HISTOLOGICAL STUDY OF THE DEVELOPMENT AND SEX-DIFFERENTIATION OF THE GONAD IN THE EUROPEAN EEL

The histological structure of the gonads was studied in yellow eels sa mpled from a coastal lagoon and from stocks reared in an aquaculture p lant showing different sex ratios. Gonad development related to body s ize rather than to age and underwent an intermediate stage characteriz ed by a structure of an early testis but containing oogonia and oocyte s. This gonad was called the Syrski organ and the stage juvenile ambis exual. Ovaries were found in eels from 22-30 cm in length, possibly de rived from undifferentiated gonads or from Syrski organs. Fully differ entiated testes were found in eels >35 cm, derived from Syrski organs. These observations support the results of previous research. From elv ers and in eels up to 15-16 cm in length, growth of the gonadal primor dium is due to primordial germ cell migration. In eels >15 cm multipli cation of primordial cells begins. Oogonial clones were found in eels >18 cm in length, while spermatogonium B clones were observed in eels >30 cm in length. The dynamics of sex differentiation was different am ong stocks with different ultimate sex ratios: ovaries were found in s horter eels in stocks with a prevalence of females, in longer eels in stocks with a prevalence of males. This result supports the hypothesis of a metagametic (environmental) sex determination. The somatic cells in contact with germ cells and those in the interstitium appeared ear ly during gonad development and preceded germ cell differentiation. Th is suggests that somatic cells are the targets of the environmental fa ctors influencing sex differentiation. (C) 1996 The Fisheries Society of the British Isles