GONADAL DIFFERENTIATION, INTERSEXUALITY AND SEX-RATIOS OF EUROPEAN EEL (ANGUILLA-ANGUILLA L) MAINTAINED IN CAPTIVITY

Migrating European eel (Anguilla anguilla L.) glass eels were collecte d annually in the Garonne Basin (France) during 1987-1989 and subseque ntly raised under freshwater conditions. In 1990 gonadal development w as determined and after about 1 year it was re-examined to check if se x ratios had changed. Undifferentiated gonads were seldom found, and w ere mainly present in the youngest year sample 89-1+ (numbers refer to year of catch followed by freshwater age), This gonadal stage can dev elop directly into an ovary. Differentiation of male sex proceeded via an intersexual stage with male and female sex cells. Phenotypic expre ssion of female gonadal sex was considered as being gonochoristic and differentiated. The female gonad gradually developed towards a fourth stage ovary, characterised by the presence of follicles containing ooc ytes arrested in the late prophase of meiosis. Yolk formation in oocyt es was not observed. The intersexual progression to males was termed p re-maturational sex change. The testes of full grown males contained n umerous tubules with spermatogonia in mitotic arrest, The percentage o f intersexual eels in 1990 of the youngest year sample (89-1 +) was ve ry high (91%). Most of these animals differentiated into males in 1991 (89-2 +). The percentages of undifferentiated and intersexual eels in the older year samples of 1988 and 1987 examined in 1990 were low, sh owing that sex differentiation in most animals had occurred after 2-3 years. The sex ratios expressed as males/(males + females) were 64% (8 8-2 +) and 76% (88-3 +); 84% (87-3 +) and 83% (87-4 +), Comparison of these sex ratios suggests that sex reversal did not occur, Selective m ortality and migration, population density, feeding conditions and tem perature are reviewed as possible factors controlling sex differentiat ion in European eel. (C) 1997 Elsevier Science B.V.