The photic environment and scotopic visual pigments of the creek chub,Semotilus atromaculatus and white sucker,Catostomus commersoni

Summary Scotopic visual pigments measured in the creek chub and the white sucker are porphyropsins with mean _max values located at 538.3 and 536.5 nm, respectively. There is a shift of the _max towards shorter wavelengths during the winter in both of these species coinciding with similar changes in the quality of downwelling light. _max is significantly correlated to the P₅₀ and spectrum width of the downwelling light and dissolved oxygen. An analysis of variance shows that there are significant differences between the _max values of: fish in the two lakes, fish at different times, the two species at different times and fish in different lakes at different seasons. The offset visual pigments of both species appear to be well adapted to their photic environment in terms of the contrast hypothesis. This improvement of contrast detection is discussed in relation to their feeding habits.Abbreviations _max wavelength at which absorbance is maximum - P₅₀ wavelength which halves the total number of photons between 400 and 700 nm, a measure of spectral quality - PAR photosynthetically active radiation - MSP microspectrophotometric - SE standard error