Phenotypic sex differentiation of blue tilapia under constant and fluctuating thermal regimes and its adaptive and evolutionary implications

Oreochromis aureus exposed juring the first 28 days of exogenous feeding to constant 35 degrees C, or fluctuating temperatures (day at 35 degrees C, n ight at 27 degrees C, and vice versa) showed significantly (P<0.05) faster growth, least size heterogeneity and better survival rates than siblings un der constant 27 degrees C. Constant high temperatures had a strong masculin izing effect (M : F sex ratios of 7.33-19.00 : 1.00 v. 0.75-0.82 : 1.00 in controls reared at 27 degrees C). Fluctuating temperatures had less masculi nizing 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 p ressure for the selection of thermolabile sex-determinism in this species, and presumably in other Oreochromis spp. The evolutionary advantage of ther mosensitivity in Oreochromis spp. is discussed. considering a framework whe re individual advantages oppose, to some degree, to the population or speci es interest. (C) 2000 The Fisheries Society of the British Isles.