Biology of Pachygrapsus crassipes —HlATT 
143 
The trio of closely associated grapsoid crabs 
along the central California coast (P. crassipes, 
H. nudus, and H. oregonensis) exhibit distinct 
preferences for certain types of substrate, not¬ 
withstanding the fact that the habitats preferred 
overlap to a certain minor degree. In order to 
ascertain more specifically the underlying causes 
for this habitat preference, an analysis of the 
preferred physical features of the environment 
of each species was made, after which certain 
pertinent structural, physiological, and behavior¬ 
istic comparisons among the crabs were made. 
It was found that H. oregonensis prefers a 
muddy, silty substrate; H. nudus is found more 
commonly on a sandy or gravelly bottom; and, 
as indicated above, P. crassipes shows a prefer¬ 
ence for the hard, non-silty substrate. A clue to 
this distributive pattern may possibly be found 
in a study of the structures concerned with ex¬ 
ternal respiration, inasmuch as it is necessary 
that a constant supply of clean water be car¬ 
ried through the branchial chamber. An exam¬ 
ination of the abundance and position of setae, 
both in the vicinity of the incurrent openings 
to the branchial chamber and on the mastigo- 
branch of the third maxilliped, was made. 
A thick mat of very fine setae on the branchi- 
ostegite immediately above the coxae of the 
pereiopods is easily observed in H. oregonensis. 
The dorsal side of each coxa likewise bears a 
thick tuft, which, with the setae of the branchi- 
ostegite, forms a fine strainer covering the incur¬ 
rent openings to the branchial chamber. Func¬ 
tional evidence for this setal sieve is apparent 
since a heavy deposit of silt covers the struc¬ 
ture. The mastigobranch of the third maxilliped 
is exceedingly plumose, and serves to sweep the 
gill surfaces free of silt which does filter through 
the incurrent sieve. 
The setae on the branchiostegite and pereio- 
podal coxae of H. nudus, although comparatively 
fewer in number, are far heavier than those of 
the preceding species. They arise in clusters of 
three and four on the branchiostegite and pro¬ 
ject ventrolaterally; those near the ventral border 
of the branchiostegite extend out to enmesh 
with those which originate on the base of the 
coxae. The stiff nature of these setae seems an 
adaptation for withholding large particles such 
as sand grains. The setae are heaviest on the 
ventral border of the branchiostegite above 
coxae 3, 4, and 5; those setae farther cephalad 
are less rigid, though more numerous. The 
mastigobranch of the third maxilliped is heavily 
plumose. 
P. crassipes, on the other hand, exhibits fewer 
and finer setae. The branchiostegite, with the 
exception of the ventral border, is virtually de¬ 
void of setae. Those setae located on the ventral 
border of the branchiostegite enmesh with the 
relatively sparse, fine coxal setae to cover the 
opening to the branchial chamber. The mastigo¬ 
branch of the third maxilliped has comparatively 
fewer setae than the two species previously 
described. 
It is evident that these morphological dif¬ 
ferences in the rigidness and abundance of setae 
participate in establishing the character of the 
environment selected. The fine, dense mat of 
setae covering the incurrent channels of the 
branchial chamber in H. oregonensis is well 
adapted for straining out particles of a silt-like 
nature. Therefore, among other contributing 
factors, this structural adaptation enables this 
species to subsist on the soft, muddy banks of 
estuaries where it is especially common, and a 
habitat wherein the remaining two crabs under 
discussion cannot become established. H. ore¬ 
gonensis is rarely found associated with P. cras¬ 
sipes (except in the third biotope mentioned) 
probably because the former species cannot 
withstand the extended periods of desiccation 
which the latter type undergoes in its position 
high in the intertidal zone. However, in those 
estuaries wherein the fasciation of P. crassipes 
is lower than the highest tide level, and where 
sufficient cover is available to maintain rela¬ 
tively moist refuges, H. oregonensis does become 
a competitor for places of refuge; but, because 
of the diverse feeding habits the two species 
show little interspecific competition for sub¬ 
sistence. Associations of this type are to a 
