RETINAL TRANSFORMATION AT METAMORPHOSIS IN THE WINTER FLOUNDER (PSEUDOPLEURONECTES-AMERICANUS)

Winter flounder (Pseudopleuronectes americanus) are hatched as bilater ally symmetric larvae which live near the ocean surface. At metamorpho sis, they become laterally compressed, one eye migrates to the opposit e side of the head, and they live the remainder of their lives lying o n their blind side on the ocean floor. The present study characterizes and quantifies retinal cell distribution throughout the larval period and contrasts it with the adult retina. Based on light- and electron- microscopic analyses, retinas of larval flounder contain only a single cone-like photoreceptor type, arranged in a hexagonal array. In contr ast, after metamorphosis, the adult retina has three types of photorec eptors: rods, single cones, and double cones. Rod photoreceptors are n umerous in the ventral retina and decrease in density dorsad. The cone photoreceptor density, in contrast to rods, is higher in the dorsal r etina decreasing ventrad. Adult cone photoreceptors are arranged in a square mosaic with four double cones surrounding one single cone. The differences in larval and adult retinal morphology reflect the distinc tly different habitat each occupies.