Circadian Rhythm of Extraretinal Photosensitivity in Hatchling Alligators, Alligator Mississippiensis

Hatchling American alligators, Alligator mississippiensis, display a diel rhythm of extra‐retinal (non‐pineal) photosensitivity. There is a bimodal pattern in the threshold fluence rate required for initiating and effecting extra‐retinally‐mediated phototaclic responses. Significantly greater threshold fluence rates are required in the photophase than the scotophasc of a 12 h light: 12 h dark cycle. Under constant darkness there continues an endogenous, apparently circadian rhythm of extra‐retinal photosensitivity. In all cases there is a significantly greater photosensitivity at lower temperatures.