1Division of General Education, Dohto University, Kitahiroshima, Hokkaido 061-1196, 2Graduate School of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido 041-8611, Japan
The reproductive ecology of the neptune whelk, Neptunea polycostata, was investigated in Erimo, Hokkaido, from April 2000 to April 2001. Special emphasis was placed on maturity size, reproductive cycle (seasonal change in body organ index), and sex ratio. The sexual maturity size was 120-130 mm shell height in the male and 150-160 mm shell height in the female. The main copulation period was surmised to be from February to May. On the other hand, spawning was estimated to take place from March through August. Sex ratio changed seasonally. The shell height frequency distribution differed between male and female. From the data that the proportion of males sharply decreased after the maturity size of females (155 mm in shell height), the difference is considered to be also caused by the difference in mortality rate between male and female, besides the difference in growth between the two sexes.