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

Life histories of two sympatric assimineid gastropods, Assiminea japonica a nd 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. M ating behaviour of A. japonica was observed mainly from February to June, w hile that of A. castanea was observed from May to August. The diameter of d eposited eggs of A. japonica was 200 to 250 mu m. whereas that of A. castan ea was 125 to 150 mu m The length of the veliger larva of A. japonica at ha tching was approximately 200 mu m, whereas that of A. castanea was approxim ately 125 mu m. Longevity, age at sexual maturity and opportunities for rep roduction during life time were estimated in the field. For A. japonica, se xual maturity was established by the age of 17 months and longevity was abo ut three years. Opportunities for reproduction of A. japonica occurred twic e during a lifetime. For A. castanea. sexual maturity was established by th e age of 10 months and longevity was almost five years. Opportunities for r eproduction of A. castanea occurred five times during a Lifetime. The diffe rences of life-history traits and sexual dimorphism between species and pos sible explanations of evolutionary factors are discussed.