Abstract
The spawning habits of two closely related river-sculpins, Cottus amblystomopsis and C.nozawae were compared. The spawning period partially overlapped between the two species though C.amblystomopsis began to spawn slightly before C.nozawae. There were no significant differences between the two species in micro-environmental conditions of the nest sites, such as the depth of water, current velocity, bottom substratum and the size of stones used as the nest.However, the spawning area in the course of the rivers was clearly geographically separated according to the species in most rivers studied; C.amblystomopsis spawned in the lower course of the river, while C.nozawae spawned in the upper course, distinctly apart from the spawning area of C.amblystomopsis. No noticeable differences were revealed between the two species in the pattern of spawning behavior, or the spawning time within the day. However, in mate preference tests, no mating occurred in heterospecific combinations between the two species, though conspecific combinations succeeded in mating in both species. From the present results, it appears that the two closely related species are reproductively isolated from each other, and it is suggested that geographical and ethological isolation may serve as important devices as isolating mechanisms.
