Sexual Allocation in a Simultaneously Hermaphroditic Coral Reef Fish

Abstract

There has been some question about sexual allocation in outbreeding simultaneous hermaphrodites. In particular, under what circumstances should they invest equally in male and female functions? An attempt was made to determine the pattern of allocation of reproductive effort in the hermaphroditic teleost fish, Hypoplectrus nigricans (Serranidae), and to elucidate the factors influencing that pattern. Most of the gonad of H. nigricans is comprised of ovary, the testis forming only a thin band of tissue on the outside of the gonadal capsule. On a given day of spawning, eggs comprise most of the biomass of the gametes released. Courtship behaviors are at least as much female as male functions. Thus, most of the reproductive effort is spent on female functions. This pattern is not accounted for by inbreeding, low density, or partial parthenogenesis, but by the structure of the mating system. Hamlets "trade eggs," giving up eggs to be fertilized in exchange for the opportunity to fertilize those of another individual. Together, egg trading and the pattern of allocation of reproductive effort account for the evolutionary stability of the hermaphroditism by providing a fecundity advantage similar to that of parthenogens over sexual organisms.