Plasticity in the mating system of the longnose filefish,Oxymonacanthus longirostris, in relation to mate availability

Abstract

The mating system of the longnose filefish,Oxymonacanthus longirostris, was examined on coral reefs of Okinawa, Japan. This species has been shown previously to be monogamous. Fish were usually found swimming together in heterosexual pairs with the male and female sharing the same feeding territory. However, both monogamous and polygynous (bigamous) males were found in the present study. Polygynous males, which were larger than monogamous males, visited and stayed several minutes in turn with each female within the territories. Although most males were monogamous in the early breeding season, over 20% of males mated polygynously in the late breeding season. The adult sex ratio in the former was unbiased, but became slightly female-biased toward the end of the breeding season because of the higher disappearance rate of males. The higher disappearance rate may be due mainly to a higher mortality rate of males resulting from a greater deterioration of physical condition during the breeding season. Thus, the mating system varied with the change of the adult sex ratio. Plasticity in the mating system of this species may be the outcome of male mating tactic depending on local mate availability.