Abstract
The egg-laying season and monthly change in egg capsule production of Clithon retropictus (Prosobranchia : Neritidae) were studied by immersing concrete blocks (39 cm×19 cm×12 cm) in the upstream portion of brackish-water zone in the Naka river of Izu Peninsula, central Japan. Number of egg capsules laid on the surface of a concrete block (39 cm×19 cm) was counted monthly from June 1989 to October 1990. Egg-laying continued from February till September. In this egg-laying season, change in the number of egg capsules laid showed two peaks, suggesting two times of egg-laying by the same female. Although the size of egg capsules was not measured, the comparison of photographs taken monthly revealed that the size of egg capsules laid in February and late May to September were fairly smaller than that laid in late March to early May at a peak of egg-laying. Hatching of veligers from egg capsules was not observed at all during each period of monthly observation, suggesting that the development from egg-laying until hatching needs more than one month.
