144 
PACIFIC SCIENCE, Vol. II, July, 1948 
certain extent competitive in that the maximum 
number of each species present is probably not 
as high as it would be if only one species were 
present. It seems to be essentially an ecological 
compromise in which both species suffer quan¬ 
titatively in the struggle for subsistence—further 
evidence of the great halobiotic pressure of the 
strand. 
The rigid, clumped setae of H. nudus seem 
well adapted to withhold large grains of sand; 
an examination of the setae substantiates this 
assumption. It is apparent that the sparse but 
heavy setae are ineffective in straining out sus¬ 
pended silt; consequently, if the animals were 
placed on a muddy substratum respiratory dif¬ 
ficulties would undoubtedly occur. This species 
and H. oregonensis are, therefore, only infre¬ 
quently found associated. Where a close associa¬ 
tion exists, the substratum will normally be 
transitional between sand and silt. Both species 
in such an association are doubtlessly near the 
frontiers of their respective tolerances. H. nudus 
is often found associated with P. crassipes where 
the latter occupies a position relatively low in 
the intertidal zone. However, at the high-tide 
level where P. crassipes is most abundant, very 
few individuals of H. nudus are found. Under¬ 
lying causes for the virtual absence of H. nudus 
from areas with comparatively long exposures 
to air are obscure. No significant difference 
between the abilities of the two species to with¬ 
stand desiccation was found. However, a clue 
to the inherent preferential differences with 
respect to habitat may be gained from the 
hydrotactic responses exhibited by each type. 
P. crassipes is negatively hydrotactic; whereas, 
H. nudus and H. oregonensis are positively 
hydrotactic. This fact may readily be demon¬ 
strated by placing representatives of the three 
species in an aquarium which is tilted to pro¬ 
vide deep water at one end and a dry area at 
the opposite end. The crabs will move to their 
preferred stations immediately; the latter two 
species submerge in the deep end, and the 
former species takes a position at the dry end. 
This behavior pattern of P. crassipes was utilized 
in the laboratory where the aquaria were tilted 
to simulate tide-pool conditions—deep water 
at one end and none at the other. The crabs 
spent most of each day at the shallow or dry 
end. Those reared in large tanks containing 
projecting rocks were observed upon the rocks 
the greater part of the time until they were 
disturbed, in which case they would dart under¬ 
neath the rocks. This reaction to disturbance 
is typical of their normal littoral life, which will 
be discussed later. This negative hydrotactic 
response of P. crassipes is a prominent factor 
underlying its success, because this species alone, 
among littoral crabs, wanders onto the rocks 
high above the water to reach the uncontested 
food supply found there. 
Preliminary experiments designed to test dif¬ 
ferences in the ability of these three local shore 
crabs to withstand desiccation were performed 
as follows: Representatives of each species of 
equivalent sizes in stage Q (see section on 
Intermolt Cycle, p. 146) were deposited in 
individual battery jars and placed in the same 
room to insure comparable external environ¬ 
mental conditions. Observations were then made 
at 15-minute intervals from 4:00 A.M. to 10:00 
P.M. The interval of time which elapsed be¬ 
tween the initiation of the experiment and the 
subsequent death of each animal was recorded. 
It was found that H. oregonensis possessed 
much less ability to withstand desiccation than 
either H. nudus or P. crassipes. All individuals 
of H. oregonensis were moribund in 6 hours 
and all representatives of this group succumbed 
in 10 hours. None of the specimens of H. 
nudus and P. crassipes was moribund until 13 
hours had elapsed, most of them lasting for a 
minimum of 18 hours. All were dead in 24 
hours. No differences in tolerance to desicca¬ 
tion were apparent between P. crassipes and 
H. nudus. 
A second experiment along similar lines was 
performed to test further the relative abilities 
of P. crassipes and H. nudus to withstand desic¬ 
cation. This experiment was performed at the 
