178 
PACIFIC SCIENCE, Vol. II, July, 1948 
evidence for the hypothesis set forth above for 
brachyurans concerning the correlation of speed 
with habitat may be obtained by comparing 
the speed of the former with that of its more 
sluggish, near relative, H. nudus, located at a 
lower level on the strand. 
Swimming is rarely undertaken by P. cras- 
sipes and is poorly executed because of the lack 
of structural adaptation for this method of 
locomotion. Infrequently, crabs were observed 
to leave the top of a submerged rock by giving 
a powerful kick; the ambulatory appendages 
then move at a very rapid rate until the animal 
attains the opposite bank. At no time has swim¬ 
ming been successfully undertaken over dis¬ 
tances in excess of 1 to 2 feet. Movement 
through the water is retarded, notwithstanding 
the fact that the appendages are moved swiftly. 
On one occasion a crab swam forward by mov¬ 
ing the limbs in an anteroposterior direction 
instead of the normal lateral flexing. This 
method seemed more efficient than the usual 
lateral swimming because the broad surfaces 
of the legs provided greater resistance to the 
water. 
Food and Feeding Habits 
The food of P. crassipes consists, in order of 
decreasing importance, of (1) live algae, both 
matted and frondal, (2) detritus left by reced¬ 
ing tides and fishermen, and (3) living littoral 
animals. 
By far the most important method of food 
procurement is the scraping of the minute 
algal mat from the bottom and sides of tide 
pools, damp crevices, and the tops and sides of 
boulders. The excavated tips of the chelae (Fig. 
3) are highly adapted to this method of forag¬ 
ing. The algae most commonly eaten are listed 
on page 141. 
Nocturnal foraging is preferred and large 
numbers of crabs are found at night high on 
the rocks in the splash zone where they scrape 
the algal film. Nocturnal foraging is not pecu¬ 
liar to P. crassipes and seems to be prevalent 
among Brachyura in general; most land crabs 
(Cowles, 1908; Andrews, 1909; Cott, 1929; 
Hara, 1933), some spider crabs (Milligan, 
1915), and undoubtedly many others are more 
active nocturnally. Diurnal feeding, however, 
is common with P. crassipes, particularly on 
warm days. The crabs do not venture far from 
the pools or refuges, and are content to forage 
on the algal film growing adjacent to their 
places of concealment. When the tide recedes, 
they frequently engage in consuming the short 
fronds of Ulva which cover the boulders in 
many areas of the strand. 
The lack of pugnacity among these com¬ 
paratively belligerent animals during their for¬ 
aging on live algal food seems to indicate that 
intraspecific competition for this type of sub¬ 
sistence is negligible. It has been mentioned 
previously that crabs frequently appear to be 
so completely absorbed with foraging activities 
that they are unaware of approaching danger 
from predators. 
Other workers who have recorded observa¬ 
tions on this species (Hewatt, 1936; Ricketts 
and Calvin, 1939) have placed emphasis on 
detritus as the principal source of nutriment. 
Inasmuch as their observations were divided 
among all the littoral forms and perhaps directed 
toward these crabs only at infrequent intervals, 
it is probable that they were impressed with 
this type of food material which seems, from 
far more extensive study, to be secondary in 
importance. While it is true, as Ricketts and 
Calvin (1939) state, that these crabs and the 
beach hoppers are the most active scavengers 
in this particular ecological association, it is 
evident from the information at hand that food 
secured from this source contributes but a minor 
portion of the total diet. All available carrion 
is quickly consumed, but the amount of carrion 
deposited on the rocks by the receding tide is 
almost negligible; indeed, observations of crabs 
eating food of this nature are infrequent. 
The least significant food source (Pearse, 
1931, to the contrary), but the most interest¬ 
ing from the point of view of behavior, is other 
littoral animals. Variations as to the extent and 
type of predacious behavior were disclosed 
among crabs in a given locality as well as be- 
