Abstract
An amphidromous sculpin, Cottus hangiongensis, distributed freely in a natural river system, exhibited clinal differences in population density, sex ratio, age composition and life-history. In a second river, in which the distribution was restricted to a narrow area in the lower reaches by a notched weir, the species was characterized by high population density and a similar population structure in two different habitats. Little life-history variation was evident. Field observations suggested that clinal life-history variations in C. hangiongensis were not genotypic, but environmentally-induced, phenotypic responses. Rearing experiments demonstrated that the maturity of one-year old males was delayed under low density conditions, and that no relationship existed between the timing of maturity and the sex ratio . This suggests that phenotypic life-history variation in C. hangiongensis is induced primarily in response to population density. Such phenotypic plasticity in life-histories of individual C. hangiongensis populations may be an adaptation for exploitation of broad and heterogenous river habitats.
