Diffuse reflectance of retinal tapeta lucida, with special reference to drums (Sciaenidae)

Abstract

Reflection of light by ocular tapeta lucida of drums (Sciaenidae) was studied in Cynoscion arenarius Ginsburg, C. nebulosus (Cuvier), Micropogon undulatus (L.) and Leiostomus xanthurus Lacèpéde. The tapetum is located in the pigment epithelium of the eye, is matt white, and behaves as a uniform diffuser. The pigment epithelium contains densely packed tapetal spheres; these are randomly arranged, have a diameter of about 0.4 μm and contain a lipid that has a refractive index of 1.50. Because of their dense packing they backscatter much of the incident light. Diffuse reflectance is about 50%. Measured values of reflectance are compared with those computed for a model system having the same physical characteristics, and they are in good agreement. The optical density of the visual pigment at the wavelength of maximal absorption (λ 510 nm) is 0.33, and the tapetum increases light absorption in the dark-adapted eye by a factor of 1.3. Reflectance of croaker tapetum is compared with reflectances of tapeta of Lepisosteus, Arius, and Notopterus afer, which contain different reflecting materials.