Institute of Oceanology, Polish Academy of Sciences, Sopot
What is an ostracod?
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Last updated
20 May 2020
Southampton Oceanography Centre
What is a halocyprid ostracod?
The full taxonomic hierarchy of the planktonic species Conchoecia magna is
as follows: -
Kingdom Animalia
Phylum Arthropoda
Subphylum Crustacea Brünnich, 1772
Class Ostracoda Latreille, 1802
Subclass Myodocopa Sars, 1866
Order Halocyprida Dana, 1853
Suborder Halocypridina Dana, 1853
Superfamily Halocyproidea Dana, 1853
Family Halocyprididae Dana, 1853
Subfamily Conchoecinae Müller, 1912
Genus Conchoecia Dana, 1849
Species Conchoecia magna Claus, 1874
All the planktonic ostracods belong to the subclass Myodocopa, and most belong to the order Halocyprida, which lack obvious eyes. A few belong to the suborder Myodocopina Sars, 1866, which either have large lateral compound eyes (Macrocypridina), or large central naupliar eyes with parabolic reflectors (Gigantocypris). All the ostracods have the body totally enclosed within a carapace (or shell) and the adults typically have seven pairs of limbs. The limbs are only fully developed in adults; the earlier juvenile instars have either fewer or only partially developed limbs. The following description is for the halocyprid species; the modocopids have distinctive characteristics.
Carapace. <click for diagram> consists of two valves that are hinged along the dorsal line. Their shapes range between being quite globular, to being cylindrical or laterally compressed. The carapace shapes and dimensions show marked sexual dimorphism in adults, and there are allometric differences between the larval instars. The surface of the carapace can be smooth or incised with longitudinal or concentric striae or covered with elaborate sculpturing of lines and rectangles and even spines. Anteriorly each valve is developed into a rostrum that overlies and protects the first antennae and frontal organ. The rostrum is produced into a long process in some species (e.g. Conchoecilla daphnoides), but reduced almost completely in others (e.g. Halocypris spp). Below the rostrum is the incisure through which the exopodites of the second antennae project laterally during swimming to provide the propulsive force. The ventral margin of the carapace curves posteriorly and form either a rounded or abruptly angled corner with the posterior margin. In some species there are tubercles at the posterior ventral corner (genus Conchoecissa). Likewise the posterior margin may be more or less curved and can be armed with spines. It extends up to the posterior dorsal corner, which may be rounded or developed into spines on one of both valves of the carapace. The posterior half of the dorsal margin is more or less straight, but the anterior part is often arched forming the shoulder vaults that cover the large muscular protopodite of the second antenna. In a few species the shoulder vaults are sharp edged (B. antipoda and P. spinifera) or even winged (Alacia alata). There is a large gland on each carapace valve, which are of taxonomic importance. In the subfamilies Archiconchoecinae and Halocypridinae these large glands are placed symmetrically on the posterior margin of the carapace. In the Conchoecinae, they are asymmetrical. On the right valve the usual position for the gland to open is at, or close to the posterior ventral 'corner'. In Conchoecilla it is placed just below the rostral incisure, and in Metaconchoecia species just ventral to the posterior dorsal corner. On the left valve the usual position for the gland is on the dorsal margin close to the posterior dorsal corner, but is often displaced anteriorly, notably in Metaconchoecia species in which it opens close to or even on the rostrum. In males, there a corresponding asymmetry in some of the characteristics of the endopodite of the second antenna and the intromittent organ is a single appendage on the right side of the caudal furca. Note that in the myodocopids the intromittent organ is a paired appendage and there are no comparable asymmetries.
In males there is usually a group of glands on each valve that open just below the posterior dorsal corner, where the ends of the long terminal setae of the sixth pair of limbs project. There are numerous smaller glands arranged along the ventral margins of the valves that secrete mucus. Glands are often present on the lower edge of the incisure and also where the posterior margin is arched into an exhalent siphon. These glands release bioluminescence into the respiratory flow of water. This flow, driven by the beating of the epipodial appendages of the fifth and sixth limbs, enters the carapace below the rostrum, and is discharged through the exhalent siphon on the posterior margin. Species in which the rostrum is elongated into spines (e.g. Conchoecilla daphnoides) and/or there are large spines or tubercles at the posterior or ventral dorsal corners (e.g. Conchoecissa symmetrica) usually contain additional glands at the tips which generate bioluminescence, which is retained within the glands.
