# Histology and Microspectrophotometry of the Photoreceptors of a Crocodilian, Alligator mississippiensis

A. J. Sillman, S. J. Ronan, E. R. Loew

## Abstract

The photoreceptors in the retina of Alligator mississippiensis were examined by means of in situ, single cell microspectrophotometry and scanning electron microscopy (SEM). The retina is characterized by a system of four cone visual pigments. One type of single cone contains a visual pigment with absorbance maximum ($\lambda _{\max}$) at 535 nm. A second type contains a visual pigment absorbing maximally at 444 nm. The principal member of the double cone has a visual pigment with $\lambda _{\max}$ at 566 nm whereas the accessory member's visual pigment absorbs maximally at 503 nm. There is only one rod pigment and it is virtually identical to that within the accessory cone, absorbing maximally at 501 nm. All visual pigments are based on the retinal or vitamin A$_{1}$ chromophore. SEM confirms that the alligator retina is duplex with rods dominating but cones present in significant numbers. There is a wide range of photoreceptor shapes and sizes throughout the retina