The rapid synthesis and destruction of photoreceptor membrane by a dinopid spider: a daily cycle

Abstract

The receptive segments of the posterior median eyes of Dinopis subrufus are hexagonal in transverse profile, and each of the six faces is composed of a rhabdomere. Supportive cell processes are reduced to strands at the junction of the rhabdoms, and each of the six rhabdomeres, therefore, is contiguous with the rhabdomere of a different receptive segment; this arrangement allows the possibility of both optical and electrical coupling between cells. During the day, receptive segments are short, and rhabdo­mere membrane occupies only a small proportion of their volumes. At nightfall, the segments lengthen, and novel membrane is added in a rapid burst of synthesis almost to fill them. At dawn, the sequence is reversed, and membrane is removed as pinocytotic vesicles which are assembled into multivesicular bodies and lysed in the inter-rhabdomeral cytoplasm and in the swollen receptor axons which underlie the retina. Synthesis and destruction of membrane are shown to be controlled in part by immediate states of retinal illumination, superimposed upon a daily rhythm. It is argued that the evolution of this metabolically extravagant system is unlikely to be primarily concerned with the manipulation of states of adaptation, and some alternative hypotheses are proposed to account for it.