82 
PACIFIC SCIENCE, Vol. XXI, January 1967 
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TIME OF DAY 
Fig. 6. The average daily indices of the darkly pigmented chromatophores in medium-sized Ocypode cera- 
tophthalma when the crabs were maintained in the normal photoperiod on white sand for 4 days with eye- 
stalks covered with clear or red nail polish. 
the chromatophores concentrates at night 
(Parker, 1948:51). On white sand this concen- 
tration of pigment at night enabled the crabs 
to blend with the white background. On black 
sand there was less blanching at night. This 
definite change in the normal pattern of chroma- 
tophore responses would indicate that the re- 
sponses are flexible and are influenced not only 
by the light intensity but by the color of the 
background as well. 
Smith (1938:252) observed in Ligia that a 
sudden change in the color of the background 
was frequently accompanied by a chromatophore 
response in which the animal gradually adapted 
to the new background color. Studies on Uca 
(Brown and Sandeen, 1948:366) showed a 
greater dispersion of dark pigment in animals 
on a dark background than on a light back- 
ground when light intensity was the Same. 
Constant laboratory conditions were used for 
the reversed photoperiod experiments. The 
chromatophore rhythm which was established 
corresponded to the artificial night and day, 
with the animals being darkest during the arti- 
ficial day and lightest during the artificial night, 
indicating that the rhythmic chromatophore 
changes can be reversed. 
Brown (1961:510) reported that a daily 
rhythm of chromatophore responses will persist 
under conditions of constant darkness in many 
species. In the experiment reported here the re- 
sults showed that a diurnal rhythm was main- 
tained, although at a lower level on the Hogben 
and Slome scale when the crabs were main- 
tained on white sand. The normal environment 
of these crabs is on white sand and in constant 
darkness, for they are active at night and spend 
the day underground. The conditions of the 
experiment conducted in total darkness were, 
therefore, quite similar to those of their natural 
surroundings. During the experimental period 
the crabs were very active, behaving as they 
normally do at night. Although the chroma- 
tophore rhythm was not destroyed under the 
conditions of constant darkness, it was evident 
toward the end of the last day that the pattern 
had started to decay. The crabs maintained in 
total darkness on black sand, however, showed 
very little change in pigment concentration 
with respect to time. Because the black sand is 
much coarser than the white, one must include 
the possibility that substrate characteristics other 
than reflectivity might influence chromatophore 
responses. 
