Effects of temperature and salinity on spawning of the brackish water bivalve Corbicula japonica in Lake Abashiri, Hokkaido, Japan

The abundance of planktonic larvae (veliger) of Corbicula japonica in Lake Abashiri, which is near the northern limit of this species' range, was extremely low in 1995 and 1996, compared to other years (1989, 1990, 1994 and 1997). In histological examinations, the female genital tubules were filled with the atretic oocytes after the spawning season of 1995. Therefore, it was concluded that most C. japonica failed to spawn in 1995. The effects of temperature and salinity on the spawning of C. japonica were examined in a spawning induction experiment under laboratory conditions. Probability of spawning by C. japonica in the experiment was described by the equation: 1/( 1 + exp[-(-15.57 + 0.57 T- 12.64S- 0.55Gam + 0.69 T.S- 0.1T.S.Gam)]) (T: temperature, S: salinity, Gam: dummy variable of 1 or 2, i.e. 1 denotes each group to which the gamete suspension was added and 2 each group to which the gamete suspension was not added). The probability of spawning within the group to which the gamete suspension was added (i.e. Gam =1) explained well both annual and seasonal fluctuations of larval occurrence in Lake Abashiri in past years. The spawning of C. japonica may have been extremely low in 1995 and 1996 because of low temperature and low salinity. Such unfavorable conditions for spawning are not exceptional and occurred in 10 out of 21 years for which data were available. In C. japonica, reproduction succeeds less frequently in northern than in southern populations because the probability of spawning depends markedly on the temperature.