Effects of oestradiol—enriched diet and of feeding with porcine testicular tissue on macroscopic gonadal sex in European eels

Abstract

Oestradiol–enriched food (25 mg kg ‐1 oestradiol) fed for 91 days to small eels (2 or 7 g) with undifferentiated gonads (UD) significantly favoured development of gonads with the macroscopic appearance of ovaries (♀‐gonads) during the next 8 months. Fifty‐four and 51% of control eels and 75 and 78% ofoestradiol‐treated eels developed ♀‐gonads. Minced porcine testicular tissue fed to 2‐g eels for up to 371 days had effects similar to oestradiol‐enriched food (68% with ♀‐gonads), whereas feeding for only 91 days had no effect during the next 8 months. Oestradiol‐enriched food was also fed to larger eels with already macroscopically differentiated gonads (30–70 g; 95% with ♀‐gonads, 5% with ♀‐gonads). After 27 days significantly more eels with a ♀‐gonad or with a mixture of ovary‐like and testis‐like regions in the gonads (♂+♀‐gonads) were found. After 96 days there were 44% with ♀‐gonads, 40% with ♂+♀‐gonads and 16% with ♀‐gonads. Oestradiol thus had a feminizing effect, not only on morphologically undifferentiated gonads but also on morphologically differentiated ♀‐gonads. The presence of sex steroid hormones or their precursors in porcine testicular tissue may also exert a feminizing influence. In all experiments the hormone‐fed groups showed a tendency (not significant) towards increased growth rate. In small eels early rapid growth and differentiation of ♀‐gonads were clearly correlated, both in hormone treated and in control eels. Otherwise no correlation was found between growth rate and gonadal sex.