Unusual cases of adult intersexuality in the European eel Anguilla anguilla

Two cases of intersexuality are reported for the first time in European eel, Anguilla anguilla (at the beginning of the silver eel stage), within 140 fish caught as glass eels in the south-west of France and reared in tanks at 17°C. Cysts containing spermatozoa were observed in ovaries with pre-vitellogenic oocytes. This feature is very uncommon, especially owing to the fact that male cells do not normally reach this stage in captivity, and an environmentally controlled transdifferentation process may not be excluded. Besides, the expression of the gonadal aromatase gene (cyp19a1a) was found to be higher in these 2 intersexual fish compared to normal females, although these results must be considered with caution since only 2 intersexual fish were available. A possible feminizing effect of this 'abnormal' upregulation of aromatase is discussed.