Chromatophore Responses — Little 
81 
Figure 4 shows the results of maintaining 
crabs in constant darkness. In crabs maintained 
on white sand a daily chromatophore change 
rhythm similar to the one displayed during the 
normal photoperiod was observed, though it 
was at a lower level on the Hogben and Slome 
(1931) scale (compare with Fig. 1). The 
chromatophore rhythm of crabs maintained in 
constant darkness on black sand became irregu- 
lar (Fig. 4). 
When crabs were maintained on a white back- 
ground under constant illumination, a small 
rhythmic chromatophore change was observed 
the first day, but then it began to decay. In crabs 
maintained under similar light conditions on 
a black background the rhythm died out (Fig. 
5). As in preceding experiments, crabs on the 
dark background displayed a consistently greater 
degree of pigment dispersion. 
Figure 6 shows the results of covering the 
eyestalks of medium-sized crabs with either dark 
red nail polish or clear nail polish and maintain- 
ing them on a white background in the normal 
photoperiod. When the chromatophores were 
indexed at night, the two observed indices were 
very close, but when the chromatophores were 
indexed in the light, the crabs with the clear 
nail polish on their eyestalks showed a greater 
degree of pigment dispersion (Fig. 6), at least 
during the first two days. Toward the end of the 
experiment the difference in response between 
the two experimental groups diminished appre- 
ciably. This phenomenon was attributed to 
chipping of the red nail polish. 
DISCUSSION 
Other studies have been conducted on the 
existence of persistent rhythmic physiological 
chromatophore changes. An endogenous rhythm 
in the fiddler crab Uca has been reported by 
Brown and Sandeen (1948:370). The persis- 
tence of this rhythm in total darkness has been 
tested (Brown, Fingerman, Sandeen, and Webb, 
1953:36), and Webb (1950:336) found that 
the rhythm could be altered by artificially chang- 
ing the normal time of night and day. 
The results obtained in these experiments 
suggest that the Hawaiian ghost crab exhibits a 
daily rhythm of chromatophore changes under 
normal conditions, with maximum concentra- 
tion of the dark pigment at night and maximum 
dispersion of this pigment during the day. In 
most chromatic decapods the dark pigment in 
X 
LU 
Q 
Z 
LU 
oc 
0 
1 
Q_ 
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h- 
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2 
0 
DC 
1 
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TIME OF DAY 
Fig. 5. The average daily indices of the darkly pigmented chromatophores in Ocypode ceratophthalma 
when the crabs were maintained in constant fluorescent illumination for 4 days on a black or a white back- 
ground. 
