Developmental changes in the cone visual pigments of black bream Acanthopagrus butcheri

The spectral absorption characteristics of the visual pigments in the photoreceptors of the black bream Acanthopagrus butcheri Munro (Sparidae, Teleostei), were measured using microspectrophotometry. A single cohort of fish aged 5-172 days post-hatch (dph), aquarium-reared adults and wild-caught juveniles were investigated. During the larval stage and in juveniles younger than 100 dph, two classes of visual pigment were found, with wavelengths of maximum absorbance (λ_max) at approximately 425 nm and 535 nm. Following double cone formation, from 40 dph onwards, the short wavelength-sensitive pigment was recorded in single cones and the longer wavelength-sensitive pigment in double cones. From 100 dph, a gradual shift in the λ_max towards longer wavelengths was observed in both cone types. By 160 dph, and in adults, all single cones had a λ_max at approximately 475 nm while the λ_max in double cones ranged from 545 to 575 nm. The relationships between the λ_max and the ratio of bandwidth:λ_max, for changes in either chromophore or opsin, were modelled mathematically for the long-wavelength-sensitive visual pigments. Comparing our data with the models indicated that changes in λ_max were not mediated by a switch from an A₁ to A₂ chromophore, rather a change in opsin expression was most likely. The shifts in the λ_max of the visual pigments occur at a stage when the juvenile fish begin feeding in deeper, tannin-stained estuarine waters, which transmit predominantly longer wavelengths, so the spectral sensitivity changes may represent an adaptation by the fish to the changing light environment.