DEEP-SEA ADAPTATION OF THE EPIPELAGIC CHAETOGNATH SAGITTA-ELEGANS IN THE JAPAN SEA

The Japan Sea is a semi-isolated marginal sea connected to the Pacific by 4 shallow straits. The only important current flowing into this se a today is the warm Tsushima Current entering through the southern str ait. Between 20 000 and 10 000 yr ago, the cold Oyashio Current flowed into the Japan Sea through its northern straits. The epipelagic chaet ognath Sagitta elegans was introduced into this sea during this period , but shallow straits prevented the invasion of deep-sea chaetognaths. All other chaetognath species were introduced into this sea by the wa rm current within the last 10000 yr and inhabit the epipelagic layer. S. elegans is distributed as deep as 1000 m or more, but feeding activ ity is low in the deep water. The size and number of grasping spines a nd teeth of the Japan Sea specimens are larger than those from the Pac ific. They have also developed intestinal tissue containing oil drople ts, like meso- and bathypelagic species. These features are adaptation s to the deep-water environment. The vertical dispersal of S. elegans is facilitated by the absence of competitors such as Eukrohnia hamata, E. fowleri and S. macrocephala in this marginal sea.