Structural features and adaptive properties of photoreceptors in the skate retina.

In the duplex (rod/cone) retinae of most vertebrates, electrical responses associated with nocturnal vision can be recorded in relative isolation only over a limited intensity range before there is encroachment by signals from the cone system; e.g., at mesopic levels of illumination, the rods begin to saturate, and the cone mechanism is brought into play. On the other hand, the retinae of some sub-species of skate appear to contain only one class of photoreceptors, namely rods. Nevertheless, skate photoreceptors, as well as other retinal neurons, are able to respond to incremental stimuli presented on background fields so intense that 90% or more of the available rhodopsin is bleached during the exposure. These and other findings raise some doubts as to whether the skate has, in fact, an all-rod retina. In this paper, we present a body of evidence--based on the results of photochemical, anatomical, and electrophysiological studies--to support the view that only a single class of photoreceptor subserves vision in the skate retina. In addition, recent findings will be described that: a) demonstrate how the visual cells transform from sluggishly responding rods to brisk, cone-like elements, and b) may provide some insight into the functional significance of this unusual adaptive property.