Hypersensitivity to light of the iris (Sphincter pupillae) of the albino axolotl (Ambystoma mexicanum).

As is common for amphibians, the sphincter pupillae of the axolotl contracts in vitro in response to illumination with visible light. 1. In a comparison of photomechanical responses of albino and normally pigmented axolotls, similar time courses and maxima of force development were found. 2. The dependence of isometric active force development on the length of the sphincter pupillae is similar to that of other smooth muscles. 3. The action spectrum of the axolotl is similar to the absorption spectrum of frog rhodopsin. 4. At low stimulus strengths, the increase of normalized, isometric, active force with increasing stimulus strength is approximately seven times as great in albino axolotls as in normally pigmented ones. 5. Melanin appears to decrease the light sensitivity of the irises of normally pigmented animals by acting as a simple light shield.