Changes in deep-sea metazoan meiobenthic communities and nematode assemblages along a depth gradient (North-western Sea of Japan, Pacific)

The Sea of Japan is a very young sea which has been newer connected with the deep Pacific. In this connection, a big portion of shallow-water taxa are expected to inhabit the deep part of this sea. Meiofaunal communities were investigated for the first time on 13 stations at depths from 500 to 3700 m within four deep-sea transects in the north-western part of the sea. The densities were very low ranging from 4.80±1.17 ind./10 cm2 to 177.42±14.3 ind./10 cm2. There was a significant decrease in the density of meiofauna with depth at all locations. Taxonomic structure of metazoan meiobenthos is plentiful in the whole area and totaled 20 groups. Nematodes were the numerically dominant taxon, constituting 49.6–84.1% of total abundance. At the stations with depth below 3000 m, polychaetes were the dominant taxon and nematodes constituting only 12–36.9% of the total meiofaunal density. On portions of nematodes and polychaetes, deep-sea meiofauna of the Sea of Japan was quite unique in comparison with surrounding deep-sea regions. In total, 92 different nematode species, belonging to 75 genera and 32 families were found. The number of nematode species strongly reduces with depth. Species richness and evenness in nematode assemblages greatly decreased with depth and were very low at depth below than 2000 m. No specific deep-sea nematode fauna was found.