Plastic life-history variation along the course of a steep mountainous river in male Cottus hangiongensis (Pisces: Cottidae)

To elucidate the causes of clinal life-history variation in male freshwater amphidromous Cottus hangiongensis found along the river courses, we examined the genetic composition of upstream, midstream, and downstream groups using microsatellite DNA markers and different growth patterns among juvenile individuals before and after they ascended a small, steep mountainous river in southern Hokkaido, Japan. Microsatellite allele frequencies indicated that the samples collected from three different habitats appeared to represent a panmictic population and no genetic differences were found among the three groups. Ecological field surveys showed that juveniles greatly depended on the timing of their ascent into the river from the sea, and that larger juveniles were able to disperse to more upstream areas and grew larger during the period after they migrated up to the mouth of the river. These results suggested that clinal life-history variations may be induced through phenotypic plasticity and that earlier ascent of juveniles from the mouth of river may be an important source of variation.