Ostracoda
Zoogeography and vertical distribution

Few wide-ranging zoogeographic studies have been conducted on halocyprids in the South Atlantic; the data given in the tables in the Higher Taxa module are derived from a compilation of all literature records, and so may give a slightly false impression of the normal distributional ranges of many species. In the South Atlantic the zoogeographic patterns are likely to be analogous to those described by Angel and Fasham (1975) along the 20°W meridian in the North Atlantic. There cfhanges in the community composition were related to boundaries associated with the main hydrographic circulation. At high latitudes (i.e., >50°N) the vertical stratification of the populations inhabiting the water column down to depths of 2000 m is weakly subdivided into three depth zones. Whereas at lower latitudes, where thermal stratification is better established, the ostracods were also more clearly organised into five bathymetric zones. A widely-spaced latitudinal transect along 20°W revealed that a major faunal boundary occurs between 40°-42°N. Sampling across this boundary showed a poleward reduction of 30% in the numbers of halocyprid species caught at all depths down to at least 2000 m (bathymetric profiles). At the same time, dominance of the assemblages by a very few species within each depth stratum increased. Similar trends occur in other pelagic taxa and Angel (1991) postulated that the changes are associated with the southern limit to which winter-mixing extends deep enough to replenish surface nutrients to result in a well-developed spring bloom in the phytoplankton (i.e., the Subtropical Convergence in the South Atlantic). Recently mosaics of remotely sensed images of ocean colour have visualized patterns of chlorophyll concentrations in the surface waters and clearly show this boundary, particularly in the boreal summer (Campbell and Aarup, 1992).

In the North Atlantic the subtropical circulation is dominated by the subtropical gyre involving the Gulf Stream and the Canary Current. Peterson and Strama (1991) have provided a detailed description of the upper circulation and the distribution of major oceanographic fronts in the South Atlantic. These fronts coincide with the boundaries of biogeochemical provinces (Longhurst, 1995) within which there are characteristic seasonal cycles of productivity.

Poulsen (1977) noted that most epipelagic and mesopelagic species occur in all oceans, whereas many bathypelagic species have more restricted geographical ranges. This may be an artefact of poor sample coverage since deeper-living species described from the Southern Ocean (i.e., Archiconchoecia versicula and Metaconchoecia arcuata), and the North Pacific (i.e., Loricoecia acutimarginata and Proceroecia vitjazi) have subsequently been found in the North Atlantic. However, species of Mollicia and Paramollicia do have restricted ranges, and there are examples of bipolar sibling species pairs (Boroecia borealis and Boroecia maxima in the Arctic and B. antipoda in the South; Obtusoecia obtusata in the Arctic and Obtusoecia antarctica in the South). (bathymetric profiles).

Some species have small low latitude and large high latitude forms, sometimes showing clinal changes (e.g., Discoconchoecia elegans), or sometimes disjunct distributions (e.g., Archiconchoecia cucullata). There is a similar size and bathymetric separation between Paraconchoecia mamillata and Paraconchoecia nanomamillata. Angel (1982), when redescribing the two species of Halocypris inflata and Halocypris pelagica (which had been synonymised as Halocypris brevirostris), suggested that the size and depth ranges of the co-occurring juveniles could be interpreted as exemplifying character displacement.

In the oceanic waters at high latitudes, planktonic ostracod numbers are generally low in the surface 100 m, except in late summer. They occur at maximum abundance (5-10 ind. per mö3) at depths of 200-300 m. Below 300 m, along with other taxa, their abundance declines with increasing depth. However, ostracods are usually ranked second in numerical abundance in the macroplankton throughout the water column. But because of their relatively small size, their contribution to the total standing crop in the water column is relatively less important. Their abundances increase quite sharply within 100 m of the sea bed in the benthopelagic zone (i.e., in the benthic boundary layer).

Within the water column the halocyprids are bathymetrically zoned. Vertical ranges tend to be smaller at shallow depths than in deep water. Epipelagic species inhabit the euphotic zone and the top of the seasonal thermocline extending down to 250-300 m; the depth at which the light field becomes totally symmetrical regardless of the sun's elevation. The shallow mesopelagic zone extends down from 250-300 m to the depth at which the down-welling daylight becomes dim relative to the bioluminesence produced by the organisms themselves, at about 700 m in the clearest oceanic water. The deep mesopelagic zone extends from there down to where daylight ceases to become detectable by the animals, usually at around 1000 m. The bathypelagic zone extends from 1000 m to around 2500-2700 m, where it gives way to the abyssopelagic zone. The boundary between the bathypelagic and abyssopelagic zone does not seem to be related to a specific physical parameter, but may be related to the depth at which fishes cease to be a major component of the midwater community (Angel, 1983). The lower boundary of the abyssopelagic zone is at the top of the benthopelagic zone about 100 m above the bottom in the North Atlantic, but its height above the bottom in some parts of the South Atlantic may be higher as a result of the greater incidence of benthic storms in the Argentinian Basin. There are few regions where soundings exceed 6000 m where novel hadal communities may occur. These deep near-bottom zones have hardly been studied in the South Atlantic.

Many species have bathymetric ranges that only roughly correspond to this zonation. An individual species may migrate vertically between these zones either daily or seasonally. Ontogenetic migrations result in a species changing zones during its development. A common feature in several species is for adult females to have tails to their vertical distributions which extend into far deeper water than either juveniles or adult males. For example, Mikroconchoecia curta is predominantly a shallow mesopelagic subtropical species often migrating up into the neuston at night, but adult females are regularly caught in small numbers as deep as 2000 m.

Angel (1979) analysed the spectra of carapace lengths of halocyprid assemblages throughout the water column off Bermuda. By day, as the sampling was extended into deeper water, the size range of the ostracod assemblage progressively extended to include larger and larger animals, while the numbers caught declined. At night, vertical migration resulted in the bathymetric distribution of sizes becoming more uniform.

Some species always appear to be non-migrants (e.g., Paraconchoecia inermis). Several species undertake extensive diel vertical migrations, synchronised with the cycle of day and night. for example Paraconchoecia spinifera and Conchoecissa imbricata migrate up 300-400 m at dusk - equivalent to 2x10ö5 body lengths. Along 20°W in the northeastern Atlantic, the greatest expression of diel vertical migration occurred in the oligotrophic waters around 30°N, and become less evident at both higher and lower latitudes. Repeated sampling throughout much of the year in the southern Bay of Biscay indicated that both the extent of diel migrations and the number of animals participating varied markedly with season (Angel, unpublished). There are also major changes occurring throughout the year in the bathymetric ranges of the species as a result of seasonal migrations.