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Biol Bull 101: 71-83. (August 1951)
© 1951 Marine Biological Laboratory
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STUDIES OF THE DAILY RHYTHMICITY OF THE FIDDLER CRAB, UCA. MODIFICATIONS BY PHOTOPERIOD

FRANK A. BROWN JR. 1 and GROVER C. STEPHENS 1

1 Department of Biological Sciences, Northwestern University, Evanston, Ill., and Marine Biological Laboratory, Woods Hole, Mass.

1. It has been shown that if the diurnal rhythm of the black chromatophores of Uca is treated as a periodic excursion into the dispersed phase, the magnitude of this excursion is significantly influenced by the length of the daily photoperiod to which the animals are subjected.

2. That persistent alterations are induced by different photoperiods is clearly seen in the levels at which the rhythm stabilizes after the animals are placed in constant darkness subsequent to experimental treatments with light.

3. The time of an 18-hour period of illumination between 1 A. M. and 1 A. M. appears not to influence the induced magnitude of the rhythm. The time of a 6-hour period of illumination between 7 A. M. and 7 P. M. is similarly immaterial. However, it has been shown that variations in the time of day within these limits of the period of illumination can produce corresponding and persistent shifts in the relation of the phases of the rhythm to solar time. These alterations may be in either direction from an arbitrary zero point.

4. In addition to the induction of persistent modifications of the rhythm by photoperiod, the character of the rhythm has been shown to be influenced also by illumination and by darkness. These latter effects are transient when the animals are no longer exposed to alternating periods of light and darkness.

5. An analysis of the data clearly indicates the operation of two centers of rhythmicity, such that the activity of the first is influenced by the length of the photoperiod to which the animal is exposed while the activity of the second is influenced by the duration and intensity of light to which the animal is exposed daily, by the duration of the daily dark period, and by the activity of the first center.

6. An adaptive significance for this duality of the control of the rhythm is suggested.




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