BOTH-WAYS SEX-CHANGE IN MONOGAMOUS CORAL GOBIES, GOBIODON SPP

We confirmed both-ways sex change in the coral-dwelling gobies Gobiodo n micropus, G. oculolineatus, G. quinquestrigatus and G. rivulatus riv ulatus by mate-removal experiment in the field and by the aquarium exp eriment of keeping two consexual fish in a coral. Eight species of Gob iodon were found in Acropora corals on the reef flat of Sesoko Island, Okinawa, southern Japan. The 4 species mentioned above bred in monoga mous pairs composed of a male and a female matched by size, and the ma le took care of eggs deposited on the coral branch. In G. quinquestrig atus and G. rivulatus rivulatus males were larger than females in newl y formed pairs, and females grew faster than their mates until breedin g. The growth-rate advantage in females seems to be the major factor i n the evolution of female to male sex change. The gobies strongly depe nded on host corals, but they moved between the corals after mate loss or coral death to form new pairs. This provides opportunities for the evolution of male to female sex change; the ability to change sex in both directions reduces the frequency of risky movement between host c orals to form new pairs. These conditions are very similar to those re ported in the both-ways sex change of another coral-dwelling goby Para gobiodon echinocephalus.