A re‐investigation and re‐interpretation of the cumacean photoreceptor

Abstract

Evidence for and against the view that the singular eye in Diastylis rathkei (Cumacea) represents a regressed compound eye is summarised and supplemented with new ultrastructural observations. Neither in the adult nor in the larval manca stage are even traces of a facetation found. The eye consists of two closely apposed eye halves and the four lenticular complexes in each appear to increase in size with age along an isometric growth curve. Each lenticular complex consists of a lens rich in glycogen‐like particles and a rhabdom made up of regularly aligned microvilli 0.075 μm in diameter. No more than three retinula cells contribute to each lenticular compkx. Retinula cells contain presumed carotenoid bodies 0.4–0.5 μm in diameter and clusters of electron‐opaque glycogen matcrial near the proximally‐located nuclei. The bulk of the eye is occupied by cells crowded with reflecting vesicles of approximately 0.8 μm diameter. Though it appears too early to offer a final and decisive conclusion as to the nature and origin of the eye of D. rathkei, comparisons with compound eyes of other peracaridan crustaceans and anatomical parallels to isopod and other malacostracan ocelli make clear that the long‐held view that the cumacean photoreceuptor represents a regressed compound eye is not necessarily the correct one.