DARK-ADAPTIVE CONE ELONGATION IN THE BLUE ACARA RETINA IS TRIGGERED BY GREEN-SENSITIVE CONES

In a dichromatic teleost species, we determined the intensity of light of various wavelengths required to prevent cone elongation by exposin g fish at the time of their normal ''dusk'' phase to monochromatic lig ht (479, 623, and 660 nm) at eight to ten different intensities for 75 min. The positions of single and double cones were measured in tangen tial sections and expressed as cone indices. At all wavelengths, the s pectral responses of both cone types were virtually identical. Further more, the sensitivity of the blocking effect was highest at shorter wa velengths. When comparing the relative quantal sensitivities of myoid elongation for the two cone types to the spectral sensitivities of the three types of Aequidens pulcher photoreceptor, we found the closest match between the action spectrum and the absorption spectrum of the g reen-sensitive single cones. This may indicate that this cone type is capable of reacting directly to decreasing levels of illumination. On the other hand, the identical sensitivity of both cone types argues fo r an indirect control mechanism of dark-adaptive cone elongation, poss ibly via a neural pathway involving the inner retinal layers, compleme ntary to the neural control of light adaptation. Green-sensitive singl e cones are well suited to trigger this response, since (1) their sens itivity is inferior to that of double cones; (2) waters inhabited by t he blue acara transmit best at long wavelengths; and (3) at dusk, long -wavelength radiation dominates over other parts of the spectrum. Ther efore, green-sensitive cone threshold will be reached first at dusk.