Adaptive Significance of Temperature-Dependent Sex Determination in a Fish

Abstract

In the Atlantic silverside, Menidia menidia, temperature during the larval development affects primary sex determination. Most offspring produced under lower temperature regimes of the early breeding season become females while most offspring produced at the higher temperatures prevailing during the late breeding season become males. The end result is a sexual dimorphism in size because females are provided with a longer growing season by virtue of earlier birth. The adaptive significance of TSD in M. menidia appears to be related to this sexual dimorphism and the way in which size affects the relative fitnesses of males and females. Based on a survivorship schedule that includes a winter period of severe, size-selective mortality, selection favors large individuals of either sex in survival to the breeding season. However, large size is apparently more of an advantage to female than male RS during breeding. Atlantic silversides are group spawners that openly broadcast their gametes with those of many others in mass spawning events that are closely timed with recurring environmental cues. Neither aggressive interactions nor assortative mating by size seem to occur and epigamic sexual selection appears to be minimal. In this type of mating system where sperm competition is intense, the RS of males and females should depend primarily on the number of gametes produced. If gonad weights adequately reflect the relative fecundities of each sex, females gain more than males by being large because ovarian weight is a more steeply ascending function of body size than is testicular weight. Some evidence is offered suggesting that the low correlation of testicular weight with male body size may be due to a trade-off between reproductive activities and feeding. TSD in Menidia supports the Charnov-Bull model for the evolution of environmental sex determination. Offspring produced early in the breeding season enter a large patch and will maximize relative fitness if they become female. Similarly, offspring produced late in the breeding season can maximize relative fitness by becoming male. Temperature itself provides no direct benefit to differentiation as a male or female but acts only as a cue to whether offspring will experience a long or short growing season. TSD provides a mechanism by which a sexual dimorphism can arise without sacrificing growth rates in either sex. ESD should also occur in other fishes having a sexual dimorphism in size, prolonged breeding seasons, and maturity early in life.