Reflectivity of the Chorioidal Tapeta of Selachians

Abstract

The reflecting layer or chorioidal tapetum in the eye of many selachians was examined to determine, especially, whether it is a set feature or whether it can be occluded.Many selachians have a tapetum under the entire fundus of the eye. It consists of highly reflecting cells, towards the outer ends of which there is a layer of melanophores. In active pelagic sharks of the neritic zone (e.g. Squalus, Mustelus), the melanophores send out pigment over the reflecting cells so as to conceal them when the fish is illuminated; the pigment retreats in darkness. In benthic species of the neritic zone, notably Scyliorhinus, the fundus has a black ventral area; a tapetum occurs elsewhere and is not occlusible. The eyes of rays have a black ventral field, and pigment migration over the tapetum is slight. The tapetum is not occlusible in deep-sea squaloids and a chimaerid Hydrolagus affinis.Reflectivity of the dark-adapted tapetum of Squalus acanthias amounts to about 85%. Full expansion of pigment in the light and retreat in darkness occupy about 2 hr in this fish.Pieces of tapeta, freed of retinae, were mounted in Ringer solution, observed, and photographed by oblique illumination from above. The tapetal pigment migrates inwards over the reflecting plates in such preparations of tapeta from fishes that have occlusible tapeta.The suggestion is offered that the tapetum is concealed, either by closure of the pupil or by migration of pigment, so as to avoid displaying eye-shine. Deep-sea squaloids, living in dimly lit environments, do neither.