An Endogenous Crepuscular Rhythm Of Rainbow Trout (Salmo Gairdneri) Photomechanical Movements

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Abstract
1. The position of the epithelial pigment and cones in the retina of Salmo gairdneri was determined during extended periods of darkness in fish entrained to both artificial and natural light/dark cycles. 2. An endogenous rhythm of such photomechanical movements, unique among species so far examined, was observed in both groups of fish, with two peaks of light adaptation coincident with dawn and dusk. 3. It is suggested that such an apparently non-adaptive physiological rhythm is related to the behavioural pattern of trout and reveals a basic crepuscular organisation. 4. No endogenous rhythm was observed in continual light. 5. These results suggest that control of photomechanical changes in rainbow trout has two components: an endogenous component, that causes the bimodal pattern in maintained darkness, and a direct effect of light, that maintains light adaptation throughout a normal day. Note:

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