Single and Multiple Visual Systems in Arthropods

Abstract

Extraction of 2 visual pigments from crayfish eyes prompted an electrophysiological examination of the role of visual pigments in the compound eyes of 6 arthropods. The intact animals were used; in crayfishes isolated eyestalks also. Thresholds were measured in terms of the absolute or relative numbers of photons/flash at various wavelengths needed to evoke a constant amplitude of electroretinogram, usually 50 uv. Two species of crayfish, as well as the green crab, possess blue- and red-sensitive receptors apparently arranged for color discrimination. In the northern crayfish, Orconectes virilis, the spectral sensitivity of the dark-adapted eye is maximal at about 550 m[mu], and on adaptation to bright red or blue lights breaks into 2 functions with Amax respectively at about 435 and 565 mu, apparently emanating from different receptors. The swamp crayfish, Procambarus clarkii, displays a maximum sensitivity when dark-adapted at about 570 mu, that breaks on color adaption into blue-and red-sensitive functions with Amax about 450 and 575 mu, again involving different receptors. Similarly the green crab, Carcinides maenas, presents a dark-adapted sensitivity maximal at about 510 mu, that divides on color adaptation into sensitivity curves maximal near 425 and 565 mu. Each of these organisms thus possesses an apparatus adequate for at least 2 color vision, resembling that of human green-blinds (deuter-anopes). The visual pigments of the red-sensitive systems were extracted from the crayfish eyes. The horse-shoe crab, Limulus, and the lobster each possesses a single visual system, with ^max respectively at 520 and 525 mu. Each of these is invariant with color adaptation. In each case the visual pigment had already been identified in extracts. The spider crab, Libinia emarginata, presents another variation. It possesses 2 visual systems apparently differentiated, not for color discrimination but for use in dim and bright light, like vertebrate rods and cones. The spectral sensitivity of the dark-adapted eye is maximal at about 490 mu, and on light adaptation, whether to blue, red, or white light, is displaced toward shorter wavelengths in what is essentially a reverse Purkinje shift. In all these animals dark adaptation appears to involve 2 phases: a rapid, hyperbolic fall of log threshold associated probably with visual pigment regeneration, followed by a slow, almost linear fall of log threshold that may be associated with pigment migration.