Yellow filters and the absorption of light by the visual pigments of some amazonian fishes

Abstract

The visual pigments of twenty four species of Amazonian fishes have been studied by measuring the spectral absorbance of small areas of intact isolated retinas. In some species the absorbance of the corneas and lenses was measured as well. In 15 species measurements were made at more than one retinal location, and in no case was there any evidence that the visual pigments were different in different parts of the retina. The spectral absorbance of the retinas of cichlids and characins varied considerably between species, but these variations were not related in any clear way to the spectral characteristics of the different bodies of water from which they came. In one species ( Aequidens tetramerus) however, three specimens caught in highly coloured, moderately coloured and very clear water respectively, had retinas absorbing at progressively shorter wavelengths. Evidence was obtained suggesting that this variation was due to alterations in the relative proportion of A 1-based and A 2-based visual pigments in the retina, in response to changes in light intensity. This effect, which has been described previously for several other species, may well be common in Amazonian fishes, and could allow rapid adaptive alterations in spectral sensitivity in response to changes in the light environment. Many of the cichlids studied had yellow corneas. The depth of the pigmentation increased towards the dorsal edge of the cornea, and varied greatly between species. Those species that had yellow corneas frequently also had yellow pigments in the lens and retina. Such yellow filtering pigments, which occur in many other fishes apart from cichlids, and also in many other groups of animals as well, make any attempt to correlate the spectral absorbance of visual pigments with the spectral characteristics of the environment very difficult. In contrast to the cichlids and characins, the retinas of siluriform fishes showed little interspecific variation. The retina of a freshwater sting-ray ( Paratrygon motoro) absorbed at short wavelengths compared to those of other bottom living fishes, but the tapetum of this species was golden in colour, which will shift the effective absorption of the retina to longer wavelengths.