigmented, bivalved carapace marked anteriorly with a rostrum and large antennal notch. Pair of black lateral eyes are visible through the carapace. Caudal furca is extremely large and visible extending through the ventral valve opening. At 0.12 in (3 mm), is relatively large for ostracod.

distribution

Pacific coast of central Japan.

habitat

Very common and easy to collect on sandy bottoms in moderately shallow water.

behavior

Stays in sediment in daytime and is most active at night, either hopping across the substratum or swimming in lower part of water column. Swimming and digging in the sediment are accomplished by synchronous strokes of the antennae. In probable escape response to predators, it used its rather large furca to push itself into sand. Escape from sand was also accomplished

by a sudden push from the caudal furca, launching it into overlying water. In general, it burrowed only a few millimeters into the sediment.

feeding ecology and diet

In laboratory, it has been observed to attack living prey such as polychaete annelids, to scavenge on fish carcasses, or to eat artificial aquarium foods. The mandibular palps are used primarily to hold the food while it uses its fourth limbs (maxillules), caudal furcae to open the integument or body wall. Endites transfer food items to opening of the esophagus.

reproductive biology

Observations are few, but it appears to spend 30–60 minutes off the bottom in a precopulatory mode with the male grasping the female. Mating occurs on the bottom with the pair lying in reverse ventral position. Male transfers spermatophore to female. Eggs are brooded under carapace of female. After hatching, there are five instars leading to the adult. Youngest juveniles were able to crawl, swim, and dig into the bottom sediment.

conservation status

Not listed by the IUCN.

significance to humans

As with most myodocopids, it is preyed upon by fish.

No common name

Heterocypris salinus

order

Podocopida

family

Cyprididae

taxonomy

Heterocypris salinus (Brady).

other common names

None known.

physical characteristics

Valves are conspicuously pigmented with vertical brown streaks on either side of the muscles' scars.

distribution

Known from Europe, the Azores, North Africa, and western Asia. (Specific distribution unknown; no map available.)

habitat

Brackish water pools along the coast as well as inland saline waters. Tolerance experiments showed that they tolerate 10 parts per thousand salinity the best, especially at lower temperatures. In the field, tolerance to wide ranges of salinity have been seen.

behavior

Active only during the day. At low light levels, they creep towards deeper parts of the pools in which they are found. They walk about the pool constantly with no resting period observed during daylight hours.

feeding ecology and diet

Feeds exclusively on small green alga cells, especially desmids and Chlorella-like algae.

reproductive biology

Thirty to 40 red to orange oval eggs are generally attached to the substratum or to thalli of green algae. Three generations are produced during the summer in the Swedish Baltic, with the third generation being an over-wintering generation. The eggs of the third generation are always laid in the bottom sediments so as to be protected from the winter ice.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Books

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.

Periodicals

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.

Les Watling, PhD