Biology of Pachygrapsus crassipes —HlATT 
181 
may continue for several seconds; one of the 
crabs usually withdraws from the contest. The 
proximity of food accentuates the pugnacity, 
and contests occur among all sizes and sexes. 
Crabs of this species have never been observed 
to lock their chelae on their adversaries in any 
manner, although it seems likely that this be¬ 
havior might occur as it does in the fiddler 
crabs (Pearse, 1912). These conflicts, although 
frequent, result in little if any dismemberment 
of individuals; no casualties have been observed, 
although innumerable contests have been wit¬ 
nessed. 
Defense against predacious enemies is accom¬ 
plished both by flight and by action of the 
chelae. The latter behavior is manifest when 
the crabs are picked up; the chelae provide 
vigorous opposition to predators through their 
strong, vise-like grip. In addition to pinching 
action, the articulation between podomeres en¬ 
ables the cheliped to undergo considerable tor¬ 
sion, contributing greatly to the discomfort of 
the predator. 
Crabs which have been cornered by an adver¬ 
sary take a stance designated by Bethe (1897) 
as the "Aufbaumreflex.” The animal elevates 
itself upon the dactyls; the chelae are raised 
and spread widely; and the body proper is held 
well above the substrate. This behavior is 
primarily a bluff to deter the antagonists, be¬ 
cause the crab will take flight if an opportunity 
for concealment is at hand; however, lacking 
that opportunity, the defiant animal will fiercely 
thrust the chelae at the attacker. 
Chelae seldom participate in the reproduc¬ 
tive activity of P. crassipes. Throughout copula¬ 
tion the chelae of both sexes are generally 
flexed against the oral field; infrequently, those 
of the males assist in grasping the female. No 
nuptial activity is assigned to the chelae although 
such activity has been suggested for male fiddler 
crabs (Alcock, 1892). The sole indication of 
a nuptial activity was a slight up-and-down 
movement of the flexed cheliped of a male 
which became separated from a female during 
copulation. 
Activity under Varied External Conditions 
The diurnal activities of this species in a 
tide pool attain a maximum intensity on bright, 
sunny days; their movements perceptibly slacken 
on dull and cloudy days. To place the measure¬ 
ment of activity on an objective basis, a record 
was made of the number of complete move¬ 
ments made by the chelae from the substrate to 
the mouth while the crabs were scraping the 
algal mat. The animals selected for observation 
were scraping and feeding on the algal mat 
immediately below the surface of the water 
near the perimeter of a tide pool. Temperatures 
were taken of the water near the crabs. Move¬ 
ments of the chelae of each crab were recorded 
over 4-minute intervals. 
These data, together with extensive field 
observations on territorial associations, indicate 
that activity of the crabs is correlated with the 
temperature; activity seemed significantly slack¬ 
ened below 65° F. and accelerated when tem¬ 
peratures increased beyond this figure. On foggy 
days during which the sun was continually 
obscured, the temperature of the tide pools 
was generally lower than that of the air and 
crab activity within the tide pools was slight. 
The rapid rise of tide-pool temperature during 
periods when the sun is unobscured tends to 
benefit the crab population by providing condi¬ 
tions under which this species is most active. 
The conspicuous inactivity of crabs under 
shaded ledges was in unequivocal contrast to 
the intense activity which occurred in adjacent 
tide pools. On a relatively warm day tem¬ 
peratures of the outside air, of air under a 
ledge in which crabs were concealed, and of 
tide-pool water were found to be 60°, 58°, and 
84° F., respectively. It would seem, therefore, 
that this temperature differential would satis¬ 
factorily account for the differences in activity 
between tide-pool and under-ledge crabs. Obser¬ 
vations which extended over several consecutive 
hours showed that even during the warmer days 
of the year the crabs under ledges exhibited only 
a minimum of movement. 
