Glass eel (Anguilla anguilla) acclimation to freshwater and seawater: morphological changes of the digestive tract

Glass eels (Anguilla anguilla, L. 1758) caught during ascent at the mouth of the River Tiber were kept in aquaria with freshwater and full strength salinity (35 %) for four months. Morphological features of glass eels at capture and after four months of experimental rearin are described. The structure of the gut, an important osmoregulatory organ, observed in glass eels in seawater experimental rearing suggests that they undergo an irreversible process of adaptation to freshwater, despite the fact of survival in seawater.