The outer retina and tapetum lucidum of the snook Centropomus undecimalis (Teleostei).

Abstract

The snook Centropomus undecimalis is a nocturnal fish inhabiting turbid estuaries, bays, and inshore waters. Its retina is duplex, containing rods and short single, long single, and double cones. Cones are arranged in a square mosaic; the rods are extremely numerous and outer segments are short and staggered in depth. Ellipsoid mitochondria undergo morphological alterations along the vitreo–scleral axis including the appearance of elongate cristae and the development of electron-dense matrices which nearly obscure the cristae. Peripheral mitochondria give off tubular elements in the calycal processes. Pigment epithelial cells have long processes which extend vitread to the cone ellipsoids and contain tapetal spheres (0.3 to 0.5 μm). The processes form a diffusely reflecting tapetum about 80 μm thick. In the scotopic eye the tapetum is uncovered when the retinal pigment retreats into the cell bases. The reflecting material is a triglyceride, largely glyceryl tridocosahexaenoate. Its high refractive index (n = 1.50) and the close packing of the spheres cause light to be backscattered. Reflection from the white lipid tapeta is about 50%; quantal catch in the retina because of the tapetum is enhanced by a factor of about 1.5.