Limbs. <click for diagram> If the adductor muscles holding the carapace valve together are sliced through, one of the carapace valves can be folded back to reveal the arrangement of the limbs. The largest limbs are the second antennae, whose muscular protopodite occupy a third to half the carapace length and provide the motive power for swimming via the exopodite. The terminal eight expodite segments carry a series of pairs of long setae with secondary filaments that spread during the propulsive stroke of the exopodite and feather during the recovery stroke. On the inner surface of the protopodite is an endopodite. These endopodites are sexually dimorphic. In females there are two segments, and terminal segment carries five long setae. In males there is a third segment, which is developed into a hook appendage. The two longest endopodite setae are carried on the second segment, whereas the three subequal thin-walled sensory setae are attached to the basal hasp of the hook. The hook appendage on the left limb is larger and more strongly developed than on the right.
Between the basale of the second antennae are the two of first antennae with the frontal organ between them. Both first antennae and the frontal organ are sexually dimorphic. In males the stem of the frontal organ is anchored to the second segment of the antenna by a dorsal seta from the second segment that curls around it, and there is always a clear division between the stem and the terminal capitulum. In females the stem and capitulum of the frontal organ are fused in several genera and the seta on the second segment of first antenna is straight. During swimming the terminal setae of the first antennae are splayed out in front of the animal. In males the limb segments of the first antennae not fused and the longest terminal seta (the e seta) carries an armature of spines. The numbers and arrangement of the spines of this armature are important taxonomic characters. In females the limb segments of the first antennae are either fully or partially fused in many of the genera, and the four of the five terminal setae are thin-walled and sensory. In Mikroconchoecia these sensory setae are bifid.
Posterior to the second antennae and flanking the mouth are a pair of mandibles, with well-developed exopodites that are used to manipulate food items at the mouth. The basal segments are developed into an elaborate system of tooth rows, which are also useful in identification. The mandibles show no sexual dimorphism.
Next are the maxillae that show only small differences between the various species. Their exopodites manipulate food particles at the mouth.
Posterior to the maxillae are the fifth pair of limbs, which again function to manipulate food particles between the gape of the carapace valves. They show little sexual dimorphism and have not been used in identification despite their setation and the relative lengths of the terminal claws vary between species and genera.
The sixth pair of limbs is strongly sexually dimorphic. In males the limb is more strongly developed and muscular. Its three terminal setae are long and filamentous and usually carry long secondary hairs. It has been suggested that the male glands situated just below the posterior dorsal corner release their secretions onto these long setae, which play some role in mating behaviour. In females the terminal setae are claw-like and help to manipulate food oarticles. Both the fifth and sixth limbs have lateral plates (epipodial appendages) that carry three groups of between 4 and 7 long setae, forming the 'vibratory plates'. In living animals these setae can be seen beating rhythmically and driving the respiratory flow of water through the carapace.
The seventh pair of limbs are vestigial, and reduced to two segments. The terminal segment carries two setae, one long, one short. In myodocopids the seventh limb are multi-segmented and carry an elaborate array of bristles. These limbs are in a continuous state of writhing motion and probably function to keep the inner surfaces of the carapace clean.
The caudal furca consists of two flanges each armed with pairs of hook setae. The first longest pair is off-set from the others, which diminish in size posteriorly. The earliest juvenile instar has just two pairs of hook setae and a further pair is added at each ecdysis. So after the six ecdyses leaving to maturation the number pairs of claw setae rises to with eight. The anus opens in the fold anterior to the base of the furca. In males, the copulatory appendage is mounted on the right side of the furca; this asymmetry is probably related to the asymmetries in the openings of the carapace glands and the second antennal endopodites. In myodocopids, there is a symmetrical pair of copulatory appendages at the base of the furca, and the limbs and carapace characteristics are symmetrical.