Biotransformation enzymes catalyze oxidation of a diversity of xenobiotics, doing a key role in its detoxication. CYP1A and GST expression, induced by pollutants, seems to be highly conserved across vertebrate taxa. Since this system acts as a defense mechanism, the intra or interspecies differences have been attributed to adaptations to different habitats and trophic strategies. Thus, the main goal of this study was to evaluate the presence and inducibility of biotransformation enzymes in Petromyzon marinus (agnathan), because a clear causal link between toxicological effects on individuals and population responses have demonstrated reduced abundance in wild populations. Sampling occurred at the beginning of the sea lamprey downstream migration in three Portuguese river basins: Lima, Vouga, and Mondego
