Yellow intraocular filters in fishes.

Yellow intraocular filters are common among the teleosts, especially highly diurnal species. This yellow pigmentation may be uniform, more dense dorsally, or localized to a narrow dorsal ring near the limbus. Certain species possess occlusable yellow corneas and can vary the corneal colour in response to the level of illumination. Yellow lenses and corneas function as hi-pass filters, with the cutoff points varying depending on species. Thus, the amount of short-wavelength light reaching the retina can be regulated. Three distinct yellow pigments may be present in each of the lens, cornea and the retina of certain South American cichlids. The spectral absorbance of the yellow corneal pigment bears a close resemblance to that of beta-carotene. Possible functions of these yellow filters are: a reduction in chromatic aberration, the reduction of glare and dazzle, the improvement of detail by the absorption of "blue haze", the improvement of contrast vision, and the rendering of bioluminescence more conspicuous. Yellow intraocular filters may result in a loss of scotopic sensitivity due to absorption of short wavelengths. Various adaptations in diurnal teleosts to avoid the loss of sensitivity resulting from a yellow filter are presented. Normally, bottom-dwelling fishes lack yellow filters. These filters cause the effective absorbance maximum of scotopic visual pigments to be shifted to longer wavelengths. No correlation has been found between the presence of such filters and the water colour, diet or spectral absorbance of the visual pigment. A possible explanation for the lack of correlation with visual pigments is discussed. Investigation of cone spectral sensitivities may possibly reveal such a correlation.