Seasonal change in male reproductive investment of a fish

Many animals are sexually dimorphic, whereby males may display brighter body coloration and more distinctive ornamentation than females. Fishes in temperate regions markedly change their energy allocation toward reproduction in response to the seasonal environment. Seasonal change in reproductive investment affects the expression of sexually dimorphic traits in males through gonadal weight change. Here, we report seasonal changes in body size, testis weight and sexual dimorphism of the fins (anal fin length and dorsal fin length) in the northern medaka Oryzias sakaizumii. Fish were collected periodically from a wild population (Aomori). Gonad weight increased from May to July in both males and females, corresponding to the reproductive season. Moreover, during this period, the degree of sexual dimorphism in fin length increased. To investigate the relationship between testis weight and individual differences in male fin length, we analyzed relationships among morphological traits using structural equation modeling. In the reproductive season, increased testis weight was associated with longer fin length in males, but the relationship disappeared after the reproductive season. These observations suggest that the sexually dimorphic fin in this fish is a mating signal. Results from a mating experiment also support this view. Males with larger size and/or longer fin attracted more mates than those with smaller fins, suggesting that sexual selection operates through females choosing to mate with males having longer fins.