Back to Home Page

The most distinctive feature of an ostracod is the calcareous bivalved carapace which can totally envelop the body and limbs, but from which various appendages are protruded for locomotion, feeding and reproduction. The word ostracod (some prefer to spell it: 'ostracode') is derived from the Greek word ostrakon ('a shell'); this shell or carapace has numerous morphological characters which allow taxonomic and phylogenetic studies to be made on living and fossil specimens.

Species with strongly calcified carapaces are readily fossilized and ostracods are abundantly represented in sediments from the Ordovician period (approximately 500 million years ago) onwards. The chitinous appendages are rarely fossilized, but phosphatized specimens have occasionally been found which are preserved in their entirety. Such examples from Palaeozoic, Mesozoic and Cenozoic rocks provide valuable insights into the detailed morphology of early ostracod groups. Fossil ostracods are regarded by palaeontologists as important stratigraphical and environmental indices, with important applications in oil and gas exploration; they also make excellent subjects for studies of evolution.

Back to top of page

Back to Home Page

Back to top of page

Back to Home Page

Back to Home Page

Ostracod carapaces may be spheroidal (mainly pelagic forms), elongate, laterally or vertically compressed, or possess lateral wing-like protuberances (alae), or have posterior extensions (caudal processes). Valves are sometimes crossed by depressions (sulci). The external surface of the valves may be smooth and shiny, or may have a hairy appearance due to an abundance of sensilla; many, particularly among the marine cytheraceans, are ornamented, however, and the pattern and development of the surface ornament is of great taxonomic value. The ornament may consist of pits, ranging from tiny foveolae, through puncta, to relatively large fossae. Walls separating fossae are termed muri, while long, narrow walls are carinae. Valves may be reticulate, tuberculate, carinate, spinose, or bear a combination of any or all of these ornaments.
Outer margins may be smooth or denticulate. Some forms bear on each valve an anterior or anteroventral incisure and accompanying rostrum. Ornament may vary in its degree of development within a single species, but its pattern is usually constant enough to be used as a specific characteristic. Some species have phenotypic tubercles which are developed only under certain ecological conditions. The ornament of juveniles is usually similar to, but more subdued than that of their adults; in a few species, however, marked changes in ornament and shape occur in the final moult. Ostracods often exhibit sexual dimorphism of the carapace, usually expressed in differing shape and size.

Back to top of page

Back to Home Page

The first pair of appendages, the antennulae (first antennae of some authors), are in all ostracods uniramous, consisting of between five and eight podomeres, the exopodite being absent or reduced to a single inconspicuous seta. In swimming species the antennulae bear long, thin, flexible setae with numerous setules (natatory setae), while those of crawling, climbing or burrowing species have shorter, claw-like (chelate) setae. In Myodocopida the setae are often dimorphic and especially adapted as sensory devices in the male.

The second pair of appendages, the antennae (second antennae of some authors), are primarily adapted for locomotion. The three Orders of living ostracods, Myodocopida, Platycopida and Podocopida, are distinguished on the structure of these appendages. In all groups they are biramous, but differ conspicuously in the development of the exopodite and endopodite. The protopodite usually consists of a single podomere, but in the Platycopida there are two. In the Myodocopida the exopodite is well-developed and bears long natatory setae; the endopodite may also bear natatory setae (suborder Cladocopina) or is reduced and in some species developed as a clasping organ in the male (Myodocopina and Halocypridina). Conversely, in the Podocopida it is the endopodite which is well developed, consisting of three or four articulated podomeres bearing a variety of setae and with terminal claws (chelate setae). The exopodite is reduced and in the Cytheracea it usually forms a long, hollow process (spinneret seta) through which runs the duct of a large gland situated in the forehead (the spinneret gland). This secretes a fine thread which supposedly facilitates movement over smooth surfaces; some species are known to use this thread for the construction of a coccoon in which ecdysis takes place. In the Platycopida the exopodite and endopodite are nearly equally developed, much flattened, and equipped with stout chelate setae.

The mandibles are the third pair of appendages and in most ostracods each has a strong, heavily selerotized coxa, provided ventrally with a number of teeth. These teeth meet centrally in the ventral side of the body in the atrium, while the dorsal end of the coxa bears on the fulcral point on the inside of the valve. The basis and three endopodite podomeres form a tubular palp which is furnished with a variety of setae for feeding, crawling or digging. The basis also bears an exopodite which is rudimentary and often bears a group of feathery setae forming a small branchial plate. The number of exopodite setae is useful in classification. In the Halocypridinae the basis may also be modified to bear teeth while in the Platycopida and some primitive Podocopida (Darwinuloidea) it bears a comb-like row of setae. Among the podocopids, some Paradoxostomatidae are exceptional in that the protopodite is styliform, forming a long, thin tube like a hypodermic needle, through which plant or animal fluids may be sucked.

