Comparative Morphology of the Eye (with Particular Attention to the Retina) in Various Species of Cardinal Fish (Apogonidae, Teleostei)

Various parameters of the eye dimension and structure have been compared in 15 species of cardinal fish (Apogonidae), including both nocturnal and diurnal forms, mostly inhabiting rocky habitats in tropical and subtropical regions. In general, in the nocturnal forms the eye and retina are larger than in the diurnal fish of similar dimensions. In the nocturnal species, eye diameter to body length is ca. 12-13%, whereas in the diurnal species it is less than 10%. Retina size in adult fish of the various species varies from 20 mm² to 183 mm². Cytological examination of the studied retinas revealed that they are composes, additional to rods (20-40 μm), of both bulbous and slender double cones, as well as single cones. These cones form a mosaic comprising four double with one single in the center, a pattern that is less prominent at the periphery of the retina and more so in the fundus. The rod ellipsoids reveal normal mitochondria, whereas the cones bear ellipsoids featuring opaque and unusual, ellipsosome-like mitochondria. The number of rods in a retinas varies from 15 to 128 million, and the number of cones from 460,000 to 5,673,000. As revealed in cardinal fish of similar dimensions, the number of visual cells found in the retina is much higher in the nocturnal than in the diurnal species. The ecological and developmental aspects of the observed phenomena are discussed.