Shell Selection of Hermit Crabs— ORIANS and King 
305 
a crab of the other two species from a shell, 
even when it had a decided size advantage. Im- 
munity from eviction was conferred only when 
a P. hirmtius cuius was in a Thais shell too small 
to Interest the other crabs. No clear dominance 
was found between P. samuelis and P, granosi- 
manus . Size appeared to be the critical factor 
in outcomes, but more experiments are needed 
to confirm this tentative conclusion. Interspecific 
dominance in hermit crabs in which species 
membership is more important than size has 
also been reported by Reese (1961). 
In attempting to evict a crab from its shell, 
invaders of all three species follow a general 
behavior pattern. A crab without a shell or with 
an unsuitable shell, upon making contact with 
a shell, immediately tests the size of the shell 
with its chelipeds and quickly moves into a 
position where it is able to probe the aperture. 
Eviction of a crab already occupying the shell is 
effected by grasping ...either the legs or the an- 
tenna with the large cheliped and smoothly 
sliding the occupant from the shell. Then while 
holding the evicted individual at arm’s length, 
the intruder slides out of its own shell, if it has 
one, and into the newly vacated shell. During 
the probing, shell-less crabs frequently twitch 
their abdomens violently. 
It is interesting that a human is unable to 
pull either of the larger species from their shells 
with forceps without severely injuring the ani- 
mals and yet an intruding crab is able to do so 
with ease (see also Alice and Doughs, 1945; 
Brightwell, 1952 ) . One must therefore conclude 
that the evicted crab releases its hold and, 
though there are no data to test this, it is possi- 
ble that the striking difference in behavior has 
important survival value. A larger crab may be 
able to exert enough force to sever the cheliped 
from the body or otherwise injure the inhabi- 
tant. If evicted, however, another shell may be 
found and the probability of survival increased. 
In contrast, if the attack is by a large predator, 
eviction probably means certain death, so that 
chances of surviving would be enhanced by 
remaining in the shell at all costs. 
SIGNIFICANCE OF SHELL SELECTION 
It is possible to observe a relationship be- 
tween the types of shells selected and the habitat 
utilized by the crabs in California. Pagurus hir- 
sutiusculus , which under laboratory conditions 
prefers smaller shells, is much more mobile than 
the other two species and is able to climb over 
the algal growth in the pools better than the other 
species. Both P. samuelis and P. granosimanus , 
with their heavier shells, restrict their activities 
to the bottoms of the pools. This may be ex- 
plained either by comparative ability to climb 
as related to shell weight or as the result of the 
aggressive dominance of P. samuelis and P. 
granosimanus over P. hirsutius cuius which was 
discussed earlier. Thus, though P hirsutius cuius 
and P. samuelis are found together in most of 
the tidal pools in Horseshoe Cove, their dis- 
tribution within the pools differs significantly 
as tested by the Fisher Exact Probability Test 
(Siegel, 1956). P. hirsutius cuius was the most 
common pagurid in 11 pools with good algal 
growth, but was the most abundant species in 
only 3 pools with poor algal growth. In contrast, 
P. samuelis was most common in 9 pools with 
few plants, but was the most abundant crab in 
only 2 pools with good algal growth ( Table 4 ) . 
Coffin (1954) also noted the importance of 
rocky bottoms in tidal pools containing P. 
samuelis and was able to greatly reduce crab 
population densities by removing rocks. 
From the results of the survey of shell utili- 
zation in natural populations of hermit crabs, 
supported by the results of shell selection ex- 
periments, it can be concluded that not only 
is there active shell selection by hermit crabs 
which differs from species to species, but also 
TABLE 4 
Hermit Crab Abundance and Tidal Pool 
Conditions at Horseshoe Cove, California 
NATURE 
OF POOL 
NUMBER OF POOLS 
IN WHICH THE 
MOST ABUNDANT 
HERMIT CRAB WAS 
p 
Pagurus 
hirsutius cuius 
Pagurus 
samuelis 
Good to dense 
algal growth 
11 
2 
Plants few and 
scattered 
3 
9 
.01 
Sandy bottom 
3 
1 
insig. 
Rocky bottom 
9 
7 
