クロダカワニナの分布と成貝および新生貝の種内変異 Distribution and Intraspecific Variation in Adult and Newborn Shells of Semisulcospira kurodai (Prosobranchia : Pleuroceridae)
The distribution of a freshwater snail, Semisulcospira kurodai was surveyed at 16 rivers, including type locality, and 5 ponds in the Tokai district and adjacent area, Japan. Intraspecific variation of the shape of the adult snails and the size of newborns was investigated. Shell width-length relationships of the adult snails from 8 localities were classified into 3 types. The mean size in the newborns, i.e. 1.4 mm of shell diameter and 2.1 mm of shell height, showed little variation except 2 localities, Chikaraishi River and Ichiunsai River, in the east part of their distribution. The mean size of the newborns in Chikaraishi R., which were 1.2 mm of shell diameter and 1.9 mm of shell height, were smaller than that in the type locality. The mean size of the newborns from the females having smaller diameter than 11 mm in Ichiunsai R., which were smaller than that in type locality, increased with the size of the mother. Average numbers of embryonic shells in brood pouch of the female with 9-14 mm of shell diameter were estimated 17-72 at 6 localities. Average numbers of newborns during one year at 3 localities were estimated 86-113 in the standard size of newborns and 253-370 in the small size. Distribution and dispersion process of S. kurodai were discussed in relation with the fossils of the gastropods in pleistocene, the fluctuation of the sea level in the past and the ancient river system of Tokai district.