現生種腹足類キサゴUmbonium (Suchium) costatumおよび祖先種 Umbonium (Suchium) obsoletumの成長・生息場所・分布

抄録

In this study, we obtained an oxygen isotope (δ18O) profile for the trochid gastropod, Umbonium (Suchium) costatum and confirmed that the prominent growth lines were annual rings formed in summer as suggested by a previous study based on the population dynamics. We also compared the growth history between U. (S.) costatum and its inferred ancestral species U. (S.) obsoletum from a previous study, using the Walford plot. The specimen, U. (S.) obsoletum, analyzed in this study is identical with that analyzed in our previous report under the name, U. (S.) obsoletum arenarium. No significant difference was recognized in the rate and pattern of shell growth between the two species. However, U. (S.) costatum appears to have invaded inner-bay environments as compared to the habitat of the ancestor. The most significant difference between these two species probably lies in their latitudinal distribution, reflecting adaptation to low temperature. U. (S.) costatum is currently widely distributed from Kyushu (32°N) to Hokkaido (43°N). On the other hand, U. (S.) obsoletum has been reported from fossil localities of the subtropical Kakegawa Fauna, from Kyushu (31°N) to Kanto (35°N). This indicates that U. (S.) costatum adapted to a higher-latitude, colder water habitat with evolution from its ancestor.