日本海の好冷性介形虫相の変遷と海洋環境(<特集>日本海の生物相の変遷と環境変動-過去,現在そして未来へ向けて)

抄録

This paper overviews the faunal changes of cryophilic ostracods (Crustacea) in the Japan Sea since the early Pleistocene, in relation to oceanographic environments. Summaries of water temperature-salinity ranges for living relict-species of the families Hemicytheridae, Cytheruridae and Eucytheridae in and around this sea provided significant information on their ecology and survival through Pleistocene environmental fluctuations. Using summer temperature-salinity habitat requirements, 14 representative species could be divided into three groups. The winter temperature-salinity of areas inhabited by these three species-groups falls into a single range. The ability of all these species to tolerate a low winter temperature around 5℃ (or less than 5℃) is considered critical for their survival. Their wide temperature-salinity tolerances between summer and winter are considered to be the most advantageous factor for survival through Pleistocene environmental fluctuations in the Japan Sea, caused by glacio-eustatic sea-level changes. Palaeo-habitats and the Pleistocene extinction-history of cryophilic species in this sea are recorded in coastal strata. The presence of 16 now-extinct species from the Omma Formation (1.5 Ma), central Japan, indicates that such species predominated in a shallow, open sea environment. Tolerance ranges of salinity for these now-extinct species are inferred to have been narrower than those of most extant cryophilic species that live in open water as well as in brackish inner-bays. The salinity decrease in shallow-areas, as the result of glacio-eustatic changes and the closure of shallow straits around this sea owing to the low sea-level, might have caused their extinctions during glacial periods with increased sea-level amplitude since 1 Ma.