Unusual A1/A2-visual pigment conversion during light/dark adaptation in marine fish

Changes in visual pigments were studied in two marine fish species, the masked greenling Hexagrammos octogrammus and the prickleback Pholidapus dybowskii. A microspectrophotometric (MSP) analysis showed that the rods and cones of the fish collected from the natural marine environment in summer or kept in a tank at a high illumination level predominantly contained porphyropsins based on chromophore A2. As a result, λ_max of the double cones significantly shifted to longer wavelengths, reaching 625 and 609 nm, respectively. After several weeks of dark adaptation, the spectra of all the photoreceptor types shifted to shorter wavelengths, as the A1:A2 ratio switched to A1. The MSP data from the fish kept under controlled light conditions were confirmed by chromatography (HPLC), which showed that the changes in the chromophore ratio were reversible and independent of the water temperature. After the preliminary deep dark adaptation, the first noticeable shift in the pigment ratio from A1 to A2 occurred within two weeks of exposure to bright light. A novel finding in this study was a reverse polarity of A1/A2 changes, unlike the case in most other fish species, where A2 chromophore predominated after the dark exposure. This demonstration of the unusual phenomenon of visual pigment transformation suggests a modification or a new way for the activation of specific biochemical mechanisms of A1:A2 conversion at both high and low illumination levels.