GLASS EEL (ANGUILLA-ANGUILLA) ACCLIMATION TO FRESH-WATER AND SEAWATER- MORPHOLOGICAL-CHANGES OF THE DIGESTIVE-TRACT

Glass eels (Anguilla anguilla, L. 1758) caught during ascent at the mo uth of the River Tiber were kept in aquaria with freshwater and full s trength salinity (35 parts per thousand) for four months. Morphologica l features of glass eels at capture and after four months of experimen tal rearing are described. The structure of the gut, an important osmo regulatory organ, observed in glass eels in seawater experimental rear ing suggests that they undergo an irreversible process of adaptation t o freshwater, despite the fact of survival in seawater.