Planktonic ostracods are a neglected component of mesoplanktonic communities, despite being almost ubiquitous throughout the World's oceans. Currently the number of planktonic ostracods described from oceanic waters is 203. In addition there are 34 species of the family Thaumatocyprididae, which are predominantly either benthic or cavernicolous. We present a listing of the currently recognized species. In inshore waters, particularly in the vicinity of coral reefs, there is a great diversity of myodocopid ostracods that can be sampled in the water column especially at night. Most of these myodocopid species are not holoplanktonic, entering the water column at certain stages of their life histories. This site only covers the holoplanktonic species and does not deal with any of the neritic species.
In oceanic waters the vast majority of planktonic ostracods are halocyprids. They are often very abundant, and rank second to copepods in numerical abundance particularly in subthermocline waters. They may occur almost everywhere from the surface to abyssal depths. However, they are seldom encountered in the upper 100-200m of the water column at high latitudes (>50 deg), for example they are infrequently sampled by the Continuous Plankton Recorder, which is towed at 8-10m (Williams, 1975). However, their relatively small size (0.5->3mm) means their contribution to the overall planktonic biomass is usually small (<5%). Most species appear to be detritivores, seemingly adapted to exploit marine snow and other sinking particulates. The group frequently goes unreported, because there is a perception that they are difficult to identify, and so they are overlooked. This difficulty is more the result of a lack of good manuals of identification, rather than any greater intrinsic difficulty of identifying them compared with most other components of the plankton. As well as the halocyprids, there are eight very striking species of carnivorous deepwater myodocopids (i.e. Gigantocypris and Macrocypridina ) that are planktonic, but only two have been recorded from the Southern Ocean.
In preparing this atlas, we have compiled a complete inventory of published records for all known species that can be geo-positioned with reasonable accuracy. These data have been supplemented with a considerable body of unpublished data derived from material collected by Discovery Investigations mostly in the 1930's (identified by MVA). Our aim is ultimately to generate a global atlas for all the species. Here as an initial step we have focused on the species that have been recorded from the Southern Ocean . The criterion whereby we have chosen to include species is a single reliable record from south of 52 deg S. This reduces the number of species that are predominantly associated with low latitudes, but are advected into higher latitudes via mesoscale eddies. Other authors have used lower latitudes to delimit Southern Ocean species, for example Razouls et al. (2000) in their study of the biogeography of Antarctic copepods used 45 deg S as the northern limit of the Southern Ocean. Our compilation for these 47 species illustrates crucial gaps in geographical coverage, notably in the North and Central Pacific and in the Southern Indian Oceans. A similar mapping exercise was undertaken by Hillman (1969) and included data from the expeditions of the era of heroic Antarctic exploration together with data from the Eltanin cruises conducted in 1983-1984. Hillman noted that only five halocyprid species were principally restricted to south of the Antarctic Convergence, but a further 17 species had been recorded there. His atlas charted the distributions of eleven species that had been taken regularly in plankton tows. Some of the data plotted by Hillman have been included in our plots, but we could not reconcile all his plotted positions with the listing of Eltanin stations. Similarly some of the records reported posthumously by Deevey (1983) could not be included. These omissions have not lead to a serious gap in coverage.
Our maps illustrate the dominant influence of the principle oceanic fronts and flows on the distributions of the individual species, and this highlights their possible value as biological markers. The maps also reveal some inconsistencies in the data. For example when plotting the data for Boroecia antipoda, a few records from tropical seas derived from material collected during the Dana Expedition (Poulsen, 1975) were clearly anomalous. We have re-examined these specimens and found then to be a new species, albeit very similar to B. antipoda, so these records have not been included in the map for this species.