Brachyura and Anomura of Puget Sound— KNUDSEN 
7 
small holes formed along mud banks, from 
which the crab will venture out to foray along 
the entire expanse of the muddy beach. Schmitt 
(1921:275) suggests that H. oregonensis is 
quite common in dredge hauls ranging from 
114 to 3J4 fathoms. The substrate from which 
these hauls were made consisted of shelly bot- 
toms or mud bottoms. 
Within the southern Puget Sound system of 
waterways one will encounter a variety of sub- 
strates beginning with large coarse boulders 
where the current is rather swift, such as the 
Port Orchard area or the Tacoma Narrows. Ad- 
jacent to this the substrate gives way to rock 
over sand where the boulder size may become 
progressively smaller, until finally one encoun- 
ters in the numerous small bays and inlets a 
pure mud or fine sand substrate intermingled 
here and there with gravel in areas where 
seepage is quite common. 
The substrate of coarse to fine sand overlain 
with a fair supply of rocks and boulders makes 
up the greatest expanse of the intertidal area. 
To a collector first visiting this type of a habi- 
tat it would appear as if the two species of 
Hemigrapsus share the identical habitat, for 
both abound in very large numbers. However, 
H. nudus prefers the upper reaches of the inter- 
tidal zone and is found abundantly during pe- 
riods of low tide under large boulders situated 
over a coarse sand or broken shell substrate. 
Ricketts and Calvin (1952:208) note that H. 
nudus is found in a "middle zone” in California, 
where temperatures are cooler than in the upper 
zones, while it is found in the upper zone in 
Puget Sound. The temperatures in these two 
zones may approach being isothermal and thus 
explain the pattern of distribution. H. oregonen- 
sis is most common in the lower reaches of the 
intertidal zone. A careful check of the substrate 
will show that this crab is found where there 
is a very fine silty sand or mud present beneath 
the rocks, and that frequently coarse gravels 
make up a substrate between the rocks. This 
species seems to prefer shallow depressions 
where the gravel or mud is saturated with water, 
a sharp contrast to the very well-drained sub- 
strate on which H. nudus is often found. There 
are numerous invertebrate burrows in this habi- 
tat and these are frequently occupied by H. 
oregonensis. H. nudus also visits this lower 
zone from time to time, especially during the 
colder parts of the winter months when both 
sexes move down in the intertidal zone, or in 
the spring when females carrying eggs will 
occasionally be found hiding there. There are 
no data, however, which suggest that females 
move to this zone in order to keep the eggs 
moist, for equally large numbers are found at 
low tide in the very high and extremely dry 
parts of the intertidal area. When H. oregonen- 
sis is found in the uppermost reaches of the 
intertidal area, it is in situations which are kept 
moist by surface run-off or in depressions where 
very shallow tide pools form. Thus, the pref- 
erence seems to be that H. nudus will select 
larger boulders with relatively good drainage 
beneath and that H. oregonensis will select any 
situation in which the body may be bathed al- 
most continuously with water. 
As one moves from this typical habitat to 
the more rocky headlands, he finds that H. 
oregonensis begins to drop out in population 
counts but that H. nudus remains a dominant 
member of the fauna. Conversely, as one moves 
from the rock and sand situation towards that 
of a fine mud and gravel, he finds that H. nudus 
drops from the population count but H. ore- 
gonensis becomes extremely abundant. This 
habitat is perhaps more suitable for H. ore- 
gonensis since, because of its diminutive size, 
this crab can hide even between pieces of 
gravel. The fine sand grains or mud tend to 
hold more water, thus meeting another require- 
ment of this species. Where the water is ex- 
tremely swift and the substrate is composed 
exclusively of large boulders, the only member 
of the genus likely to be found is H. nudus. 
Associated with the habitat of this genus is 
the growth of various phyla of microscopic algae 
upon the rocks. Such a micro-turf may consist 
of diatoms, desmids, very young growths of 
the green algae, some of the finer species of 
the brown algae, etc. Such a turf is essential, 
being the chief food of members of this genus. 
Like Pachygrapsus species, crabs of this genus 
spend a tremendous amount of time gleaning 
the micro-algae from the rocks by means of 
their large chelae. At night or when the crabs 
appear to be undisturbed, they may be seen in 
large numbers foraging along the tops of rocks. 
It is interesting to note that the males frequently 
