The Effect of Screening Pigment Migration on Spectral Sensitivity in a Crayfish Reflecting Superposition Eye

Abstract

ABSTRACT In crayfish, there is some evidence that visual pigments in the eye affect the spectral sensitivity, both at the receptor level (Goldsmith & Fernandez, 1968; Kong & Goldsmith, 1977) and in the optic fibres (Woodcock & Goldsmith, 1970). However, there is a discrepancy between the absorption spectrum of rhodopsin as measured by microspectrophotometry (λmax is 530–550 nm, Goldsmith, 1978) and published measurements of spectral sensitivity [e.g. electroretinogram (ERG) peaks of 565–570 nm (Kennedy & Bruno, 1961) and intracellular recordings of 556 nm and 600–620 nm (Nosaki, 1969) and 538–634 nm (Waterman & Fernandez, 1970)]. Woodcock & Goldsmith (1970) suggested that sensitivity maxima in the orange and red were due to the non-alignment of the stimulating beam with the optical axis of the cell, thereby allowing light to filter obliquely through red and brown proximal screening pigment surrounding the rhabdom. Goldsmith (1978) showed (theoretically) that the red-brown proximal screening pigments of crayfish and lobster are indeed capable of shifting the effective absorption of the rhabdoms enough to account for the discrepancy between the absorption peak of the visual pigment and the published measurements of spectral sensitivity. However, the effect on spectral sensitivity of the migration of the proximal screening pigment has not been documented.