Effects of Behavioral Interaction on Sex Determination in the Midas Cichlid.

Abstract

Some teleost fishes can change sex according to social conditions, a trait that has been proposed to have evolved through a change in developmental timing. I reviewed literature and found that cichlids exhibit different expressions of lability at each of three life stages, supporting this idea. In Midas cichlids, Amphilophus citrinellus, relative body size within a group has been reported to determine sex at the juvenile stage, with larger individuals differentiating as males and smaller fish differentiating as females. In contrast to this report, I found that sex was not associated with either behavioral interaction or relative body size in any of 10 small groups of Midas cichlids that were grown to maturity in the laboratory. In Lake Apoyo, Nicaragua, large juveniles at the onset of sex differentiation exhibited no association between body size and sex in either of two natural social groups. In lab experiments that restricted available space and numbers of competitors, juveniles behaved territorially and performed elevated levels of aggression. Therefore, subsequent lab experiments tested the effects of relative body size on sex determination at larger group sizes and in larger tanks. When eight groups from four different lineages were grown for much longer periods, sex was unrelated to body size initially, but as the fish matured males began to grow faster than females. Maturation in isolation did not affect sex determination. Of 25 wild-caught Midas cichlids of various ages from the same locality as those used in the original reports, none had bisexual gonads although these are often present in sexually labile species. In juveniles, there was no significant difference in body size between females and males, but in adults males were much larger than females. In each investigation, results were not consistent with a hypothesis of socially controlled sex determination. Larger body size in adult males compared to adult females is attained not because the largest juveniles differentiate as males, but because males experience greater post-maturational growth than females. Sex determination in Midas cichlids therefore does not support any ypothesis regarding the evolution of functional sex change in sequentially hermaphroditic fishes.