184 
PACIFIC SCIENCE, Vol. II, July, 1948 
returned as the tide receded to the deep crevices 
below. The foregoing behavior pattern seems 
contrary to their previously mentioned penchant 
for remaining out of water. Crabs which follow 
the receding tide return as the tide ebbs and 
begins to rise. It seems apparent that the deep 
crevices just below high-tide level offer for 
this species the most desirable refuge against 
wave action. 
Relationship of P. crassipes to Certain Other 
Species in the High Littoral Zone 
General interrelationships with respect to 
the biota of the strand at Monterey Bay, Cali¬ 
fornia, have been set forth by Hewatt (1936). 
The present observations serve to supplement 
those recorded by Hewatt for the littoral species 
more or less closely associated with P. crassipes. 
Although this species is generally found in a 
fasciation relatively uncontested by other forms, 
its extensive vertical range brings it into inti¬ 
mate association with several other brach- 
yurans which provide potential competition. 
The association of P. crassipes and H. nudus 
is perhaps the most important because of the 
overlapping refuge places in certain regions 
throughout much of their geographical ranges. 
It has been found that a differential interspecific 
tolerance exists in different types of biotopes. 
At Monterey both species were commonly found 
just below high-tide level, under rocks which 
rest on a solid substrate, or on coarse gravel. In 
addition, both occurred in tide pools which lie 
below high-tide level. Although there was no 
interspecific antagonism displayed on the boul¬ 
der beach, the individuals which occupied the 
tide pools frequently engaged in combats for 
certain desirable crevices. At no time were the 
two species found in juxtaposition. The rela¬ 
tively larger chelae of H. nudus generally en¬ 
abled these crabs to drive off P. crassipes of com¬ 
parable size. 
Extensive collecting soon demonstrated that 
H. nudus remained in refuges which were lower 
on the strand and generally cooler than those of 
P. crassipes. Inasmuch as temperature appears 
to be a significant factor in the distribution of 
littoral animals, the temperature was recorded 
in the localities which seemed to contain most 
of each of the two species of crabs at midday. 
The air temperature during this investigation 
was 69.5° F. 
Temperatures of 10 rock crevices containing 
P. crassipes ranged from 58.6° F. to 61.2° F., 
with a mean temperature of 60.0° F. Tempera¬ 
tures of the moist sand under 12 rocks harbor¬ 
ing H. nudus ranged from 57.6° F. to 59.8° F., 
with a mean temperature of 58.0° F. Thus, the 
differential of temperature between the rock 
crevices and surface sand below rocks harbor¬ 
ing P. crassipes and H. nudus, respectively, is 
slight and probably does not account for the 
different locations selected by each crab. Mid¬ 
day temperatures of eight high tide pools 
frequented only by P. crassipes ranged from 
68.2° F. to 84.5° F., with a mean temperature 
of 72.0° F.; whereas temperatures of eight lower 
tide pools containing mostly H. nudus ranged 
from 56.5° F. to 59-3° F., with a mean tem¬ 
perature of 57.5° F. or 14.5° F. lower than the 
average of the higher pools. Thus, the tempera¬ 
tures of tide pools may have considerable influ¬ 
ence upon the segregation of these species, H. 
nudus being as characteristic of the zone of the 
rockweeds (2.0 to 4.0 tide level) as P. crassipes 
is of the naked zone higher up (over 4.0 tide 
level). Although their zonation on the strand 
overlaps to some extent, very few of the indi¬ 
viduals of one species ever have contact with 
those of the other. 
Foods consumed by H. nudus and P. crassipes 
differ sufficiently to reduce to a minimum inter¬ 
specific competition for sustenance. The former 
subsists primarily on detritus and infrequently 
on algal fronds; whereas members of the latter 
species sustain themselves on scrapings of the 
algal film and on detritus. During the intensive 
nocturnal feeding, P. crassipes climbs to the tops 
and sides of boulders to eat young LJlva fronds, 
while H. nudus remains below the boulders, 
seldom attempting to climb them. Because of 
dissimilar feeding habits, the association of these 
species within the same tide .pool is not strictly 
competitive. 
