Knowledge of how the optimum temperature for growth (T opt ) varies during ontogeny, and how close it is to the temperatures that induce phenotypic masculinization is fundamental to the understanding of the evolution of thermolabile sex determinism (TSD) in fishes. In blue tilapia Oreochromis aureus, T opt is 32·6 C at the start of exogenous feeding (10 mg fish) and it decreases by c. 1 C each time that the fish body mass increases by an order of magnitude. Temperatures <35 C are not sufficient to induce complete phenotypic masculinization. Based on a multiple-regression model (r 2 =0·938) plotting growth against body mass and water temperature, genotypically female tilapia living at high temperatures during the thermosensitive period (21-28 days) and being reversed into phenotypic males would incur an initial growth disadvantage over fish living at T opt , but not over those living at slightly colder temperatures (27-29 C). This initial disadvantage would be later compensated for by faster growth because of between-sex growth dimorphism to the detriment of phenotypic females. These arguments suggest that there is no definite pressure against the selection of TSD in blue tilapia and probably other Oreochromis spp.

Knowledge of how the optimum temperature for growth (T opt ) varies during ontogeny, and how close it is to the temperatures that induce phenotypic masculinization is fundamental to the understanding of the evolution of thermolabile sex determinism (TSD) in fishes. In blue tilapia Oreochromis aureus, T opt is 32·6 C at the start of exogenous feeding (10 mg fish) and it decreases by c. 1 C each time that the fish body mass increases by an order of magnitude. Temperatures <35 C are not sufficient to induce complete phenotypic masculinization. Based on a multiple-regression model (r 2 =0·938) plotting growth against body mass and water temperature, genotypically female tilapia living at high temperatures during the thermosensitive period (21-28 days) and being reversed into phenotypic males would incur an initial growth disadvantage over fish living at T opt , but not over those living at slightly colder temperatures (27-29 C). This initial disadvantage would be later compensated for by faster growth because of between-sex growth dimorphism to the detriment of phenotypic females. These arguments suggest that there is no definite pressure against the selection of TSD in blue tilapia and probably other Oreochromis spp.