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The temperature sex determination (TSD) mechanism in the European sea bass (Dicentrarchus labrax L.) was studied in respect to: a) the TSD sensitivity during the different developmental stages; and b) the intrapopulation correlation of sex determination with the growth rate up to the end of the TSD-sensitive period. At the stage of half-epiboly, eggs from the same batch were divided into four groups and subjected to different thermal treatments: a) 15°C (G15 group) and b) 20°C (G20 group) up to the middle of metamorphosis stage; c) 15°C up to the end of yolk-sac larval stage and subsequently to 20°C (G15-5 group); and d) 15°C up to the end of the preflexion stage and then to 20°C (G15-10 group). At the end of the treatments, size grading was applied and four additional populations were established from the upper (L) and lower (S) size portions of the G15 and G20 populations: G15L, G15S, G20L, and G20S. During the following growing phase, all populations were subjected to common rearing conditions. The sex ratios of each population were macroscopically determined at 190–210 mm mean total length. Female incidence was significantly affected (P < 0.05) by the different thermal treatments: 66.1% in the G15, 47.1% in the G15-10, 37.6% in the G15-5, and 18.1% in the G20 group. In addition, sex ratio was correlated with the growth rate of the fish up to the end of the TSD-sensitive period, with the larger fish presenting a significantly higher (P < 0.01) female incidence than the smaller fish in both thermal regimes tested: 73.1% in G15L vs. 57% in G15S, and 36.6% in G20L vs. 22.5% in G20S group. Results provide, for the first time, clear evidence that the sea bass is sensitive to TSD during all different ontogenetic stages up to metamorphosis, and that sex ratio is correlated with the growth rate of the fish well before the differentiation and maturation of the gonads. J. Exp. Zool. 292:573–579, 2002. © 2002 Wiley-Liss, Inc.

To test the hypothesis that sex determination in the European sea bass (Dicentrarchus labrax L.) Can be affected by the incubating temperature during the very early developmental stages, eggs from the same batch of spontaneously spawned broodstock were divided at the stage of half-epiboly into three groups according to rearing temperature: G13 = 13°C, G15 = 15°C, and G20 = 20°C. Temperature treatment lasted until the middle of metamorphosis (17–18 mm total length, [TL]), and, with the exclusion of water temperature, all biotic and abiotic conditions were identical for the three experimental groups. The on-growing phase was performed under ambient photoperiod and temperature conditions for all groups. Sex proportions were determined by histological examination of the gonads of fish at 308, 467, and 568 days posthatch (DPH). At 308 DPH (TL: 135–201 mm), 100% of the specimens had differentiated into males and females. A significantly higher (P < 0.01) proportion of females was found in groups G13 (72–74%) and G15 (67–73%) than in group G20 (24–28%). At the final sampling there was no statistically significant difference in body weight between the experimental groups. However, in all groups, female fish were larger than males (P < 0.001). Results provide for the first time clear evidence that temperature during the very early developmental stages is the crucial factor affecting the process of sex differentiation of the sea bass, with low rearing temperatures (13 or 15°C) resulting in sex proportions consistently skewed in favor of females. J. Exp. Zool. 287:225–232, 2000. © 2000 Wiley-Liss, Inc.