The body of an ostracod is enclosed between two valves and consists of two main parts, the head (or cephalon) and the thorax, separated by a slight constriction. The abdomen is absent or fused with the thorax. Up to 8 pairs of appendages mark the existence of ancestral segmentation of the head and thorax. The body is entirely enveloped in a cuticle, secreted by the epidermis.
The two valves, or carapace, develops from two lateral folds in the epidermis, each with an outer lamella and an inner lamella. The space between these lamellae is a continuation of the body cavity and may house certain reproductive and digestive organs (plaat pag 1C-cross section). The epidermis of the outer lamella and of the peripheral part of the inner lamella lays down calcareous material to form hard lateral valves. Thus two valves hinged dorsally form a carapace. In some groups no special hinge structure is present and the dorsal margins of the valves are connected by a ligament, while in others there may be a complex structure of intermeshing teeth and sockets.
Between the valves, appendages are protruded for feeding, locomotion and reproduction, which can be completely withdrawn. The valves can be closed by means of adductor muscles attached to the inside of each valve. Muscles from appendages are also attached to the inner surface of the valves, mostly dorsally, or to a framework of chitinous rods within the body. A fulcral point or notch, on which the mandible pivots, is often associated with the central muscle scar field.
The peripheral, calcified part of the inner lamella may be wholly or partially fused with the outer lamella, forming the marginal zone. When only partly fused, a vestibulum is formed between the outer lamella and the free part of the inner lamella. The edge of the valve is termed the outer margin, and the inner edge of the calcified inner lamella is the inner margin; the inner edge of the fused zone is called the line of concrescence. The marginal zone is crossed by marginal (or radial) pore canals through which pass sensilla. The calcified inner lamella may bear a number of ridge-like structures; a prominent ridge is called the selvage, less prominent ones are called lists (plaat 9/4-Internal view of valve).
Normal pore canals carry sensilla through the outer lamella, opening on its outer surface to form external pores. The number and type varies, the most usual being simple pores and sieve pores. A third type of pore, which does not bear sensilla but is associated with the process of moulting is called the exocrine pore. Normal pores may be flush with the valve surface, raised on prominent pore conuli, or recessed as celate or funnel pores.
The carapaces may be spheroidal (mainly in 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 are ornamented 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. 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.
Colour is often a characteristic and sometimes a diagnostic feature of living ostracods. The colour is usually provided by pigmented epidermis lining the calcified outer lamella; this may be evenly distributed or forms distinct patches.
A number of genera are characterised by valves with conspicuous opaque patches when seen in transmitted light. These patches are useful diagnostic criteria as they maintain constant shapes and positions within a species.
In the family Xestoleberidae a crescent-shaped marking, the Xestoleberis -spot, is found behind the eye in both valves. This spot appears to be formed by the modification of the inner surface of the calcified outer lamella and it occupies about half the total thickness of the valve. There is no external surface expression, but it appears as a raised scar internally. Its function is unknown.
The body and appendages of an ostracod may be readily seen by removing one of the valves. Body segmentation is hardly discernable, although its ancestral existence is marked by paired appendages; recognition of homologues of ostracod body segments with those of other crustaceans is therefore extremely difficult, and the names of appendages vary in literature. For the comparisons of the appendages of the major groups of ostracods found in European waters, the following appendage terminology is used (from front to back): antennula, antenna, mandible, maxillula, fifth limb, sixth limb, seventh limb, furca (Figs 8, 10, 11). For the cypridacean and cytheracean families, genera and species the fifth, sixth and seventh limbs are referred to as the first, second and third legs.
Additionally, male ostracods possess a pair of copulatory appendages situated between the seventh limbs and the furcae, and (in cytheraceans at least) a brush-shaped organ, situated between or in front of the fifth pair of limbs, which is regarded as the vestige of another pair of limbs.
The appendages vary considerably in shape and function and are adapted to different habitats and modes of life. Ostracod appendages are jointed and composed of a number of articulated segments called podomeres. Typically, a crustacean appendage has a basal protopodite, comprising a coxa and a basis; the basis gives rise to two rami, an inner endopodite and an outer exopodite, both of which consist of a number of podomeres. Inner branches of the protopodite podomeres are called endites, whilst outer lobes are referred to as epipodites (Fig. 9). However, in many ostracod appendages it is difficult to recognise the biramous form of the ancestral crustacean limb. Fusing of adjacent podomeres and extreme reduction of one or other ramus is common. Many of the podomeres bear setae of varying size and shape which may themselves bear secondary excrescences called setules, imparting a feathered appearance to individual setae. The numbers and relative proportions of the podomeres and setae are often extremely useful in classification.
The first pair of appendages, the antennulae (antennae 1), are in all ostracods uniramous, consisting of between 5-8 podomeres, the exopodite being absent or reduced to a single inconspicuous seta (Figs 10). In swimming species the antennulae bear long, thin, flexible setae with numerous setules, while those of crawling, climbing or burrowing species have shorter, claw-like (chelate) setae.
The second pair of appendages, the antennae (antennae 2) are primarily adapted for locomotion (Figs 10). The three orders of living ostracods 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 mandibles are the third pair of appendages and in most ostracods each has a strong, heavily sclerotized coxa, provided ventrally with a number of teeth (Figs 10, IIb). 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 fourth pair of appendages, the maxillulae (maxillae 1) lies immediately behind the mandibles and has two functions, feeding and respiration. They have developed in many different ways and a generalised description of their structure is difficult. In the Podocopida the protopodite bears up to three endites, all of which terminate in several short setae (Fig. 10). There is also an extremely well developed epipodite branchial plate with radiating feathery setae posteriorly; in some groups a number 'reflexed' setae point forwards. 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, 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 pair of appendages (maxillae, maxillae 2, maxillipeds or the first thoracic legs) 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. In the simplest arrangement 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 addition the protopodite may bear a large epipodite branchial plate. The structure of this appendage in the cypridaceans, 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.
The sixth limbs (first or second thoracic legs, depending on the status of the fifth pair of appendages) may serve as ancillary respiratory organs or for locomotion. In the some taxa, these are simple, uniramous legs bearing only a few marginal setae and terminating in a long, chelate seta. In other groups, they may be sexually dimorphic and asymmetric in males. The sixth limb may also be rudimentary and lamelliform with a clasping organ in males, but reduced in females so that only a small branchial plate remains. These appendages may be similar to the fifth limbs, but in some groups, they are completely absent.
The seventh limbs (second or third thoracic legs) are well developed in some groups. In others, the appendage is reduced to a small process bearing one or two setae, or is absent.
The paired furcae have been considered to be homologous with the telson of other crustaceans and are therefore not true appendages. The position of the furcae varies: they can be located dorsal or ventral to the anus. The furcae are never jointed and are fused at their base. In some taxa they are lamelliform with strong chelate setae on their posterior margins. Other groups have furcae with long rami and terminal claws or setae, although in some groups they are much reduced or absent.
The brush-shaped organs are paired lobes bearing numerous fine setae, which are found in some male, 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.
Source: Athersuch et al., 1989.