Blackwell Publishing Journal Backfiles 1879-2005
Starting from 66 days post hatching (dph), European sea bass Dicentrarchus labrax were graded successively to create a fast growing (L-extreme) and a slow growing (S-extreme) population. The L-extreme population grew significantly larger (ANOVA, n = 89–101, P 〈 0·01) attaining twice the wet mass of the S-extreme population at 300 dph (130·9 ± 1·8 v. 66·7 ± 0·9 g, mean ± s.e.). When the two populations were sexed, the L-extreme consisted of 96·5% and the S-extreme of 30·2% females, while the ungraded control had 59·2% females. Sex differentiation began first in females at a total length (LT) of 97 ± 4 mm and wet mass of 9·4 ± 1·2 g (150 dph), and was completed when fish reached 166 ± 6 mm and 53·4 ± 6·4 g (250 dph) in both sexes. Precocious maturation in males was positively correlated to growth. Gonad oestradiol (E2) was significantly higher in the female-dominant population at the onset of ovarian differentiation (ANOVA, n = 10, P 〈 0·05) and in the plasma after the appearance of the first primary oocytes (P 〈 0·01). Gonad testosterone (T) increased in both populations after sex differentiation (ANOVA, n = 10, P 〈 0·05), while plasma levels were significantly higher in the male-dominant population (P 〈 0·001). Both gonad and plasma 11-keto testosterone (11-KT) were significantly higher in the male-dominant population (ANOVA, n = 10, P 〈 0·01) reaching maximal values at spermiation. The results suggest that E2 is closely related with ovarian differentiation and the onset of oogenesis, while T and 11-KT is more related to spermatogenesis and precocious maturation.
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