Ostracoda
Morphology
The best general accounts of the morphology of planktonic ostracods are by Iles (1961) for the halopcyprid Boroecia (Conchoecia) antipoda and Cannon (1940) for Gigantocypris (both based on Southern Ocean material). This description focuses on halocyprids, drawing heavily on Iles (1961).
The ostracod body is completely enclosed within a bivalved carapace which is hinged along the mid-dorsal margin. In most species, the carapace is developed anteriorly into a rostrum, below which is a notch known as the incisure, through which the exopodites of the second antennae extend during swimming (diagram halocyprid ostracod). While the animal is swimming and behaving normally, the carapace is slightly agape, but it can be clamped shut by adductor muscles inserted close to the midpoint of each valve. The shapes of planktonic ostracods range from subglobular to subcylindrical. In many species the carapace is compressed laterally and raked anteriorly. Spines or tubercles may ornament the posterior dorsal corners and/or the posterior ventral corners (Conchoecilla chuni, Conchoecilla chuni 2). The rostrum may be extended as a spine, or spines may be developed elsewhere on the carapace (Fellia cornuta, Fellia cornuta 2). The carapace may also be ornamented with sculpturing which ranges from linear or concentric striations to heavy reticulations Macroconchoecia reticulata, Macroconchoecia reticulata 2). Dorsally, the anterior of the carapace is often vaulted over the powerfully muscular protopodites of the second antennae which provide the propulsive power for swimming; in a few species the vaults are sharp-edged (Paraconchoecia spinifera, Paraconchoecia spinifera 2), developed into wings (Alacia alata , Alacia alata 2), or armed with spines. An inhalant respiratory flow of water enters through the gap between the carapace valves at the incisure, passes over the body and discharges through a siphon on the posterior margin (diagram of respiratory flow).
On the carapace are several large glands whose positions are taxonomically important (diagram halocyprid ostracod). In halocyprids there are "asymmetrical" glands which typically open close to the posterior dorsal hinge on the left carapace valve, and at the posterior ventral corner on the right valve. However, in some of the genera the glands are displaced well away from these positions. For example in the genus Metaconchoecia (Metaconchoecia acuta, Metaconchoecia arcuata 2) the gland on the left valve is on the rostrum and in Conchoecilla the gland on the right valve is just beneath the incisure (Conchoecilla chuni, Conchoecilla chuni 2). In males there is a group of glands situated just below the posterior dorsal corner close to where the tips of the long setae of the sixth limb protrude through the carapace (see below). The ventral margins of the carapace are lined with mucus-secreting glands. In many species there are accessory glands which produce bioluminescence, located either on the anterior margin just below the incisure, besides the exhalant siphon, or at the tips of carapace spines or tubercles.
Adult halocyprids have seven pairs of segmented limbs. The nomenclature for the setation of the limbs derived by Skogsberg (1920) is followed herein [see illustrations below: diagrams of the limbs of halocyprid ostracods also illustrating the nomenclature of the setae. Modified from Iles (1961)]:
arrangement of limbs in female
adult female, fr. organ/1st ant
male frontal organ /1st antenna
female, 2nd antenna
male endopodite of 2nd antenna
mandible (Ostracoda)
fifth limb
female, sixth limb
male,caudal furca/cop.appendage
endopodite of the maxilla
male, sixt limb
The most anterior pair of limbs are the first antennae or antennules. These lie either side of the frontal organ (or organ of Bellonci). The frontal organ is sexually dimorphic in most species (Mollicia mollis 2), and is usually divided into two sections, the stem (segmented in some species) and the capitulum, but the capitulum and stem are fused in females of some species (see female frontal organ Porroecia parthenoda 2). In the genus Bathyconchoecia (not treated herein because this bathypelagic genus has seldom been reported from the South Atlantic) the frontal organ is totally lacking in all species but one. In living specimens, the tip of the capitulum can usually be seen either projecting forward just below the rostral opening, or down-turned across the inhalant flow of water. In females the stem is unsupported, but in males the stem is firmly anchored by a seta from the second segment of each the first antenna.
