Abstract
The properties of the visual pigments in several fish species that possess a unique physiological mechanism of changing the corneal colouration depending on the intensity of ambient illumination are described in the present paper. The absorbance spectra of the visual pigments and the spectral characteristics of the cornea have been measured in masked greenling Hexagrammos octogrammus and five fish species of the order Tetraodontiformes using microspectrophotometry. In all the species, retinal photoreceptors contain a mixture of the rhodopsin (A1) and porphyropsin (A2) pigments in different proportions, from 0 to 100%. The experiments with double cones in a chloride-free medium have revealed a shift of λmax in some of the pigments to the shortwave region (ionochromic effect). This suggests that pufferfishes have one pigment of the LWS type (λmax = A2_567 nm), while greenling has two LWS-pigments (λmax = A2_625, and A1_560 nm). The shift of the spectral absorbance of double cones towards the longwave region in pufferfish, as compared to that in most other shallow-sea fishes, is probably explained by the presence of a changeable yellow-orange carotenoid corneal filter.
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