Guanine-Type retinal tapetum of three species of mormyrid fishes

The eye of mormyrid fishes (Marcusenius andGnathonenius) contains a retinal tapetum composed of guanine crystals. InMarcusenius, the quantity of guanine is about 2 mg cm^?2 of the retinal surface area. The retina is duplex, and the cones and rods are grouped in bundles. Each bundle is surrounded by pigment epithelial cell processes which contain numerous guanine reflectors. Two kinds of reflector are present: brick-shaped and rodlet. Mormyrids may use their high sensitivity for nocturnal activities. The retinal features of mormyrid fishes were compared with those of other fish species belonging to the Notopteroidei such as the Hiodontidae, Notopteridae and Gymnarchidae, and related to the chemical nature of notopterid and gymnarchid tapetum.