Biology of Pachygrapsus crassipes-— HlATT 
207 
fluids of P. crassipes is considerably reduced in 
comparison to brachyurans which range lower 
in the littoral belt (Pearse, 1931), but it is 
higher than that of crabs which have achieved 
a terrestrial or an almost terrestrial existence. 
P. crassipes has achieved both hyperosmotic 
and hypoosmotic regulation (Jones, 1941). 
Therefore this species can regulate against the 
increasing salt concentration of the water in the 
gill chamber during periods of exposure to 
the air and can maintain a constant body salinity. 
A provision of this type is requisite to an active 
terrestrial existence. H. nudus and H. oregonen- 
sis, on the other hand, do not regulate hypo- 
osmotically toward a salinity greater than that 
of the ocean water (Jones, op. cit .), a fact 
which discloses physiological evidence pertinent 
to their restriction to lower and more shaded 
positions on the strand. Both these species 
exhibit hyperosmotic regulation in brackish 
water to a greater magnitude than was found for 
P. crassipes. This condition partially explains 
why the latter species never occurs in water as 
brackish as that in which the two former species 
occur. Jones has shown, further, that hypo- 
osmotic regulation occurs only among the Grap- 
sidae and is exceedingly variable within the 
group, ranging from complete absence (H. 
nudus and H. oregonensis) to high development 
(Uca crenulata, and all land crabs). 
Homeostasis is definitely correlated with the 
more highly advanced physiological grades 
found in the vertebrates among birds and mam¬ 
mals. Therefore it is not unreasonable to assume 
that stability of the internal physiological mech¬ 
anism of arthropods is conducive to increased 
activity on the part of the possessor. Further¬ 
more, those crabs which have highly developed 
hypoosmotic regulation (Uca, Grapsus, Pachy¬ 
grapsus, etc.) have effected the greatest progress 
toward the conquest of the terrestrial habitat; 
hence they remain exposed to air for the great¬ 
est length of time. 
Certain near-terrestrial Brachyura have evolved 
homing instincts reminiscent of purely terres¬ 
trial groups. Ocypoda arenaria , when molested, 
will return directly to its burrow from con¬ 
siderable distances (Cowles, 1908); the same 
was noted for fiddler crabs (Pearse, 1914). 
This behavior is unquestionably associated with 
their fossorial habit. Although P. crassipes does 
not burrow, several animals have been observed 
to wander short distances away from their base 
and later to return to the identical point of 
departure. To test the possibility of a homing 
instinct in this species, an experiment was 
devised in which 21 crabs were collected from 
a tide pool. The carapaces were marked with 
large numbers, and the crabs were released at 
four different loci 25 feet away from the pool. 
The releasing loci were selected to present four 
different types of routes home, varying from 
simple slopes to complex crevice formations. 
The crabs were released at night and observa¬ 
tions were begun the following afternoon. 
Seven of the crabs were found back in the 
original pool; four of six releases deposited in 
difficult terrain returned; only two of five re¬ 
leased on a moderately difficult terrain returned; 
and none returned from the locus separated from 
the pool by smooth, level rock. Several marked 
crabs were found in crevices near the releasing 
loci. It seems apparent, from the foregoing 
results, that homing behavior is nearly, if not 
totally, lacking in P. crassipes. The crabs seemed 
to be opportunists and secluded themselves in 
the best possible location. The crabs which 
returned to the tide pool probably found it 
during their usual nocturnal wandering. 
Algal types which grow highest in the littoral 
belt may be significant in luring this species 
farther toward land. The abundance of this 
food, coupled with the fact that it forms the 
major portion of the diet, is unquestionably 
one of the reasons for the success of the species 
in maintaining itself in this relatively exposed 
area. It has been suggested by Pearse (1929*0 
that crabs become herbivorous as they approach 
terrestrial life; those which attain almost true 
terrestrial existence are entirely herbivorous. 
The crabs on the coast of central California are 
herbivores, with algae comprising the predom- 
