Tapeta lucida in the eyes of the seatrout (Sciaenidae)

Abstract

Eyes of the seatrout (Sciaenidae) contain a tapetum lucidum, located in processes of the pigment epithelial cells. The tapetum is a white diffuse reflector, occluded by black retinal pigment by day, and exposed by retreat of pigment at night; threshold for pigment migration is around 10 lx. Reflexion is caused by backscattering of light from a multitude of small spheres, each about 0.4 μ m in diameter, packing the cell processes. Material has been examined by electron microscopy, employing epoxide sections and freeze-etch techniques. The spheres have homogeneous, densely staining contents and are surrounded by thin boundary membranes. The reflecting material is a lipid, it has been extracted, purified by column and thin-layer chromatography, and characterized by TLC and GLC. On TLC it runs at the same rate as triglycerides, and is demarcated according to class from wax esters, sterols and diacyl glyceryl ethers. It contains predominantly one fatty acid, n — C22 : 6 ( ≥ 95%), and appears to be largely tridocosahexaenoin. The tapetum contains 1.5 mg of triglyceride per square cm. The relationship of the tapetum to the photoreceptors and some characteristics of the latter are described, and reference is made to the occurrence of similar lipid tapeta in other families of fishes.