Retinal fine structure in the European eel Anguilla anguilla. VI. Photoreceptors of the sexually immature silver eel stage

This report deals with the fine structure of the photoreceptors of the sexually immature silver eel and compares these findings with previous observations on the glass and yellow eel stages of the life cycle of the European eel. The photoreceptors of the sexually immature glass eel are readily categorized as either rods or cones. No multiple cones are observed. The rod:cone ratio is about 100:1, which is greater than that of either the glass or yellow eel stages. Rods and perhaps cones also are capable of retinomotor responses. In the light-adapted condition, rod inner segments are the same width as the outer segments but narrow abruptly below the ellipsoid to form the myoid. Rod outer segments in the immature glass eel are longer but much the same diameter as that noted for the glass and yellow eel. Cone inner segments are much wider than the outer segments which taper distally. Both rods and cones display organelles in the inner segment region indicative of protein production. Most of the cones observed at this stage showed some degenerative signs mostly as empty vacuoles within their cytoplasm. Cone nuclei are large and vesicular and in the light-adapted state are invariably located sclerad to the external limiting membrane while rod nuclei are small and dense and located vitread to this membrane. Both rods and cones display both invaginated and superficial synaptic sites. The most obvious changes noted in the immature silver eel photoreceptor population are a marked rise in the rod:cone ratio and a lengthening of rod outer segments.