Biology of Fabia subquadrata — PEARCE 
upon which to settle. Therefore, it does not 
seem improbable that the settling first stage 
crabs and the postswarming Stage I crabs might 
use similar metabolic "targets” in selecting their 
hos s. This not only would account for the lack 
of infestation in the poorly nourished, and 
hence slower metabolizing, mussels found in the 
deeper waters, but also could explain, as pre- 
viously discussed, the propensity to infestation 
of smaller, and probably more rapidly meta- 
bolizing, mussels by the early postplanktonic 
first crab stages. 
Finally, the author does not discount the fact 
that the hazards encountered by these crabs, in 
settling in deeper water, are much greater and 
that hence the low levels of infestation found in 
such conditions may only reflect the loss incurred 
during the extended settling period. 
Houghton (1963:257) reports that there is 
a correlation between the incidence of infesta- 
tion of Mytilus edulis by P. pisum and the tidal 
level at which the host mussels were collected. 
He suggests that this is because the first true 
crab stage of this species is photonegative, and 
hence it is not likely that mussels found on the 
shore or at the surface on floats will be invaded. 
One further testimony to the delicate balance 
of this relationship is the fact that double in- 
festations occur only very rarely. As reported 
elsewhere in this paper only three such cases 
were noted in this study. This is dramatically 
different from the condition reported by Stauber 
(1945:281) and Christensen and McDermott 
( 1958 : 155 ), who found that multiple infesta- 
tions by immature P. ostreum, even of spat, were 
very common during certain periods of the life 
cycle. However, they did not ever observe two 
posthard crabs occurring in the same host. Using 
the same survey techniques as those employed 
during my investigation of F. subquadrata, they 
recently observed that frequently a single, adult 
Mytilus edulis is infested with up to six pre- 
hard P. maculatus as well as with an adult fe- 
male. In this respect, then, the behavior of F. 
subquadrata differs markedly from that of both 
P. ostreum and P. maculatus. 
The strong tendency toward single infestation 
observed in the case of F. subquadrata certainly 
suggests some mechanism which selects against 
multiple infestation of a host organism that is 
27 
unable to accommodate the activities of more 
than one pinnotherid. It appears obvious that, 
in the case of the F. subquadrata-M. modiolus 
relationship, an infestation by two adult crabs 
would reduce the food gathering surface of the 
ctenidia to a level below the minimum required 
to sustain three organisms. Since double infesta- 
tions are not ever observed between the host 
and two immature crabs, it would appear that 
the supposed mechanism operates below the 
adult level, i.e., at the first stage and/or Stage I 
levels. 
As noted by Wells (1940:34), Stage V fe- 
males display a marked hostility toward each 
other when placed together in finger bowls. 
However, the present investigation has re- 
vealed that immature forms (even Stage IV in- 
stars ) , similarly situated, do not demonstrate the 
marked aggressive behavior which characterizes 
the adults’ relationships. It is notable, however, 
that when two Stage I crabs are placed in a 
mussel one, and sometimes both, will imme- 
diately vacate the host. This was true in six 
replicate trials. On the other hand, when a single 
crab is inserted it will remain within the host. 
The relationships between the mussel crab 
and the alternative, smaller species of bivalve 
hosts already mentioned are not known as yet. 
A cursory examination of these hosts did not re- 
veal extensive damage to gills or other parts. 
The infesting crabs were mostly immature pre- 
hard crabs (94 out of 120 such infestations, or 
78%), and extensive damage probably would 
not have had time to occur. 
F. subquadrata has also been recorded as oc- 
curring in Mytilus edulis (Wells, 1928:289), 
M. calif ornianus (Wells, 1928:289; Ricketts 
and Calvin, 1952:164), and Saxidomus sp. 
( Hart, personal correspondence ) . Ricketts and 
Calvin report that the mussel crab is found in 
3 % of the full grown California mussels. Giles 
(personal correspondence) has found F. sub- 
quadrata in only 6 out of 805 M. calif ornianus 
collected from Bodega Bay and Tomales Bay, 
California. This is less than 1% infestation. The 
present author has examined some 300 M. edulis 
and 104 M. calif ornianus taken from the inter- 
tidal zones of San Juan Island without finding a 
single mussel crab. 
Hart (personal correspondence) has collected 
