The spectral sensitivity of the turtleChrysemys picta picta

The scotopic spectral sensitivity curve ofChrysemys picta picta was determined, using an instrumental technique. The results showed a maximum in the region of 460–500 nm, with a second maximum at about 640 nm in the red. There was also a significant interaction between the effects of wavelength and intensity on the rate of response. At low intensities, the rate of response was highest in the blue (460–500 nm), but at higher intensities it was greatest in the red (about 640 nm). These results show that the turtle retina is not a single-mechanism retina, but contains at least two receptors. One of these, most sensitive in the red, agrees with electrophysiological data on related species, and is probably a result of the oil droplets shifting the photopic sensitivity into the red. The second receptor, which has a lower threshold, is maximally sensitive in the blue.