Electrophysiological and Histological Observations on the Eye of Adult, FemaleDiastylis rathkei(Crustacea, Malacostraca, Cumacea)

Abstract

The approximately 200 µm wide eye of Diastylis rathkei consists of two closely apposed eye halves with four lenticular complexes measuring 40 µm in diameter in each. Each lenticular complex consists of a lens rich in 30 nm electron-opaque glycogen-like particles made up of smaller (5-6 nm) subunits, and a rhabdom comprising regularly aligned microvilli. The retinula cell somata, which are in a proximal location, are linked with the distally placed rhabdom via approximately 10 µm thick, cellular strands. The strands are surrounded by cells crowded with reflecting organelles of ca. 0.8 µm in diameter.Dark/light adaptational changes affect the position of uniformly spherical organelles measuring 0.4-0.5 µm in diameter and presumed to contain carotenoids, the overall size of the rhabdom, and the diameter of individual microvilli. The latter measure 75 nm in the light-adapted state and 90-120 nm in the dark-adapted state. There is ultrastructural evidence (swollen and abundant endoplasmic reticulum and widely distributed glycogen-like particles) that, under light-adapted conditions, the retinula cells are in a phase of intense metabolic activity.A multilamellar structure, similar in appearance to that found in the organ of Bellonci of other crustaceans, but also resembling a trophospongium, was noticed in close proximity to the eye within the optic lobe. Extracellular electrophysiological recordings obtained with NaCl-filled glass electrodes consisted of a cornea-negative potential change and reached a maximum amplitude of nearly 400 µV to 300 ms flashes of white light.Superimposed spectral response curves from eight different animals, based on a criterion amplitude of 50 µV, were nearly congruent in shape and displayed one single sensitivity peak to light of 512-549 nm in wave length. Intensity/response curves obtained to light of 472, 549, and 628 nm wavelengths and the single spectral sensitivity peak strongly suggest that only one type of excitatory visual pigment is involved in the visual process of D. rathkei.It is concluded that in spite of its tiny size, the eye of D. rathkei could be useful in the coordination of reproduction and synchronization of vertical migrations.