Circadian rhythm of distal retinal pigment migration in the fiddler crab,Uca Pugilator, maintained in constant darkness and its endocrine control∗∗

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Abstract
The distal retinal pigment in the fiddler crab, Uca pugilator, displays a circadian rhythm in constant darkness. The pigment approaches the fully light adapted position by day and the fully dark adapted position by night. This rhythm is presumably due at least to rhythmical release by day of a distal retinal pigment light‐adapting hormone. Extracts of eyestalks from Uca produce a light‐adaptational response in the crab, but extracts of eyestalks from the prawn, Palaemonetes vulgaris, do not produce a light‐adaptational response in the crab, but a dark‐adaptational response instead. In contrast, both types of extract produce a light‐adaptational response of the distal retinal pigment in Palaemonetes. The possibility of a species specificity in the action of distal retinal pigment light‐adapting hormones in crustaceans is discussed.