The Divided Eye of the Isopod Glyptonotus antarcticus: Effects of Unilateral Dark Adaptation and Temperature Elevation

V. B. Meyer-Rochow

Abstract

The literature on the structure and function of isopod compound eyes is briefly reviewed. Unlike other isopods studied, Glyptonotus antarcticus possesses physically separated large dorsal compound eyes and small ventral compound eyes. G. antarcticus turns upside down when it swims, and it seems that this is when the ventrally located eyes become useful. Structurally, the two types of eye are very similar: both consist of individual ommatidia, which in an adult specimen can be 80--100 $\mu $m wide and 300 $\mu $m long. Each ommatidium contains a bipartite crystalline cone and a long rhabdom, approximately 100 $\mu $m long, which is characteristically star-shaped when sectioned transversely. Sometimes five and sometimes six retinula cells contribute to the formation of the centrally located, fused and unbanded rhabdom. Dark--light adaptational changes were difficult to demonstrate and did not occur until one eye was kept covered and shielded from light for one week, while the other one remained uncovered. In eye pairs of five animals treated in this way, it was obvious that prolonged darkness leads to an outward migration of retinulascreening pigment granules, to the formation of multivesicular bodies in the retinula cells, and to an increase in size and abundance of spherical organelles in the interstitial cells. Exposure to light, on the other hand, results in an inward (towards the rhabdom) migration of retinula cell screening pigment granules, the formation of multilamellar bodies through pinocytotic processes at the rhabdom edge, and a swelling of interstitial cells. Temperature elevation alone mimics the effects of bright light with regard to pigment granule migration. It is suggested that, when pigment granules absorb radiation during exposure to light under normal environmental temperature conditions, they may heat up their immediate surroundings sufficiently to contribute to membrane damage.