180 
PACIFIC SCIENCE, Vol. II, July, 1948 
were observed both in the laboratory and field 
to be undertaking the necessary activities with¬ 
out the use of these appendages. Under these 
circumstances the first pair of ambulatory ap¬ 
pendages was utilized to perform several of 
the duties usually identified with the chelae. 
The duties of these appendages are varied but 
their major accomplishments are food procure¬ 
ment and protection, with reproductive activi¬ 
ties associated with them to a lesser extent. 
Food procurement is achieved in several ways 
with the aid of these structures. The spaded 
tips are adapted for scraping the algal mat; the 
toothed inner borders of both the dactylus and 
finger of the propodus are employed to hold 
and tear flesh and algal fronds. Upon discovery, 
fleshy food is invariably grasped by the chelae 
and conveyed to the oral region. Two methods 
of employing the chelae during scavenging were 
commonly observed: (1) one cheliped holds 
the material while the other conveys bits of 
it to the mouth; (2) both chelae hold the food 
in juxtaposition to the mouth parts while the 
mandibles chew off bits from the periphery 
of the food material. When the latter method is 
employed on a disc-shaped fragment of flesh or 
algal frond, the bites of the mandibles produce 
a scalloped pattern on the periphery of the 
food item. 
It was commonly observed that crabs dragged 
their food out of the tide pools to higher 
sections of the rocks. Frequently, other crabs 
would contest the food and a veritable tug of 
war would ensue, each animal grasping the food 
with one or both chelae. 
In addition to the prehensile use of the 
chelae, this species apparently employs them to 
aid in food selection. Crabs which slowly walk 
over the substrate often drag the chelae and 
frequently halt to scrape the substrate with one 
or both of these appendages. An examination 
of the setae on the propodus and dactylus reveals 
the presence of both the long and the short 
type of setae, which suggests that both tactile 
and chemical stimuli are perceived. At least 
one other crab is said to utilize the chelae in 
the selection of food (Pearse, 1912). There is 
little doubt that the chelae of P. crassipes have 
functional tactile receptors, but observations 
fail to establish any clue concerning their 
capacity to function for chemical perception. 
Both chelae are employed with equal facility 
in conveying food to the mouth when they are 
identical, or nearly so, in length. Observations 
were made on five crabs for periods ranging 
from 2 to 16 minutes to ascertain the number 
of movements of each cheliped per minute. No 
consistent preference was shown for either 
cheliped by these crabs; and out of a total of 
626 movements by all five crabs, 320 were 
made by the right cheliped, while 306 were 
made by the left cheliped. 
The use of either cheliped in securing frag¬ 
ments of Mytilus on a shell apparently depends 
upon the side from which the food material 
is most easily taken. One cheliped is employed 
to hold the shell, the other is utilized to tear 
the flesh and convey it to the mouth. However, 
exceptions were apparent in those animals 
which had recently regenerated an undersized 
cheliped. Invariably, the food was held with 
the large cheliped, while the flesh was torn 
away and conveyed to the mouth by the diminu¬ 
tive one, a unilateral type of feeding which is 
characteristic of male fiddler crabs (Pearse, 
1912; Schwartz and Safir, 1915). The pugnacity 
of P. crassipes is shown through the activity of 
the chelae. A substantial portion of the diurnal 
activity of this species is concerned with fre¬ 
quent clashes between individuals of the same 
and opposite sexes, with combats between males 
predominant. If the combatants differ markedly 
in size, the larger crab usually displays little 
interest in the contest and soon wanders off, 
even though he may be hotly pursued by his 
smaller antagonist. 
During a contest the crabs face each other 
and frequently box with flexed chelae by push¬ 
ing at the antagonists chelae with the flattened 
external surfaces. If this activity fails to result 
in the retreat of one combatant, the chelae are 
extended and rapid thrusts are directed at the 
opposing crab. If the contest is between two 
males of comparable size, the thrusting of chelae 
