Preliminary Research on Species Distribution of Planktonic Ostracods (Halocyprididae) in the Ross Sea, Antarctica

Abstract

ABSTRACT This paper reports the results of preliminary research on planktonic Ostracoda collected during the first Italian Oceanographic Expedition to Antarctica (1987-88). A total of 311 samples at 33 stations (31 in Terranova Bay and 2 farther offshore in the Ross Sea) was collected. At each station, samples were taken at various depths from different layers (usually 50–100 m deep), and an integrated sample from the entire water column was collected. Ostracods-all conchoeciine Halocyprididae—were found in 272 samples (87% of the total); all the specimens (54,224) were examined individually. Taxonomic identification at the species level was made for adults (6,587, i.e., 12.15% of all individuals). Four species were identified (the highest density and the relative abundance are reported in parentheses): Alacia belgicae (4.00 ind·m–3, 82.69%), Metaconchoecia isocheira (1.30 ind·m–3, 16.65%), Alacia hettacra (0.13 ind·m–3, 0.61%), Metaconchoecia skogsbergi (0.07 ind·m–3, 0.05%). Some juvenile stages were tentatively identified as Paraconchoecia cf. brachyaskos. In almost all the stations the dominant species was Alacia belgicae, which seems to be a true Antarctic endemic species. It occurred in 2 distinct size ranges. Density distribution of both total ostracods and adults of the dominant species has been analyzed considering samples collected at different depth layers. In Terranova Bay, ostracod density did not show any sharply defined spatial distribution pattern. A positive correlation (P < 0.0001) was found between ostracod density and sampling layer depth. At most stations, densities were higher in intermediate and deep layers, where ostracods sometimes even assumed the role of the dominant planktonic component in terms of numerical importance. The distribution pattern by depth seems consistent with the detritivorous and carnivorous role of Ostracoda in the trophic structure of the Ross Sea zooplankton community.