Spatio-temporal structure of the jellyfish community in the transition zone of cold and warm currents in the northwest pacific

Abstract

Species composition, diversity and biomass of jellyfish (Cnidaria and Ctenophora), with their spatiotemporal distributions, were analyzed in the upper 1500 m of the Oyashio front, the Transition zone and the Kuroshio extension, off Tohoku, northeastern Japan, between May 2005 and March 2006. Species composition and abundance differed remarkably between the shallower layer and the deeper layer at the boundary of 300–500 m depth, where water density was within the range of the North Pacific Intermediate Water. In the deeper layer, diversity reached its peak with the appearance of 27 taxa common in all the regions throughout the year, though abundance was low. Pantachogon haeckeli and Crossota rufobrunnea were dominant in the deeper layer. The possibility of diel vertical migration was suggested in two midwater species: Euphysa japonica and Atolla vanhoeffeni. In the Oyashio waters, jellyfish abundance was much higher than in the Transition waters and Kuroshio-derived waters, but with low diversity, dominated by large numbers of Aglantha digitale and Dimophyes arctica. High values of diversity were found in the Kuroshio-derived waters with various tropical and warm-water species, especially calycophoran siphonophores being present. In the Transition waters, diversity was relatively high, with co-occurrence of warm-water species and cold-water species. Jellyfish biomass tended to be high in the midwater zone due to the occurrence of large species, particularly Scyphozoa. Carbon-based jellyfish biomass calibrated with other studies exceeded that of other organism groups.