Biology of Pachygrapsus crassipes -—Hiatt 
171 
the gulls, the sudden change in light intensity, 
or a combination of both, is of interest. To test 
the effect of moving shadow alone, the writer 
undertook to cast shadows over the crabs from 
a place of concealment. The animals became 
startled but rarely concealed themselves in the 
near-by crevices. It would seem, therefore, that 
perception of the gulls themselves, rather than 
the casting of the shadows, was the stimulus 
contributing to the concealment response. 
Several isolated observations seem to sub¬ 
stantiate further the keen perception of P. cras¬ 
sipes. The following excerpts from my field 
notes will serve to describe visual acuity in this 
species: 
The sardine, which was placed near a crevice 
into which a crab had withdrawn, drew several 
flies, one of which settled at the entrance to 
the crevice; the crab moved very slowly toward 
the fly, as if to stalk it. When within 3 inches 
of the fly, the crab, with a very rapid thrust of 
the right cheliped, succeeded in capturing the 
fly. A series of quick movements of the chela 
followed as if an attempt were being made to 
kill the insect; the fly was subsequently con¬ 
veyed to the mouth and eaten. 
This activity occurred just 2 feet below my 
observation post which was a collapsible blind 
utilized to facilitate close-up observation. Its 
construction consisted of a detachable frame 
and large-mesh wire netting through which 
fronds of algae were woven. 
A second observation further substantiates 
the visual ability suggested above. 
A crab was seen to emerge from a tide pool 
and rapidly pursue a rock slater ( Ligia occiden¬ 
tals Dana) which had ventured close to the 
edge of the pool. The crab had apparently per¬ 
ceived its objective prior to its emergence from 
the pool because it moved slowly out of the 
pool, and walked cautiously toward the rock 
slater. The crab did not alter its stalking pace 
until the rock slater took flight. Both prey and 
predator ran with extreme rapidity in and out 
of crevices in the rock wall before the rock 
slater succeeded in eluding the crab. 
The constant behavior pattern of P. crassipes 
in response to food and materials tossed into 
tide pools and near uncovered ledges under 
which crabs were concealed has provided the 
basis for some simple experiments which were 
designed to diagnose partially the relative sig¬ 
nificance of the three senses involved in the 
procurement of food. Before outlining the ex¬ 
periments and discussing the data derived there¬ 
from, a preliminary account of the behavior 
of this crab in response to food, as recorded in 
field observations, will be helpful in setting 
forth the objectives of the experiments to follow. 
Without exception, providing all the ani¬ 
mals involved were present, the initial response 
to food materials tossed into a middle high tide 
pool along the central California coast was 
made by sculpins ( Oligocottus maculosus Jor¬ 
dan). Hermit crabs followed closely behind 
the sculpins. Upon rare occasions small P. cras¬ 
sipes would reach the food before the hermit 
crabs. It was frequently observed that food 
thrown into a pool failed to attract the atten¬ 
tion of the crabs unless it was first located by 
other species. 
The response of P. crassipes to floating food 
was rather constant. The following is an excerpt 
from my field notes: 
A fragment of cantaloupe tossed into a pool 
containing P. crassipes within submerged crev¬ 
ices elicited no response until the breeze had 
forced the morsel toward the edge of the pool, 
at which time a crab emerged and walked along 
the bottom of the pool directly below the food. 
As the food reached the water’s edge the crab 
climbed toward it, seized it with a cheliped, and 
pulled it to the bottom of the pool. 
Similar responses were noted regularly. It is 
significant that the food was constantly moving, 
and that the crabs at no time offered to swim 
to the surface to grasp it. 
Food tossed near exposed crevices which 
sheltered crabs was found soon after it fell. 
In one instance a crushed black turban snail 
(Tegula funebralis Adams) was tossed on a 
rock surface approximately 1 foot from a 
sequestered crab. The crab moved out, seized 
the turban, and returned to its crevice in 5 
seconds. This behavior pattern, because of the 
