Vision and visual pigments in a fish, Scardinius erythrophthalmus (the rudd)

Abstract

Two visual pigments may be extracted from the eye of the rudd, one based on vitamin A 1 with λ max at 507 nm, and the other based on vitamin A 2 with λ max at 535 nm. The proportion of the two pigments depends on daylength, VP507 1 increasing relative to VP535 2 with long daylengths and vice versa. Thresholds for different spectral stimuli were obtained behaviourally under photopic conditions, in a two choice discrimination situation, for fish adapted to 20 hr, 12 hr and 4 hr daylengths. The stimuli were superimposed on a tungsten light background, and photopic conditions were further maintained by an illuminated lid. Thresholds were also obtained for tungsten (“white”) light. The spectral sensitivity curves showed two clear maxima, at about 620 nm and 510 nm. There was a further less well defined maximum at short wavelengths. Spectral sensitivity was not affected by daylength, ever thought retinal extracts from fish kept under identical conditions showed marked changes in the proportions of the two visual pigments. This suggests that the pigments of the photopic receptors are unaffected by daylength, ever thought this has a large effect on the scotopic pigments. Two models of interaction between receptors at threshold were considered. It was concluded that an envelope model, where the threshold depends only on the most sensitive receptor, describes the results with both spectral and tungsten white light stimuli better than an additive model, in which it is assumed that the outputs of the different receptors summate at threshold.