Vertical distribution of planktonic ostracods (Halocyprididae) in the northeastern and central South China Sea: Significance of large-sized species in deep waters

The vertical distribution of species richness, abundance and biomass of planktonic ostracods down to a depth of 1000 m were studied in the northeastern South China Sea (NESCS) during March and in the central South China Sea (CTSCS) during September 2016. A total of 47 species of halocyprids were identified, with 46 species recorded in the NESCS and 28 species in the CTSCS. Most of the species observed were concentrated in the mesopelagic zone (200-1000 m), but the majority of species abundance and biomass occurred in the 25 m to 300 m water column. Results of statistical analysis showed that the vertical distribution of planktonic ostracods was associated with chlorophyll a concentration and sampling depth. The high abundance and species richness of ostracods in the NESCS were probably influenced by the Kuroshio intrusion and eddies leading to high levels of chlorophyll a concentration. The biomass of large-sized individuals with low abundance is equal to or larger than, the extremely small-sized individuals with high abundance, particularly in deep layers. The body sizes of planktonic ostracods were found to increase from surface to deeper water layers, indicating a trend of greater body length and higher biomass of ostracods in deep sea.