The fourth pair of appendages, the maxillulae (maxillae, first maxillae of some authors), lie immediately behind the mandibles and have two functions, feeding and respiration. They have developed in many different ways within the Ostracoda and are of systematic importance; a generalized description of their structure is, however, difficult. In the Podocopida the protopodite bears up to three endites (sometimes referred to as 'masticatory processes'), all of which terminate in several short setae . There is also an extremely well developed epipodite branchial plate with radiating feathery setae posteriorly. The endopodite usually forms a strong palp lying parallel to the endites. The endites and the palp assist the mandibles in feeding and removing waste particles from the mouth region. The branchial plate beats continuously, in most cases, circulating water within the body cavity and presumably assisting with respiration. In the Myodocopida the branchial plate is not developed; the exopodite may be reduced or absent, but in the Cladocopina the exopodite and exopodite are of similar proportions. In the Platycopida the first endopodite podomere bears, in addition to a branchial plate, numerous comb-like setae which are used for filter-feeding.

The fifth limbs (maxillae, second maxillae, maxillipeds, first thoracic legs of various authors) may function either as accessory feeding structures (maxillipeds) or as walking legs, in which case they may be sexually dimorphic and even asymmetric in the male. Perhaps the simplest arrangement is seen in the Cytheroidea, where the fifth limb is a walking leg with a single protopodite podomere and up to four exopodite podomeres. The endopodite is entirely absent or may be represented by a few setae. In the Bairdioidea, the structure is essentially the same, but in addition the protopodite bears a large epipodite branchial plate. The structure of this appendage in the Cypridoidea, however, is quite different; the endopodite loses its segmentation during ontogeny and in adults forms a setiferous process, while the protopodite often bears a small epipodite branchial plate. The endopodite is a palp which in the males of some species is further modified into a clasping organ. An epipodite branchial plate is also present in the Darwinuloidea. In the Myodocopida and Platycopida the epipodite branchial plate is well developed; in the former group the basis may have endites armed with chelate setae, and the exopodite may be leglike or reduced, while in the latter group the exopodite is dimorphic, developed into a strong clasping organ in the males of some species but often absent in females, and the endopodite is reduced to a small lobe.

The sixth limbs (first or second thoracic legs of various authors) may serve as ancillary respiratory organs or for locomotion. In the majority of Podocopida these are simple, uniramous legs bearing only a few marginal setae and terminating in a long, chelate seta. In some groups they may be sexually dimorphic and asymmetric in males. In the Platycopida the sixth limb is rudimentary and lamelliform with a clasping organ in males, but reduced in females so that only a small branchial plate remains. In the Halocypridina these appendages are similar to the fifth limbs, whilst the Myodocopina have a lamelliform setiferous appendage comprising protopodite and exopodite podomeres and a small epipodite; the endopodite is lacking. The sixth limb is completely absent in the Cladocopina.

The seventh limbs (second or third thoracic legs of various authors) are found only in the Myodocopida and Podocopida. In Cytheroidea it is a leg, essentially the same as the preceding one, but usually larger. In the Cypridoidea it is uniramous, consisting of a four- or five-jointed endopodite reflexed backwards and upwards and terminating in slender, chelate or pincer-like setae; it is considered to function as a cleaning organ within the carapace. A similar function may be ascribed to this appendage in the Myodocopina, in which it is vermiform and extremely flexible, terminating in a complex jaw-like structure.

The paired furcae have been considered by many authors to be homologous with the telson of other crustaceans and therefore not true appendages. The position of the furcae varies, however: in the Myodocopida they are located dorsal to the anus, while in Podocopida and Platycopida they are ventral to the anus. The furcae of the latter two groups should accordingly be regarded as uropods and are thus true appendages.

The furcae are never jointed and are fused at their base. In Myodocopida and Platycopida they are lamelliform with strong chelate setae on their posterior margins. Amongst the Podocopida, the cypridoideans  and bairdioideans usually have furcae with long rami and terminal claws or setae, although in some groups they are much reduced; in darwinuloideans they are absent. In most cytheroideans they are reduced to setiferous protuberances integrated with the copulatory appendages in males.

The brush-shaped organs are paired lobes bearing numerous fine setae, which are found in some male Podocopida , situated variously in front of or between the fifth or sixth limbs. Their function is not known but is probably sensory. They are considered to be vestiges of an additional pair of appendages.

