MORPHOLOGY AND IMMUNOHISTOCHEMISTRY OF THE CAUDAL NEUROSECRETORY-SYSTEM OF A MARINE EURYHALINE FISH, DICENTRARCHUS-LABRAX, ACCLIMATED TO LOW-SALINITY

The morphology and the immunohistochemistry of the caudal neurosecreto ry system of Dicentrarchus labrax were analysed after the fish acclima tation to fresh water. Two different sized neurosecretory cells have b een identified in seawater-acclimated fish and denoted as anterior and posterior cells. The two types of cells share similar patterns and in tensities of immunoreactivity with urotensin I and urotensin II antise ra. Posterior neurosecretory cells decreased in size in freshwater-acc limated specimens, whereas anterior cells did not show changes. Urophy ses of seawater- and freshwater-acclimated fish reacted with both anti sera. Immunohistochemistry of the urophysis did not provide evidence f or a significant difference in the activity of the caudal neurosecreto ry system between seawater- and freshwater-acclimated fish. Ho Never, on a morphological basis it was possible to suggest that the caudal ne urosecretory system of the sea bass is involved in the hyperosmoregula tion.