Physiology of the white chromatophores in the fiddler crab, Uca pugilator.

Abstract

1. Specimens of the fiddler crab, Uca pugilator, from Panacea, Florida, exhibited a daily rhythm of migration of their white chromatophoric pigment only when maintained in constant darkness. The pigment was more dispersed by day than at night. Crabs exposed to an incident illumination of 3.25 meter-candles on black and on white backgrounds showed no rhythm. 2. The white pigment of these fiddler crabs exhibited a strong background adaptation. The pigment was well dispersed in crabs on a white background and maximally concentrated in those on a black background. 3. At an incident light intensity of 3.25 meter-candles the white pigment of only 47% of the eyestalkless crabs was maximally dispersed. In 23% of the eyestalkless crabs it was in a maximally concentrated state. High intensities of illumination induced dispersion of the white pigment. 4. Evidence was presented for the first time for the presence of a white pigment-dispersing substance in the sinus glands and central nervous system of Uca pugilator. The optic ganglia, sinus glands, supraesophageal ganglia, and thoracic ganglia contain white pigment-dispersing and -concentrating substances. Extracts of these tissues prepared directly in physiological saline revealed only the white pigment-dispersing hormone. However, fractions obtained by acetone extraction of these tissues evoked white pigment concentration while the acetone-insoluble material evoked white pigment dispersion. 5. The circumesophageal connectives are, in contrast, devoid of the white pigment-dispersing substance. They do, however, evoke melanin dispersion in eye-stalkless Uca. 6. The white pigment-concentrating and -dispersing substances appear to be mutually antagonistic. 7. The question of the possible identity of the melanin-dispersing and white pigment-dispersing substances was discussed. The data suggest that this is a highly unlikely possibility.