| Budget Amount *help |
¥3,700,000 (Direct Cost: ¥3,700,000)
Fiscal Year 2001: ¥500,000 (Direct Cost: ¥500,000)
Fiscal Year 2000: ¥3,200,000 (Direct Cost: ¥3,200,000)
|
|---|
| Research Abstract |
Nineteen living species (68%) and ten extinct species (77%) have nonplanktotrophic larva among the gastropods from the Pliocene and the lower Pleistocene in the Japan Sea borderland. There is not so much differences of larval ecology between the living and extinct species. Thus, the larval ecology dose not act as a causal factor of extinction.
Many buccinids and muricids which are vulnerable for the paleoenvironmental change have been recorded from the Plio-Pleistocene of the Japan Sea borderland. They are subdivided into two taxonomic groups based on the living depth. Ancistrolepidinae, Neptunea, Buccinum (L group) mainly live in the water deeper than lower sublittoral zone while most species of Nucella, Ceratostoma, Ocinebrellus and Lirabuccittum (Ugroup) are upper sublittoral rocky-bottom dwellers. In the L group, twenty species and populations (56%) of the Japan Sea side suffered from extinction. Two species occur as fossil and living specimens only in the Japan Sea. On the other hand, the U group (ten species) includes seven species living both in the Japan Sea and the Pacific Ocean.
During the lowstands of the middle and the late Pleistocene ice age, low salinity surface water and anoxic deep-water was formed in the semi-enclosed Japan Sea. Thus, many species and populations of the L group and a few species of the U group suffered from extinction in such anoxic deep-water and brackish surface water conditions. At that age, there must be middle pure marine layer which enables for two endemic species of the L group to survive. Most species which are able to live in the upper sublittoral depth did not became to be extinct because it might be possible for these species to reinvade through a northern shallow strait. From the above discussion, the depth of each species is ecologically more important factor to the extinction in the marginal sea than the larval ecology.
|
