Scotopic spectral sensitivity of the optomotor response in the green treefrog Hyla cinerea.

Amphibians are unusual among vertebrates in having two spectral classes of rod photoreceptors, unique amphibian "green" rods and typical vertebrate "red" rods. Although amphibians have been the subject of extensive research on visual function, it is not known whether possession of two classes of rods is a general feature of Amphibia, nor is it clear to what behaviors each class of rods contributes. The Hylidae comprise one of the largest families within Amphibia but have been little studied with respect to visual function. Here, we demonstrate the presence of green and red rods in Hyla cinerea by microspectrophotometry and provide evidence for the contribution of green rods to one visually based behavior, the optomotor response. In addition, we discuss the role of green and red rods in visually based behavior in light of apparently conflicting demands resulting from the need to maximize absolute sensitivity, visual acuity, and color sensitivity.