Abstract
We studied spatiotemporal variation in the spawning patterns of a local population of the brackish water clam Corbicula japonica in Lake Abashiri and the Abashiri River, over the entire range of this species in this basin. Reproductive patterns were approximately synchronous throughout the local range of this species, with clams maturing in July and spawning in August. However, spatiotemporal variation in reproductive patterns was detected along the brackish water environmental gradient, especially at the edge of the distribution range where a delay in reproduction was observed. Such spatiotemporal variation in reproductive patterns is likely to have arisen due to factors such as water temperature, salinity, turbidity and environmental stress, which are typical of brackish water environments and which are affected by freshwater inputs upstream and tidal inflows downstream. The reproduction and viability of this population are strongly affected by local variations in temperature, salinity and anoxic conditions below the halocline of Lake Abashiri, which is located along the northern edge of the distribution range of C. japonica.
