176 
PACIFIC SCIENCE, Vol. II, July, 1948 
and aimlessly about the rock surface, going 
directly away from the water at the start. As it 
walked along, the advancing ambulatory legs 
were held out straight from the body when not 
employed for support. After 10 minutes of this 
slow, groping ambulation, the animal reached 
the water’s edge. Upon entering the pool the 
crab passed over the algal film at the edge and 
immediately proceeded to forage by scraping 
it with the tips of the chelae, exactly as normal 
crabs would do. 
Two additional blinded crabs were released 
at the locus mentioned above. One groped about 
the roCk, settling momentarily in any shallow, 
sun-swept crevice encountered, but finally estab¬ 
lished itself in a larger crevice 1 hour after its 
wanderings had begun. At this time it was but 
5V 2 feet from the point of release. The second 
crab paralleled the activity of the first but soon 
became oriented in a direction leading to the 
pool. The advancing pereiopods were employed 
as tactile structures. After the crab entered the 
pool, it descended along a ledge with the fifth 
pereiopods held high upon a perpendicular wall 
and walked with the remaining three pairs. The 
advancing appendages were alternately utilized 
for support and tactile organs. The chelae were 
elevated and spread at a wide angle, apparently 
to provide defense for any exigency. The fore¬ 
going observations seem to indicate that al¬ 
though the eyes are the chief means of orienta¬ 
tion, tactile perception can be substituted al¬ 
though with considerable forfeiture of efficiency. 
Notwithstanding the rather high development 
of the tactile sense, this species apparently does 
not hear sounds within the human range of per¬ 
ception. Crabs only 5 feet away from the blind 
exhibited no response to noises made by shout¬ 
ing, clapping hands, or the pounding together 
of rocks. Moreover, loud shouting at night 
within 2 feet of foraging individuals failed to 
elicit a response. Further observations pertain¬ 
ing to the role played by the various senses are 
incidental to the main theme of certain other 
aspects of this study and will be set forth where 
pertinent. 
Diurnal and Nocturnal Distribution 
Throughout the diurnal hours, members of 
this species tend to remain secluded in crevices, 
regardless of their position in or out of tide 
pools. If undisturbed, they may wander about 
in the tide pools or over short distances on the 
rocks. At the first sign of danger they dash 
into crevices and often literally fall down the 
rocks to conceal themselves in the deep crevices 
below. 
The movement from the refuges to the higher 
rock surfaces begins after dusk at a time when 
artificial light is required to observe the crabs. 
Many crabs align themselves along the edges 
of the tide pools to scrape the algal mat. Others 
climb high on the rocks in search of the rich 
supply of minute algae which grows within the 
splash zone. It is a rare occurrence to find a 
crab concealed in a crevice at night unless some 
disturbance has stimulated it to seek refuge. 
Artificial illumination does not disturb their 
foraging habits. However, if motion occurs in 
close proximity to them in the presence of faint 
illumination, they will move rapidly toward 
the protective crevices. Virtually all the collec¬ 
tions which have provided data for this study 
were made during the night, because a true 
representation of sizes and intermolt stages in 
the population could be obtained only at this 
time. The observed night population is many 
times greater than that observed throughout 
diurnal periods. 
This species is not unique in its nocturnal 
habits. Bateson (1889) states that most Crus¬ 
tacea are more active by night than by day, 
while Hara (1933) reports that the land crabs, 
Varuna literata and Sesarma tetragonum , are 
considerably more active at night. Drezwina 
(1908), while studying phototropic response 
in Pachygrapsus marmoratus and C. maenas, 
found that P. marmoratus invariably chose the 
darkened end of an aquarium, while C. maenas 
preferred the illuminated end. 
P. crassipes was found to prefer the darkened 
end of an aquarium if the water was constant 
in depth throughout the darkened and illumi¬ 
nated halves. When an aquarium was tilted to 
