Visual adaptations to different light environments in Amazonian fishes.

Light measurements were made at seven sites in the Amazon river system near Manaus. It was found that very little light penetrates into the white water of the Solimões because of the large amount of suspended material it contains. In lakes off the Solimões the suspended material settles out, and light penetration is increased. The black waters of the Rio Negro contain dissolved material but little suspended matter, and the penetrating light is mainly of long wavelength. Fishes were also collected from these areas, and the spectral characteristics of their visual pigments, lenses and corneas measured. The visual pigments of the different species vary considerably, fishes from clearer water having pigments absorbing at shorter wavelengths on average than those inhabiting more coloured or turbid waters, and the characins having pigments absorbing at longer wavelengths on average than the cichlids. Yellow lenses and corneas are common among cichlids, but rare in other groups and fishes with such filters were, with one exception, absent from the Solimões where the light levels are especially low. The exception was Colomesus asellus which has an occlusable yellow cornea, which while coloured in the light loses its colour at low levels of illumination. Using the date on light penetration and visual pigments it was calculated that vision would be impossible below 2.3 m in the Solimões, but should be possible at depths up to about 9 m in the clearest waters studied.