140 
PACIFIC SCIENCE, Vol. II, July, 1948 
persed in this manner. The first crab stage, 
which is no longer free-swimming, occurs ap¬ 
proximately 5 weeks after hatching. The time, 
therefore, is far too short for a successful ocean 
crossing by larvae. The possibility of the use 
of natural rafts as vehicles cannot be dismissed 
lightly in view of the habits and habitat of this 
form. However, both these hypotheses seem 
less plausible than the suggestion to follow. 
Since P. crassipes was not mentioned in the 
faunal accounts of Japan until 1890, it is not 
unreasonable to speculate on the probability 
that marine traffic between the west coast of 
America and Japan has supplied the medium 
of dispersal. The extensive reproductive season 
of this species would insure the presence of a 
quantity of zoeae swarming in the harbors. 
These zoeae may easily have been taken into 
the ballast tanks of ships which were filled on 
the California coast and subsequently emptied in 
Japanese waters. Inasmuch as the ecological 
requirements of the niche are fulfilled in Japan, 
it seems feasible that the species would become 
established; in fact, a constant intermixing of 
this nature may explain the uniformity of the 
species in the two widely separated areas. 
Habitat and Ecologic Niche 
P. crassipes is a eurytopic species which 
ranges through the depositing shore and eroding 
shore subbiochores; consequently, it is neces¬ 
sary to designate the characteristics of the habitat 
in general, and to subdivide this general habitat 
into the three biotopes and fasciations which 
the writer considers most significant. Generally 
speaking, the habitat of this crab is that region 
of the strand extending from upper low (0.0 
tide level) to highest high intertidal zone (6.0- 
foot tide level in the San Francisco Bay region) 
where there is a hard substrate containing 
crannies, crevices, or holes, free from loose 
stones, sand, or mud, and supporting a more 
or less luxuriant growth of ulvaceous or fila¬ 
mentous algae. 
With respect to the vertical distribution on 
the strand, the present data show that this 
species may be located in crevices in the adtidal 
zone (at Pacific Grove, California) as low as 
the - 0.7-foot tide level. However, the indi¬ 
viduals at this level were secluded in refuges 
at the base of nearly vertical rocky promontories 
which extend several feet out of the water at 
high tide. It is assumed that they had followed 
the receding water level, since the crevices they 
usually occupy are located in the intertidal zone 
higher up the rock. Such vertical migrations 
of a few feet are common, in contrast to a 
lack of extensive horizontal movements. Where 
no rocks protrude at high tide, the crabs are 
absent from crevices at the - 0.7-foot tide level, 
thus furnishing evidence for the preceding as¬ 
sumption. The greatest proportion of crabs are 
to be found in crevices and tide pools at the 
lower edge of the supratidal zone. Relatively 
fewer are found higher in the supratidal zone 
(from the +5.0- to the +8.0-foot tide level at 
Pacific Grove) and the area slightly above, 
except at night. Additional information on 
this phase of distribution is presented in the 
section on foods and feeding habits. 
The first and most important biotope is char¬ 
acterized by large rock promontories. These 
rocks are generally continuous and unbroken 
by tide channels from points high on the shore 
to the low-tide level. Their multiplicity of 
crevices affords the crabs refuge from danger 
during the periods of the day in which activity 
is minimal. The greatest concentration of crabs 
is found in the crevices of these rocky coasts. 
Tide pools of varying depths are frequently 
encountered, all of which provide activity cen¬ 
ters for the crabs. Much of the coast line in the 
region of central California is of this type; 
Plate 1, Figure 1, illustrates a typical rocky 
habitat at Pacific Grove, California. 
Microscopic algal forms associated with this 
biotope at Pacific Grove consist in the main of 
the following types: around the edges of the 
tide pools and in damp crevices are found 
juvenile sporelings of some ulvaceous alga, sev¬ 
eral species of non-colonial diatoms, an abun¬ 
dant supply of Oscillatoria sp., a sparse supply 
