Comparisons of life-history traits and sexual dimorphism between Assimineajaponica and Angustassiminea castanea (Gastropoda : Assimineidae)

摘要

Life histories of two sympatric assimineid gastropods, Assiminea japonica and Angustassiminea castanea, were investigated at a reed marsh in northern Japan. Size frequency distributions revealed that growth curves of females and males differed in both species resulting in sexual dimorphisms in shell size. In A. japonica, the shell height of females was larger than that of males, whereas in A. castanea males attained a bigger shell than females. Mating behaviour of A. japonica was observed mainly from February to June, while that of A. castanea was observed from May to August. The diameter of deposited eggs of A. japonica was 200 to 250 mu m. whereas that of A. castanea was 125 to 150 mu m The length of the veliger larva of A. japonica at hatching was approximately 200 mu m, whereas that of A. castanea was approximately 125 mu m. Longevity, age at sexual maturity and opportunities for reproduction during life time were estimated in the field. For A. japonica, sexual maturity was established by the age of 17 months and longevity was about three years. Opportunities for reproduction of A. japonica occurred twice during a lifetime. For A. castanea. sexual maturity was established by the age of 10 months and longevity was almost five years. Opportunities for reproduction of A. castanea occurred five times during a Lifetime. The differences of life-history traits and sexual dimorphism between species and possible explanations of evolutionary factors are discussed.