26 
PACIFIC SCIENCE, Vol. XX, January 1966 
small-crab type erosion is prevalent in hosts of 
this size range. Similarly, while the small-crab 
type of erosion may not appear to be as exten- 
sive as the adult-crab type, it may well result in 
as much, or more, relative surface area being 
removed from the ctenidia. In addition, the food 
groove in the former type of erosion may be 
mutilated over much of its entire length, and it 
is this structure which is essential to the feeding 
of the host mussel as well as the symbiont crab. 
Finally, small-crab damage, unlike adult-type 
erosions, usually affects both ctenidia. 
It is apparent from the foregoing that F. sub- 
quadrata is not the harmless commensal that it 
has been considered to be (Wells, 1940:26; 
MacGinitie and MacGinitie, 1949:312). As 
does P. ostreum this pinnotherid induces an 
actual physical damage to the mussel host which 
by its very nature must be harmful to some de- 
gree. Under normal conditions, however, the 
crab is a very effective parasite. Its presence does 
not seem to affect the growth of or cause the 
death of the host, nor does it appear to so 
weaken the host as to render its own position 
precarious. However, under other than normal 
conditions the effect of the relationship may be 
more serious. As Hopkins (1957:414) points 
out, ". . . an organism which robs its host of 
nourishment must be harmful in some degree, 
even if the host shows no apparent effect, but 
under favorable conditions it is probably not 
difficult for the host to compensate or even over- 
compensate for the loss by ingesting more food. 
Under conditions of food scarcity the same para- 
site might become harmful.” 
That such a situation may possibly occur can 
be shown by studies of the distribution of the 
crab with regard to water depth. Mussels re- 
moved from relatively shallow waters, i.e., 20- 
60 m, have a much higher percentage of infesta- 
tion than do mussels taken from waters at a 
depth of 200 m. Mussels collected from waters 
of intermediate depths (120-140 m) have a 
percentage of infestation intermediate between 
those typical of shallower and of deeper waters. 
As seen in Figure 1, mussels removed from 30- 
60 m of water are consistently more than 80% 
infested. Mussels removed from waters 200 m 
and greater in depth are rarely more than 2% 
infested and frequently less than 1%. 
Within the waters surrounding the San Juan 
Archipelago this relationship of the degree of 
infestation to the water depth appears to hold 
regardless of the geographic area from which 
the mussels are removed. Mussels from the 
shallow waters off Point Caution, San Juan 
Island, have more than an 80% infestation 
just as do mussels removed from the shallow 
waters off Point Lawrence on Orcas Island, and 
Pea Vine Pass, all relatively separated areas. 
Mussels removed from any of the deeper waters 
of President Channel always have a low per- 
centage of infestation. 
It was also found that the mussels themselves 
removed from the deep waters of President 
Channel were in very poor condition. The 
gonads usually appeared atrophied, while the 
visceral masses were, in general, very much re- 
duced. The valves of these mussels, while on the 
average the same approximate length as those 
from shallower waters, were thin, brittle, and 
much more subject to breakage than were those 
of the shallow water mussels. 
Wright (1917) reports that Pinnotheres 
never, or at the most very rarely, occurs in poorly 
nourished mussels, although it was frequent in 
those forms from areas where the host mussels 
were obviously well nourished and making rapid 
growth. 
Thus it can be concluded that when host mus- 
sels occur in an environment that is deficient 
in some factor the infestation by the pinnotherid 
crabs is reduced, either primarily or secondarily, 
by the same limiting factor. On the basis of 
known information it cannot be determined 
what the limiting factor might be in the present 
case. Considering the poor physical condition of 
the mussels found there, it is possible that there 
may be a deficiency in the amount or kind of 
available nutrients in the deeper waters. 
A number of recent papers (Blake, I960; 
Haskin, 1940, 1950; Janowitz, 1956) have in- 
dicated that certain gastropod predators are able 
to locate their prey on the basis of the latter’s 
relative metabolism. Apparently the diffusible 
end products of the metabolic processes of the 
host species form gradients up which the preda- 
tor is able to move. Similarly, Wilson (1948) 
has recently demonstrated the ability of certain 
larval forms to "select” appropriate substrates 
