Diurnal Variations of Response to Light in the Fiddler Crab, Uca

Abstract

The diurnal rhythm shown by the integumentary chromatophores of U. pugnax_is studied with respect to its response to light applied for varying lengths of time and at various times in the diurnal cycle. The frequency of the rhythm is not altered to correspond to alternating light and dark periods of 16 hrs. each, even though the animals are exposed to 9 such cycles. The animals were examined at 1 A.M., 7 A.M., 1 P.M., and 7 P.M. daily. The normal time for the most concentrated condition of the pigment to occur is at 1 A.M. It can be made to occur at 7 A.M. by exposing normal animals to illumination periods which begin in the day phase of the rhythm and end at 7 A.M. The most concentrated condition of the pigment is found to occur at 1 P.M. in animals which have been illuminated first from 1 A.M. to 7 A.M. and some days later from 7 P.M. to 1 A.M., as well as in animals which have been repeatedly exposed to illumination from 7 P.M. to 7 A.M. Some of the animals receiving this latter treatment were found to show, on return to constant darkness, a shift such that the most concentrated condition occurred at 7 A.M. The most concentrated condition is found at 7 P.M. in animals which were exposed to illumination periods that began at 1 A.M. and ended at some time in the day phase. Therefore, there are, in Uca, diurnal variations in sensitivity to light changes. These variations are discussed in relation to light changes to which the animals are normally exposed.