SOCIAL STRUCTURE OF THE PROTOGYNOUS FISH LABROIDES DIMIDIATUS

Abstract

Social relationships of the protogynous wrasse, Labroides dimidiatus, were investigated at the shallow rocky reefs of Shirahama, southern Japan, from 1972 to 1979. Individuals were tagged, and their behaviour and movements were recorded for at least one month. Home ranges of males hardly overlapped. Within a male's home range, 2 to 12 females resided and spawned with the male. Home ranges of females in a male's harem did not always overlap. Individuals whose home ranges largely overlapped (=members of a "primary group") had different social relationships, both in quantity and quality, from those between primary groups: e.g. within primary groups size-based dominance hierarchies were present, while between primary groups territoriality was found. Therefore, primary groups are regarded as the most basic social unit of the species. This unit was rather open for smaller individuals. Such social structure was primarily based on the "size principle": individuals of similar body size could not overlap their home ranges and had territorial relationships, while individuals of different body size could overlap their home ranges and when overlapped they had size-based dominance relationships. The size principle was also loosely applicable to smaller individuals. The process of protogynous sex-change is suggested to be controlled by the size principle, or the cooperating function of a dominance hierarchy within a primary group and territoriality between primary groups. In males, territoriality and dominance hierarchies seem to be directly related to the acquisition of mates. However, in females, they do not seem to be directly related to resources such as food: feeding territoriality, as has been suggested, does not seem to be the fact. The social structure of this species is compared with those of other labrids and haremic fishes, and the ecological requirements of harems and the means of social control of sex-change are discussed.