SOME ASPECTS OF THE BREEDING BIOLOGY OF PLANAXIS SULCATUS (BORN) (GASTROPODA: PLANAXIDAE)

Sex ratio and breeding habits of Planaxis sulcatus were studiedfor Japanese and other Indo-Pacific populations. On the coastof the Kii Peninsula, Japan, the gonad of P. sulcatus is developedfrom June through August, when pairing of the snails is observed.The females contain embryos in their brood pouches and releaseveligers. New recruits appear in autumn, grow during the nextspring and summer, and merge with the older-year cohort in sizein early autumn. The position on the shore of the first-yearand the older-year snails is lower in spring and early summerthan in other seasons. Sex ratio was significantly differentbetween populations, seemingly due to different percentagesof large snails which were mostly females. Inspection of thesamples from a wider geographic range, from Japan to India,detected various sex ratios among the localities; females weredominant in the samples composed exclusively of large individuals.The most developed embryos in the female brood pouches wereveligers. Parthenogenetic development and rearing of embryosup to crawling juveniles, which have been reported for the populationsin the inner Arabian Sea, does not seem to hold for these otherIndo-Pacific populations. (Received 20 February 1996; accepted 8 July 1996)