Biology of Packygrapsus crassipes —Hi ATT 
177 
provide deeper water at one end and none at 
the other, this species preferred the deep end 
regardless of its darkened or illuminated charac¬ 
ter. The foregoing data, secured from dozens 
of laboratory animals reared over a period of 3 
years, suggest that although this species is nega¬ 
tively phototropic, the hydrotropic responses 
are dominant. 
Locomotion 
Locomotion of Brachyura has been investi¬ 
gated by List (1897), Bethe (1897), and 
Cowles (1908). The initial two workers con¬ 
fined their studies to those species which are 
essentially restricted to an aquatic life and are 
not particularly adapted to locomotion on land. 
Cowles, on the other hand, limited his observa¬ 
tions to Ocypoda arenaria, a land crab which 
travels with relatively great speed. The present 
data for P. crassipes describe locomotion in an 
animal which is neither strictly aquatic nor 
essentially terrestrial, but which occurs on the 
strand in a position between those types pre¬ 
viously investigated. Bethe (1897) observed 
that C. maenas ordinarily travels sideways, but 
that to a very limited extent it can move forward 
and backward; in any event, he does not concur 
with List (1897) that Brachyura, in general, 
travel in an oblique direction. 
Like C. maenas, P. crassipes normally runs 
and walks in a direction perpendicular to the 
sagittal plane of the body. It is not unusual, 
however, for the crabs to travel in other direc¬ 
tions. Unlike C. maenas, P. crassipes moves 
obliquely sideways with little decrease in speed 
and travels directly forward with ease. It has 
been previously stated that this species can 
move backward when approaching food in a 
tide pool or when backing into a refuge. When 
speed is mandatory, the movement is invariably 
sideways; but when foraging is the chief objec¬ 
tive, the crabs can and do move in any direc¬ 
tion. Pugnacious crabs have been observed to 
approach each other by walking directly forward 
for distances up to 2 feet. The chelae are flexed 
in front of the oral area during the approach 
to combat. 
The sequence of movement of the ambula¬ 
tory legs is 2, 3, 1, and 4. Often, however, the 
third and first legs move concurrently. Varia¬ 
tions occur in the sequence of movement, but 
the order mentioned above is invariably adhered 
to during normal walking. It is significant that 
the second ambulatory leg, which is the largest, 
initiates movement; its length probably permits 
it to be utilized both as an ambulatory and 
tactile organ. 
Crabs taken by surprise when the rocks under 
which they are concealed are overturned, resort 
to speed and skillful dodging to escape capture. 
The ease with which they move in any direc¬ 
tion is exceedingly advantageous. Under no 
circumstances do they feign death, a behavior 
pattern characteristic of the closely associated 
H. nudus. It is a general hypothesis that crabs 
achieve greater agility and speed as they become 
adapted to exposed conditions. For example, 
members of the land-crab genus Sesarma have 
been estimated to run approximately 10 miles 
per hour; whereas C. magister, an aquatic spe¬ 
cies, is exceedingly sluggish on land. A study 
of the speed of P. crassipes was considered sig¬ 
nificant because of the unique transitory posi¬ 
tion of this crab between the littoral and ter¬ 
restrial areas. Although an accurate speed test 
for P. crassipes is difficult to make, several large 
crabs were placed on the starting line of a 
course laid out on a flat rock. Each crab was 
frightened and caused to run while a stop watch 
was employed to record the times involved. 
Many resorted to dodging tactics, but the writer 
was successful in obtaining some time records 
over very short distances. The distances involved 
were too short to provide accuracy, but an indi¬ 
cation of the possible speed of movement was 
obtained. The most rapid time clocked was 0.9 
second over a 4-foot course, indicating a speed 
of approximately 3 miles per hour. For all 
practical purposes this speed seemed to coincide 
with field estimates of speed. Therefore, P. cras¬ 
sipes runs approximately a third as rapidly as 
Sesarma. Despite the comparative slowness of 
P. crassipes in contrast to Sesarma, additional 
