Sex change in fishes: hypotheses, evidence, and objections

Synopsis

Ghiselin's size-advantage hypothesis has been used with considerable success to explain the adaptive significance of sequential hermaphroditism in fishes. If the expected number of offspring produced differs between the sexes with size, then an individual that changes sex can take advantage of these differences and have more offspring than an individual that remains exclusively male or female. Despite its success, the size-advantage hypothesis has been criticized as inadequate or incomplete, and some modifications have made it more consistent with common life-history theories. In general, individuals should change sex when the other sex has a higher reproductive value. Thus individuals may change sex and suffer an initial drop in reproductive success if this enhances their future expectations. The size-advantage model was formalized to identify the important variables that affect selection for sex change. Many of the assumptions made (e.g., a genetically fixed age of sex change) were for mathematical tractability and are not critical to the general premise. While the model can be used to specify the optimal size or age of sex change in a hermaphroditic species, it is necessary to state the factors likely to be affecting sex-specific reproductive values in the particular species in question. Inasmuch as these factors (e.g., mating system, local sex ratios, and size distributions) vary from species to species, the general model cannot and should not include them. For the same reasons, the model was also not intended to describe the precise cues that an animal might use to change sex. Any number of features might be used as a predictor of reproductive value, and the size-advantage model does not require that relative size be the actual criterion utilized. Several aspects of the model require further testing. Perhaps the most important problem is a refinement of the predictions for the occurrence of sex change from male to female.