Retinal fine structure in the European eel Anguilla anguilla. IV. Photoreceptors of the yellow eel stage.

The complex life cycle of the European eel (Anguilla anguilla) involves a long catadromous migration during which the eye undergoes a number of morphological alterations. This report deals with the fine structure of the photoreceptors of the yellow eel stage and is part of a comparative study of the retina of the eel during the major stages of its life cycle. The photoreceptors of the yellow eel are easily divisible into rods and single cones and the rods at least appear to be capable of retinomotor responses. Rods are more elongated and more numerous than cones. In the light-adapted state the inner segment of rods is of the same diameter as the outer segment but narrows abruptly in the myoid region. Cone inner segments on the other hand are much wider than the outer segments which taper distally. Both rods and cones display organelles in the inner segment indicative of high metabolic activity. Cone nuclei are located scleral to the external limiting membrane while rod nuclei are vitreal to this boundary. Both rods and cones display both invaginated and superficial synaptic sites. Except for an apparent increase in number and outer segment width of the rods, the photoreceptors of the yellow eel have changed only slightly from that described for the glass eel stage.