196 
PACIFIC SCIENCE, Vol. II, July, 1948 
changes within the Brachyura, which are, per¬ 
haps, the result of intense biotic pressure along 
the littoral area. It is certain that the cancroid 
crabs mentioned above could not subsist high 
in the littoral zone. P. crassipes, on the other 
hand, has become adapted to its exposed littoral 
horizon by achieving, among other things, the 
ability of facile self-amputation and rapid regen¬ 
eration. These features have undoubtedly con¬ 
tributed to its success in reaching and establish¬ 
ing itself in an area rich in food materials, 
notwithstanding the fact that it is exposed to 
countless dangers. 
PREDACEOUS AND PARASITIC ENEMIES 
OF P. crassipes 
The countless dangers encountered by P. cras¬ 
sipes include relatively few predators but each 
kind occurs in rather high frequency. Among 
the vertebrate predators, the gulls (p. 170) are 
perhaps the most important. The flight reflex 
exhibited by P. crassipes when a gull or its 
shadow passes by provides some circumstantial 
evidence pointing to the significance of these 
birds, although their successful predation on the 
crabs has never been noted. Frequently gulls 
have been observed dipping suddenly over the 
rocks where the crabs reside. The absence of 
protective coloration in this species would seem 
to make them effective targets for this winged 
predator. The numerous crab skeletons found 
on wharves below gull perches substantiate 
the predatory nature of the gulls with respect 
to the shore crabs. Rats which frequent the 
littoral area during the night probably prey 
upon these nocturnally active crabs, and the 
inability of the crabs to detect danger at night 
would seem to make them easy victims. This 
crab does not escape the extensive littoral pre¬ 
dation of man. Although the crabs are dis¬ 
regarded as a food source, small boys and even 
adults have frequently been observed in attempts 
to extricate the crabs from their crevices. 
Though most of the attempts fail, the crabs 
are often severely mutilated. To escape capture, 
P. crassipes will back deep into a crevice and 
elevate the carapace until the rough striae on 
the protogastric lobes are pressed tightly against 
the rock. It is virtually impossible to extricate 
a crab in this position because the striae serve 
as an effective resistance against the rock, mak¬ 
ing the animal practically immovable within the 
limits of the strength of the body parts. The 
chelae are freed for defense in this position, and 
continued molestation will eventually result in 
the surrender of these appendages. Usually the 
exoskeleton is crushed before the animal can 
be withdrawn from its wedged position. 
In general, it may be stated that P. crassipes 
is not easy prey for vertebrate predators—except 
at night—for a number of reasons: (1) Their 
alertness when exposed and their agility when 
disturbed often enable them to escape from 
predators; (2) they possess an exceptional 
ability to amputate the appendages that are 
grasped; (3) during diurnal hours they tend 
to remain in the vicinity of their refuges. 
Among the invertebrate predators, the larger 
sea anemones ( Bunodactis elegantissima and 
Anthopleura xanthogrammica) are known to 
consume small crabs. Portions of P. crassipes 
have been observed protruding from the stomo- 
daea of these species. It is a common occurrence 
in the high-tide pools at Monterey to find bits 
of P. crassipes integument among a Bunodactis 
bed, and feeding experiments conclusively indi¬ 
cate that anemones will readily utilize the crabs 
for food. None of the carapaces of the devoured 
crabs taken from anemones measured, or would 
have measured, over 20 millimeters in breadth. 
Large crabs thrown on a Bunodactis bed invaria¬ 
bly elude attempts at capture. Smaller crabs 
are much less successful. 
It is necessary to consider the crabs them¬ 
selves as potential enemies of each other. Can¬ 
nibalism is frequent and is a perpetual threat 
to recently molted individuals. 
The larger animal ectoparasites common to 
littoral crabs are exceedingly sparse on the 
P. crassipes found along the coast of central 
California. No macroscopic ectoparasites were 
ever found on collected specimens. A detailed 
