The Relationships Among Population Density, Individual Size, Mating Tactics, and Reproductive Success in a Hermaphroditic Fish, Serranus Fasciatus

The hermaphroditic reef fish Serranus fasciatus exhibits three types of social systems. The size of a social group is correlated with the local density of conspecifics. At very low densities, isolated pairs of individuals reciprocally spawn with each other, achieving equal current reproductive success. At intermediate group sizes, harems form, with the largest individual typically losing all of its female function and becoming a functional male. In harems, subordinate hermaphrodites obtain little male reproductive success through streaking, an alternative male mating tactic. The lone pure male maintains almost total monopolization of male reproductive success in harems, apparently due to aggressive domination of subordinates. At high group sizes, the ability of the male to monopolize all of the matings in a social group decreases, and some of the larger hermaphrodites obtain some male-role reproductive success by pair spawning with smaller subordinate hermaphrodites while continuing to spawn as females with the male. Mating partners stay relatively constant through time, resulting in a pattern of small 'sub-harems' within harems. These mating tactics are consistent with the hypothesis that dominant individuals increase their current reproductive success in this species by restricting male mating opportunities of conspecifics. Subordinate individuals spawn as males when the dominant is unable to restrict interactions between hermaphrodites that are potential mates, or when they successfully streak. The increased male reproductive success of hermaphrodites in isolated pairs and complex harems compared with hermaphrodites in harems appears to be important in maintaining a hermaphroditic subordinate phenotype in this largely non-reciprocating species.