The reproductive organs are located on the ventral side of the thorax in front of the furcae and are usually paired in both sexes. Amongst the Podocopida the gonads are usually housed entirely within the body, although in the Cypridoidea they enter the duplicature and often leave impressions on the interior of the carapace. In female podocopids the oviducts lead from the ovaries to paired uterine openings situated in the genital lobe between the seventh limbs and the furcae. In front of the uterine openings are paired vaginal openings, which in many groups are rimmed with chitinized vaginal rings; they are connected internally to seminal receptacles by a canal which in some groups (Cypridoidea, Bairdioidea) is long and coiled and often referred to as the spiral canal. An internal connection between the seminal receptacles and the uteri appears to be absent, and it is assumed that sperm pass back out of the vagina and into the uterine openings to fertilize the eggs. In male podocopids the testes vary considerably in shape. They are lobate in the Bairdioidea, but in most Cypridoidea consist of four long, coiled tubes, which like the ovaries of this group are situated partly between the valve lamellae. The testes lead, by way of the vas deferens, to the external copulatory appendages. In many Cypridoidea part of each vas deferens develops a tubular structure called Zenker's organ. It consists of a chitinous tube wreathed by rings of chitinous spines which are linked by tiny muscles, and it acts as a peristaltic pump to discharge spermatozoa during copulation. The detailed structure of this organ varies quite considerably amongst the cypridaceans and may be a useful taxonomic criterion. The external male copulatory appendages are usually relatively large and are paired in all but the Halocypridina. They are situated in front of or actually attached to the furcae. In those forms with Zenker's organs the distal tubular structures carrying the sperm are often relatively simple, whereas in those forms lacking such ejaculatory devices the copulatory appendage itself may contain complex musculature, seminal vesicles and a prominent ejaculatory duct which act together to bring about the release of sperm. Distally, the copulatory appendages are provided with variously-shaped processes or lappets.

The head region or cephalon is formed from a strong chitin framework connected by thin membranes, and comprises the forehead, upper lip and hypostome (or lower lip). The forehead is anterodorsally situated and to it are attached the antennulae and antenna, the eyes, and in myodocopids, the sensory frontal organ. The upper lip is situated below the forehead and bears no appendages; its posterior margin forms the anterior margin of the mouth which is positioned ventrally. The hypostome is positioned ventrally and its anterior end in turn forms the posterior margin of the mouth; associated with it, and situated within the oral cavity, or atrium, are a pair of rake-shaped organs, which assist the mandibles in breaking down food and passing it up towards the oesophagus. The mandibles and maxillulae are attached to chitinous support rods linked to the upper margin of the hypostome. Amongst podocopids, some Paradoxostomatidae are remarkable in that the mouth-parts have become highly specialized. The upper and lower lips have become fused to form an oral cone, below which the modified rake-shaped organs form a suctorial disc; the styliform mandibles protrude through the apex of the oral cone and are presumed to have a piercing and sucking function.

The thorax contains most of the digestive and reproductive systems and also bears several pairs of appendages. The digestive tract comprises a well differentiated atrium, oesophagus, stomach, intestine, rear gut and an anus which opens behind the furcae in Podocopida and Platycopida, in front of them in Myodocopida. In some forms there are distinct hepatopancreases or livers which lie laterally and are often situated between the valve lamellae; in cypridoideans their imprints on the inner surfaces of the valves may often be seen below those of the gonads.

The eye is situated just above the base of the antennulae near to the dorsal margin of the carapace. Frequently the eye is accommodated in the carapace by an ocular sinus in each valve, leading to a transparent eye-spot or eye-tubercle. The eye comprises three optic cups which may be more or less fused. In some ostracods (e.g. Cypridinidae) the cups form a pair of lateral eyes, whilst in the majority of podocopids there is a single (nauplius) eye. Occasionally all three cups may be separated. Many ostracods are without eyes (e.g., Platycopida and some Podocopida). The eyes are highly reflective and may be seen as bright points of light when live podocopids are examined at low magnifications.

Back to top of page

Back to Home Page

Like other crustaceans, ostracods grow by moulting (ecdysis).Moult stages, or instars, are designated A (= Adult), A-1, A-2, etc., in descending order of size. In the Podocopida there are usually eight moults between the egg and the adult; the first instar (A-8) which hatches from the egg is usually relatively short-lived, lacks thoracic appendages, has a poorly calcified shell, and is so small that it is unlikely to be seen except in laboratory cultures. A properly calcified shell appears at the A-7 stage, and each successive moult adds appendages and shell features, the larger instars showing an increasing likeness to the adult of the species; in some species a precocious sexual dimorphism can be recognized in the last one or two instars before the adult.

Back to top of page

Back to Home Page