Chromatophore Responses — Little 
83 
The observed daily chromatophore rhythm 
did not persist in the experiments in which the 
crabs were maintained in constant illumination. 
The light conditions of these experiments were 
completely foreign to the animals, and the 
crabs behaved sluggishly. There were no cues 
from the environment to aid in maintaining the 
rhythm, and it did not persist. There was no 
evidence of a persisting diurnal rhythm. The 
amount of illumination was the same for both 
groups of crabs, yet those on black sand showed 
a consistently greater degree of pigment disper- 
sion, showing that there is a response to the 
color of the background. 
In the final experiment an attempt was made 
to determine whether the observed daily re- 
sponses were responses to a visual stimulus. 
Cowles (1905:23-24) reported that after paint- 
ing the eyestalks of Ocypode with lampblack, 
no further color changes were observed. This 
observation, however, was not quantitative, for 
he observed only gross appearance and did not 
describe the condition of the chromatophores. 
In the present study the eyestalks of 10 medium 
crabs were covered with dark red nail polish. 
While it was uncertain whether this treatment 
blocked all the light transmission, the response 
suggested that the red coating effectively re- 
duced visual reception. Normally the ghost crab 
demonstrates a shadow reflex, depressing its 
eyestalks when moved from the light into the 
shade or vice versa. The crabs whose eyestalks 
were painted with clear polish did demonstrate 
the reflex; the crabs whose eyestalks were 
painted red did not. The degree of pigment 
dispersion in the red-painted crabs was much 
less. Clear nail polish may have cut out some 
of the light, which might account for the de- 
crease in amplitude of the daily chromatophore 
rhythm as compared with crabs with normal 
eyestalks. The results, however, suggest that the 
visual reception of light is an important factor 
in maintaining a daily rhythm of chromatophore 
changes. 
SUMMARY 
1. The Hawaiian ghost crab, when main- 
tained on white sand, demonstrates a daily 
rhythm of chromatophore changes with maxi- 
mum dispersion of dark pigment during the day 
and maximum concentration at night. On a 
black background the same daily rhythm of 
chromatophore changes is observed, but there 
is generally less concentration of pigment at all 
times. 
2. In an artificially reversed photoperiod the 
crabs on both black and white backgrounds dis- 
play a reversal of the daily rhythm of chromat- 
ophore changes. 
3. In constant darkness crabs on white sand 
still display the daily rhythm but at a lower 
over-all level of pigment dispersion; on black 
sand the rhythm becomes irregular in constant 
darkness. 
4. Under conditions of constant illumination 
crabs maintained on both backgrounds show 
little if any rhythm of chromatophore dispersion 
and concentration. 
5. The observed chromatophore responses 
are primarily responses to visual stimuli, al- 
though in the absence of light evidence is given 
for an endogenous rhythm and for alteration 
of rhythm by substrate. 
REFERENCES 
Brown, F. A., Jr. 1961. Chromatophores and 
color changes. In: C. L. Prosser and F. A. 
Brown, Jr., eds., Comparative Animal Physi- 
ology. W. B. Saunders Co., Philadelphia. 
Chap. 19. 
M. Fingerman, M. Sandeen, and H. 
M. Webb. 1953. Persistent diurnal and tidal 
rhythms of color changes in the fiddler crab, 
Uca pugnax. J. Exptl. Zool. 123:29-60, 10 
figs. 
and M. Sandeen. 1948. Responses of 
the chromatophores of the fiddler crab Uca 
to light and temperature. Physiol. Zool. 
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Cowles, R. P. 1905. Habits, reactions and 
associations in Ocypode arenaria. Papers from 
the Tortugas Laboratory, Carnegie Inst. 
Washington 2:3-39, 4 pis., 10 figs. 
Crane, J. 1941. On the growth and ecology 
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Fingerman, M. 1963. The Control of Chro- 
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Hogben, L. T., and D. Slome. 1931. Pigmen- 
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