78 
PACIFIC SCIENCE, VoL XXI, January 1967 
scale at 6-hr intervals with the aid of a Kyowa 
dissecting microscope (X 45) and a Spencer 
illuminator. The crabs were kept in their in- 
dividual wooden boxes with a petri dish of sea 
water to which fresh sea water was added every 
6 hr if needed. They were fed daily, either a 
small bit of raw meat or bread soaked in milk. 
The normal photoperiod was daylight and 
nighttime with the animals kept outside the 
laboratory. Two indexing time patterns were 
used. The 0600, 1200, 1800, 2400 sequence 
provided three indexing periods in daylight and 
one in the dark. The 0500, 1100, 1700, 2300 
sequence provided two daylight and two dark 
indexing periods. The animals in constant dark- 
ness were kept in a dark closet and were exposed 
to the light only for the few seconds it required 
to index their chromatophores every 6 hr. A 
15-watt Westinghouse cool white fluorescent 
light in the closet was used to maintain the 
animals in constant illumination with the ani- 
mals kept at a distance of 3.5 ft from the light 
source. The artificial light and the dark closet 
were also used to reverse the normal photo- 
period by keeping the animals in the dark for 
12 hr during the day and turning on the light 
for 12 hr at night. 
White sand from Oahu’s beaches provided 
the white background used in the experiments. 
Black sand from the island of Hawaii, origi- 
nating from black lava, was used in the experi- 
ments where a black background was needed. 
As a final experiment an attempt was made to 
see if the observed chromatophore responses 
were the result of visual stimuli. Medium crabs 
were collected and maintained on a white back- 
ground. In half of these crabs the eyestalks were 
completely covered with dark red Revlon nail 
polish. The eyestalks of the other half of the 
crabs were covered with clear Revlon polish. 
The animals were kept outside the laboratory 
in the normal photoperiod and their chroma- 
tophores were indexed every 6 hr at 0600, 1200, 
1800, 2400. 
RESULTS 
The data are presented in Figures 1-6. Time 
of day is given on the abscissa, and the chroma- 
tophore rating scale of Hogben and Slome 
(1931) is given on the ordinate. Unless other- 
wise indicated, 10 crabs were used for each 
experiment. The data have not been treated [ 
statistically. 
In the normal photoperiod, using the index- 
ing times of 0600, 1200, 1800, and 2400, 20 
crabs maintained on a white background dis- 
played a daily rhythmic chromatophore change 
with maximum pigment dispersion at 0600 and 
minimum dispersion at 2400 (Fig. 1). The 20 
crabs maintained on a black background under 
the same light conditions displayed a daily 
rhythm with a peak at 1200 and a low point at 
2400 (Fig. 1), though they displayed much 
less marked response. Crabs on a black back- 
ground were generally darker at any hour. 
Crabs were maintained under the same light 
conditions with the chromatophores indexed 
at 0500, 1100, 1700, and 2300 in order to have 
two indexing periods in the dark (2300 and 
0500) and two periods in the light (1100 and 
1700). Crabs on white sand maintained a 
rhythmic change where a peak was reached 
during the day and a low point at 2300, while 
crabs on black sand displayed a daily rhythmic 
change with a similar pattern of peaks and low 
points (Fig. 2), but with less total variation. 
These crabs were generally darker at all times. 
As shown in Figure 3, crabs maintained in 
the reversed photoperiod showed a reversal of 
their daily rhythmic changes. The crabs were 
in total darkness for 12 hr, including the 1100 
and 1700 indexing times, and in constant arti- 
ficial light for 12 hr, including the 2300 and 
0500 indexing times. On both black and white 
backgrounds the crabs displayed more pigment 
dispersion in the simulated daytime than in the 
artificial night. In comparing the reversed (Fig. 
3) and normal (Fig. 2) photoperiods, the 0500, 
1100, 1700, 2300 normal photoperiod results 
have been used so that both experiments have 
two indexing times in the light and two in the 
dark. Comparison of the chromatophore indices 
of crabs during the two photoperiods showed 
that the peaks of the normal photoperiod oc- 
curred simultaneously with the low points of 
the reversed photoperiod and vice versa, indi- 
cating that the chromatophore rhythms were 
reversible. Crabs maintained on white sand dis- 
played this reversal more distinctly than crabs 
maintained on black sand. Crabs on a black 
background displayed a greater degree of pig- 
ment dispersion at all times. 
