No abstract

Oreochromis aureus exposed during the first 28 days of exogenous feeding to constant 35° C, or fluctuating temperatures (day at 35° C, night at 27° C, and vice versa) showed significantly (P<0·05) faster growth, least size heterogeneity and better survival rates than siblings under constant 27° C. Constant high temperatures had a strong masculinizing effect (M: F sex ratios of 7·33–19·00: 1·00 v. 0·75–0·82: 1·00 in controls reared at 27° C). Fluctuating temperatures had less masculinizing potential but still produced sex ratios significantly skewed to the detriment of females (M: F sex ratios of 2·33–11·50: 1·00). This suggests that ambient temperature may have represented a sufficient environmental pressure for the selection of thermolabile sex‐determinism in this species, and presumably in other Oreochromis spp. The evolutionary advantage of thermosensitivity in Oreochromis spp. is discussed, considering a framework where individual advantages oppose, to some degree, to the population or species interest.

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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.

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