The first antennae are uniramous and sexually dimorphic. In females of some species the two basal segments are fused (see female frontal organ Proceroecia macroprocera 2), but in males the segmentation is always clear. In the females of most species there is an anteriorly pointing dorsal seta on the second segment (see female frontal organConchoecia magna 2). The two very short terminal segments carry a number of long setae. In females of the subfamily Conchoecinae there are five terminal setae; a long terminal e-seta and four shorter, thin-walled sensory setae (a-d setae). In males, the second segment carries a seta which curls around and braces the shaft of the capitulum; this seta is probably analagous to the anteriorly pointing dorsal seta of females. The terminal setae include a- and c-setae which are usually short, thin-walled and sensory, and the b, d and e-setae are long and relatively thick walled. Typically the e-seta carries an armature of spines or tubercles the number and arrangement of which is species specific. The d and b setae may also carry some armature. During swimming, these setae are carried splayed out in front of the animal so that they interlock (Angel, 1970). In species of the subfamily Halocyprinae the terminal setae on the first antennae also consist of one long and four shorter but show little or no sexual dimorphism. In the Archiconchoecinae there are six subequal terminal setae (Archiconchoecia cucullata s 2), whereas in the Euconchoecinae there are 20-250 terminal setae (Euconchoecia chierchiae 2).
The second antennae are biramous (female, 2nd antenna). The large muscular protopodite occupies the anterior third or so of the cara pace and provides the propulsive power for swimming. The exopodite usually consists of a long basal segment and eight short segments each carrying a long, feathered swimming seta. The endopodite is inserted on the inner face of the protopodite just posteriorly to the insertion of the exopodite. In Conchoecinae the endopodite is sexually dimorphic (female, 2nd antenna , male endopodite of 2nd antenna). In females it is composed of two segments. The basal segment has an anterior swelling known as the processus mamillaris (female, 2nd antenna, male endopodite of 2nd antenna) close to its base on its anterior edge - a process which is characteristically absent in the Halocyprinae. Distally is another process on which two setae are inserted (a- and b-setae). The second segment carries five setae, the longest being the f-seta, with the h-, i- and j-setae being shorter, subequal, thin-walled and sensory. In males the second segment of the endopodite usually carries two short setae (c- and d-) near its base, and five much longer setae subterminally. At the base of the f-seta there is sometimes an extremely short stubby e-seta. This segment also carries a hook appendage (or clasping organ). Examination of species of Bathyconchoecia has made it clear that the hook appendage is a modified third segment with the shank of the hook carrying one of the longer setae. There is a marked asymmetry in the size and elaboration of the hook appendages between the left and right antennae; the left hook appendage is larger and more elaborate than the right. This asymmetry is probably related to the offset of the copulatory organ on the caudal furca (male,caudal furca/cop.appendage) and the asymmetry of the carapace glands. In myodocopids the copulatory organ is placed symmetrically and there is no asymmetry of the antennal endopodites.
The third pair of limbs, the mandibles (mandible (Ostracoda) ), show no sexual dimorphism. The exopodite is reduced to a small segment often carrying a plumose seta. The endopodite is large and three-segmented, and armed with a number of filamentous setae and terminal claw-like setae. It is used in the manipulation of large food items at the gape of the carapace valves. The protopodite consists of two segments, the coxale which has a toothed edge (termed the pars incisa) and the basale which carries three lines of teeth (the tooth-lists) and a masticatory pad. During feeding, these pads and tooth lists of the two maxillae pivot against each other, reducing food items to a size that can be swallowed. Poulsen (1973) based his revision of the "super-genus" Conchoecia on characters of these mandibular tooth-lists, but even with good oil-immersion microscopy these characters are difficult to distinguish. Moreover, some of the species Poulsen considered to be congeneric differ markedly in other characters. Consequently, attempts to identify species via genera can, in several cases, be frustratingly confusing, and so for routine identifications it will often be pragmatic to ignore the genera.
The fourth pair of limbs, the maxillae (endopodite of the maxilla), are not sexually dimorphic and show few useful interspecific differences. The terminal claw-like setae are also used to manipulate food. The strong spinous setae on the anterior margin of the protopodite of the fifth pair of limbs also assist feeding by pushing food up towards the mouth.
The exopodite of the fifth limb consists of three segments (fifth limb). The first, in halocyprid genera like Bathyconchoecia, carries an extremely long seta on its dorsal surface. At this position in the cave-dwelling genera, Deeveyae and Spelaeoecia (see Kornicker and Iliffe, 1989) is a small articulated segment which caries two or three long setae. So these dorsal setae probably represent a vestigial endopodite. The third segment terminates in three long, claw-like setae whose relative lengths can be useful in distinguishing species (especially between some Archiconchoecia species). Laterally on an epipodial appendage on the base of both the fifth and the sixth limbs are three groups of long plumose setae. These setae lie along the flanks of the body within the carapace, and beat continuously driving the respiratory stream of water through the carapace. These groups of setae are sometimes referred to as vibratory plates.
