Changes in retinal morphology during growth were studied in three species of Merluccius, i.e., M. capensis Castelnau and M. paradoxus Franca from the southeastern Atlantic and M. merluccius L. from the Mediterranean. The structure of both the rod and the cone systems showed both inter-and intraspecific differences which reflect functional differences in both sensitivity and viusal acuity. The interspecific differences in estimated sensitivity recorded were related to differences in depth distribution and the extent of nighttime feeding activity. In the three species studied, the increase in sensitivity and visual acuity with growth was associated with the increase in habitat depth and changes in diet. The results obtained indicated that the morphology of the visual systems changes during the lifetime of the fish, adapting continuously to environmental conditions and behaviour

14 pages, 8 figures, 2 tables