Ostracoda
Introduction

Ostracoda are a sub-class of relatively small crustaceans which occur predominantly in aquatic habitats. The majority of species are benthic, but in oceanic waters most members of the family Halocyprididae and a few species of the family Cypridinididae are holoplanktonic. In neritic waters many species of Cyprinidinidae and other myodocopid families are meroplanktonic. Adult sizes of the oceanic planktonic species range from lengths of 0.5 to 30 mm, but most are in the range of 0.8 to 4 mm. Apart from the large bathypelagic species of Gigantocypris, planktonic ostracods have received scant attention from oceanic ecologists despite their relatively high numerical abundance (often outnumbered only by copepods) and more or less ubiquitous occurrence at all oceanic depths.

Planktonic ostracods, unlike their benthic counterparts, have carapaces with little if any calcification, so they are poorly represented in the fossil record. Since the majority of ostracodologists are palaeontologists, rather than zoologists or ecologists, the planktonic species have been neglected despite their importance in pelagic ecosystems. The earliest taxonomic descriptions to meet modern standards were by G. O. Sars (1865) who described three species from Norwegian waters. Much of the pioneering taxonomic work was carried out by C. Claus (1874, 1890, 1891, 1894) and G. W. Müller (1894, 1906, 1912), who described material collected in the Mediterranean and during the major exploratory expeditions of the late 19th and early 20th centuries on many vessels, notably the Valdivia, Belgica and Siboga. Many of these early expeditions sampled in the South Atlantic. Other expeditions that made major contributions to planktonic ostracod studies were the Swedish Antarctic expedition (see the exhaustive treatise by Skogsberg, 1920), and the Dana expedition (see the series of papers by Poulsen, 1962; 1969; 1973; 1977). Iles (1961) described some of the ostracods collected in the Benguela Current region by the William Scoresby, but disappointingly never completed the analysis of the copious material collected during the Discovery Expeditions (some of this material has been used herein), although Cannon (1940) had earlier described the internal anatomy of Gigantocypris muelleri from this material.

Models of the life cycles of Alacia hettacra and Alacia belgicae in the region of the Antarctic Peninsula. Adults (Ad), while occupying Antarctic Surface Water (AASW) in the upper 100 m of the water column are advected offshore. They migrate down 500 m into Circumpolar Deep Water (CDW) in autumn (a) where they release their eggs (e). The eggs hatch and develop through six juvenile stages (1-6) during the winter (w) and early spring (sp), while being advected back in over the shelf. The later stages migrate back up towards the surface where they develop into adults as they move back into the upper layers. Redrawn from Kock (1992): (Alacia hettacra tabel) (Alacia belgicae tabel)

There is relatively little information available about the life histories of most myodocopid species. Kock (1992) developed a model of the life-cycles of two Alacia species endemic to the Southern Ocean. The adults occur near surface in Antarctic Surface Water during the early summer and so are advected off the shelf over deep water (Fig. 1). Before hatching the eggs sink to depths of 400-500 m, where the early juveniles stages stay within the Circumpolar Deep Water which slowly moves them back over the shelf. During the winter the last two juvenile stages migrate back up towards the surface ready to restart the cycle when they mature in the spring. The development has been described for a Southern Ocean species P. serrulata by Ramirez and Perez Seijas (1981). Length tends to increase by a factor of 1.3-1.4 at each moult with little or no overlap in carapace lengths between successive instars (Paramollicia rhynchena tabel).

Some species (e.g., Conchoecilla daphnoides and Orthoconchoecia haddoni) show similar seasonality in their reproduction; at the end of March only small juvenile instars were present at 44°N, 13°W (Angel, 1977), whereas later in the Spring adults appeared (Angel, 1984). Similar seasonality can be expected at high latitudes in the South Atlantic. However, in most species, specimens at all developmental stages can be found at a locality throughout the year. There is a general, but by no means unversal trend, for the early juvenile stages to occupy shallower depths than the adults, and for any species which undertakes seasonal and/or ontogenetic migrations a false impression of absence may be gained if the sampling regime fails to cover the species's full range.

Superimposed outlines of juvenile and adult female instars of two species. Modified from Kock (1992):

Boroecia antipoda lateral
Boroecia antipoda dorsal
Alacia belgicae lateral
Alacia belgicae dorsal

Halocyprids will eat almost any dead material offered to them, but do not appear to be active predators. In fresh net samples the halocyprids can be seen feeding actively on gelatinous, damaged organisms and particles. They are clearly able to handle "particles" which are considerably larger than themselves, manipulating them with the mandibles and the fifth and sixth limbs, rotating them and finally rejecting them using the caudal furca. Under normal conditions they are probably opportunistic feeders and probably actively feed in the net while being caught. Analyses of gut and faecal pellet contents have shown that the food taken varies widely between species. Some species feed on detritus, since some bathypelagic species have large numbers of phytoplankton tests in their guts, which are probably derived from the sedimenting rain of detrital aggregates. Some species have guts containing large quantities of folded membranes which appear to be moulted carapaces of other crustaceans. In others, the contents are hard and resinous and have an amber-like consistency; their food source remains a mystery. Planktonic ostracods seem likely to be playing an important role in the re-cycling of material within the body of the deep ocean. All the myodocopids that have been studied, have proved to be carnivorous. Gigantocypris species take a wide variety of small micronektonic prey including mysids, chaetognaths, copepods and medusae. Their prey can be much the same size as the ostracod itself. Macrocypridina species are also predators of plankton, consuming large numbers of copepods.

All non-selective planktivores will inevitably include some ostracods in their diets, since the ostracods are often the second-most abundant group of macroplankton. Cyclothone species which are by far and away the most abundant fishes at mesopelagic depths, myctophid fishes, decapod crustaceans, and heteropod molluscs have all been reported as being non-selective feeders which take ostracods. There are a few species which specialise in ostracod predation, the hatchetfish Argyropelecus aculeatus predominantly eats Mikroconchoecia curta and the siphonophore Hippopodius hippopus feeds almost exclusively on halocyprids, and Diphyes dispar and Chelophyes appendiculata also prefer to feed on the ostracods. The vertical ranges of these predators match those of their prey.