Development of the retinal tapetum lucidum of the walleye (Stizostedion vitreum vitreum).

The development of the retinal tapetum lucidum within the cells of the retinal pigment epithelium (RPE) has been investigated by both light and electron microscopy in the walleye (Stizostedion vitreum vitreum) in specimens ranging in total length from 25-140 mm. In addition changes in the arrangement of the photoreceptors (both rods and cones) in both light and dark-adaptation have also been studied. At 25 mm no evidence of a tapetum is present. At about 30 mm it makes its initial appearance as granular bodies formed within the apical smooth endoplasmic reticulum (SER) cisternae of the RPE cells in the superior temporal fundus. The developing tapetum then spreads peripherally and continues to thicken in existing areas. By 90 mm it is well established throughout the fundus but always appears better developed in the superior fundus. By 125-140 mm it is essentially adult in appearance. At 60-70 mm the rods and cones begin to form bundles producing macroreceptors of 20-30 photoreceptors. In dark-adaptation the rod bundles are retracted and have one or more cone cells centrally located in each bundle, with the bundles separated from one another by melanosomes. Initially when no tapetal material is present, post-larval walleye are positively phototactic and feed on zooplankton. In the adult condition when a tapetum lucidum and large macroreceptors are present, the walleye is negatively phototactic and feeds almost exclusively on larger organisms such as other fish.