142 
PACIFIC SCIENCE, Vol. II, July, 1948 
This invasion may be attributed jointly to the 
decreased number of suitable refuges for P. 
crassipes and the muddy nature of the substrate 
in the high, flat zone which is suitable habitat, 
especially for H. oregonensis. 
The algal species contributing most to the 
general welfare of P. crassipes in this estuarine 
habitat is Ulva lactuca, whose short fronds or 
very young sporelings form the principal food 
for this crab. Ulva is abundant on the large 
boulders from middle high- to high-tide level; 
the plants become progressively smaller as the 
high-tide level is approached. 
The third and least common biotope in which 
P. crassipes is found is the muddy shore of bays 
and estuaries. Along these, restricted to high- 
tide level, are numerous individuals which take 
refuge in small holes in the mud bank. Extended 
observations have furnished evidence that the 
holes are not dug by the crabs, but are excavated 
by wave action which has removed the loose 
mud and sand from the netted roots of a dense 
stand of pickleweed {Salicornia ambigua 
Michx.) which grows shoreward from high-tide 
level. The splash zone, here an area extending 
back on a slight incline from the bank, is cov¬ 
ered by a dense growth of Ulva lactuca which 
grows on the hard mud between the stems of 
Salicornia. This growth of Ulva, the lone macro¬ 
scopic algal species present, covers a flattened 
area extending approximately 5 feet back from 
the top of a bank on a level with the high-tide 
mark. The habitat described above is typical 
of portions of the shores of Bolinas Bay, Bodega 
Lagoon, and Elkhorn Slough (PI. 1, Fig. 2). 
The vertical section of the bank varies from 
6 to 18 inches in height; the crest of the bank 
corresponds to the "upper high” intertidal zone, 
and the base is on a level with the "middle high” 
zone. Consequently, the vertical range of the 
crab habitat is restricted when compared with 
other types of terrain outlined above. From the 
base of the bank the intertidal zone consists of 
a down-sloping stretch of mud containing gravel 
and sand. At the ebb of a - 0.5-foot tide this 
slope averages approximately 25 feet in breadth, 
and a medium high tide completely submerges 
it. The receding tide serves to deposit on this 
slope a myriad of organisms killed by the exten¬ 
sive clam digging operations which occur at 
many points along these bays and estuaries. A 
wealth of animal food in addition to the abun¬ 
dant algal supply is thereby provided. P. cras¬ 
sipes has never been observed to venture more 
than 4 to 5 feet away from the refuge holes in 
the banks; hence it offers little, if any competi¬ 
tion to H. oregonensis, which likewise occupies 
holes in the bank, but characteristically forages 
over the entire expanse of the submarine slope. 
The latter species accepts any available organic 
matter and seems to forage typically as a true 
scavenger. 
P. crassipes may well be considered a eury- 
topic species because of its wide distribution 
along the entire width of the strand. The popu¬ 
lation is greatest at the typical high fasciation, 
and follows a decreasing gradient in abundance 
farther landward as well as farther seaward. In 
the first biotope mentioned, the habitat condi¬ 
tions consist of those deep crevices in or near 
tide pools at the high-tide level, with an abun¬ 
dant supply of filamentous or ulvaceous algae 
at hand. The habitat characteristic of the second 
biotope may be designated as those crevices 
under or between large boulders which ordin¬ 
arily have a dense growth of Ulva or other 
minute forms of algae on their exposed surfaces. 
The topography characteristic of the third bio¬ 
tope consists of shallow holes in hard mud 
banks, immediately above which is an abundant 
supply of ulvaceous algae. 
The primary topographic units described 
above apparently provide all the factors requi¬ 
site for the welfare of the species: a hard sub¬ 
stratum (rock or solid, packed mud), ample 
food which consists primarily of the more 
minute species of algae and secondarily of dead 
animal tissues, protection from wave action and 
predators by seclusion in crevices, and insurance 
against desiccation by close association with tide 
pools or damp crevices into which the sun’s 
rays do not penetrate. 