The sixth pair of limbs once again show marked sexual dimorphism in the Conchoecinae (female, sixth limb , male, sixt limb). The exopodite is usually four-segmented. Dorsally the segments may carry a few short setae, and ventrally the first segment has several short setae. In females the limb's three terminal setae are usually subequal and only a little longer than the third segment. Whereas in males the terminal setae are long and plumose, and their tips protrude from between the carapace valves just below the posterior dorsal corner where the male carapace glands discharge. Mating behaviour has not been described in halocyprids, but secretions released onto these terminal setae may play some role in courtship. A useful character for distinguishing males from females is the very muscular first segment of the male's sixth limb, which is clearly visible through the carapace of undissected specimens.
The seventh limbs of halocyprids are vestigial, each consisting of two short segments with two unequal terminal setae. Most female myodocopids brood eggs and developing young inside the carapace, to the side and dorsal to the body, so their seventh limbs are well developed. In Gigantocypris, for example, they are multi-segmented and carry numerous setae. In live specimens they are constantly writhing around inside the carapace, probably keeping the inside of the carapace and the brood of eggs or larvae clean.
The caudal furca (male,caudal furca/cop.appendage) in halocyprids is a paired structure which is probably homologous with the telson of other crustacean groups (it has also been interpreted as being a uropod, and hence a true appendage). Typically an adult halocyprid has eight pairs of hook setae on its furca. The first instar has just two pairs of these hook setae and a new pair is added at each instar. However, in Archiconchoecia adults of some species have fewer pairs and these species may have undergone neoteny and so may show a reduction in the number of juvenile instars. Recently Ikeda (1992) and Ikeda and Imamura (1992) have reported that Discoconchoecia pseudodiscophora reared in culture undergoes seven rather than six larval instars (the number of paired hook setae on the caudal furca remains at two for the initial two instars after hatching). The furca is used in the manipulation for food items, particularly by arching forward and forcing material away from the carapace gape. It may also function as a means of levering the animal out of the grasp of a predator.
In the halocyprids the male copulatory appendage is located ventrally on the base of the furca on the left-hand side (male,caudal furca/cop.appendage). It consists of an elongated flattened tube containing several obliquely arranged muscles. It terminates in a hollow spine through which the seminal vesicle discharges. Adult females appear to mate immediately on maturation because every mature female has a store of sperm at the base of the oviduct, which appears as a white spot at the base of the furca.
Colour is not used in the taxonomy of the planktonic ostracods because most identification is carried out on preserved material in which all colour has been lost. As in most pelagic groups, their coloration is correlated with their day-time depth of occurrence. Shallow-living species are generally translucent and virtually colourless. Many mesopelagic species have subtle yellow, orange or red coloration associated with the mouthparts and some of the carapace glands. The gut is often opaque orange-brown to black, and is the most obvious feature in daylight. In large deep-living species red pigmentation becomes more pervasive (e.g. Alacia valdiviae and Conchoecissa ametra); although juveniles still tend to be translucent. In deep-living crustaceans such red coloration is produced by a carotenoid pigment originating from phytoplankton and passed along the food-chain. The Macrocypridina species are exceptional in that the adults are chocolate-brown, a feature which shared with a very few other pelagic taxa (e.g the mysid Longithorax fuscus). Gigantocypris muelleri is also well coloured, appearing like a translucent orange-red ball; its deeper-living congener G. dracontovalis is much paler and tinted more with purple.
Most halocyprids produce bioluminescence. Luminous secretions are released into the water from glands situated either along the anterior edge of the carapace just below the rostral incisure (Conchoecia subarcuata, Conchoecia magna, Mikroconchoecia curta, Conchoecilla daphnoides, Conchoecissa imbricata), or halfway up the posterior margin of the carapace (Paramollicia rhynchena). Those on the anterior margin release their secretions into the inhalant stream of water entering the carapace. The secretion initially lights up the animal, but is then left behind in the exhalant plume as a cloud which is about the same size as the animal. The glands on the posterior margin fringe the enlarged gap between the carapace valves through which the exhalant respiratory outflow passes, and they discharge directly into the exhalant flow. In a few species the luminescence is retained within the carapace usually within elaborate structures, such as the long posterior dorsal spines, ventral tubercles and prolongations of the rostra in Conchoecissa imbricata, and the prolongations of the rostra and the postero-dorsal corners in Conchoecilla daphnoides. Similarly, in Paraconchoecia spinifera luminescence is retained along the sharp-edged shoulder vaults.