Rods trigger light adaptive retinomotor movements in all spectral cone types of a teleost fish

Action spectra for light induced cone contraction are described for the two spectral cone types of a dichromatic (red/green) cichlid species (Aequidens pulcher). Criterion response thresholds (50% of maximal response amplitude) were determined for seven wavelengths. After correcting for absorption by cornea and lens, the resulting action spectra were compared with the absorption spectra of the rod and cone visual pigments. We find (i) that the action spectra of red and green sensitive cones are almost identical and (ii) matched most closely the absorption spectrum of the rod visual pigment. We therefore conclude that light adaptive cone contraction is triggered by light absorption in rods, which in the dark adapted state are located next to the external limiting membrane and are therefore in an optimal position to capture the incident light during early dawn. Possible mechanisms of signal transfer from rods to cones are discussed.