Modifications in the Diurnal Pigmentary Rhythm of Uca Effected by Continuous Illumination

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Abstract
The persistent daily-rhythm of Uca pugnax evident in melanin dispersion by day and concentration by night continues under conditions of continuous illumination. The amplitude of the rhythm is a function of the incident illumination (decreasing amplitude with increasing intensity). When intensities of incident illumination are equal, a white background leads to greater concentration by night and a black background to greater dispersion by day. When crabs which have been maintained at a higher illumination for a time are transferred to a lower one there is an abrupt increase in amplitude to a new level which is a function of the length of time the crabs were maintained at the higher illumination (the longer the time, the less the amplitude). Studies of the characteristics of melanophore response to a light-to-dark change at various times in the persistent daily cycle led to the postulation of the operation of a black-pigment-concentrating hormone, in addition to the well-known dispersing one, in Uca.