24 
that P. pisum is occasionally capable of reaching 
adulthood in the primary host. 
THE RELATIONSHIP BETWEEN F. suhquddrata 
AND ITS HOST, M. modiolus 
By placing various stages of F. subquadrata 
within mussels which have had a "window” 
opened in one of the valves, in the manner re- 
ported by Orton (1921) and MacGinitie and 
MacGinitie ( 1949:313 ) , it has been possible to 
observe the behavior of the crabs within the 
host. 
Probably the most noticeable feature of the 
relationship is the comparative inactivity of the 
Stage V crab within the bivalve host. Most of 
the movements noted were associated with feed- 
ing activities. 
The adult female, without exception, is found 
occupying the anterior half of the mussel’s 
mantle cavity. This is generally the widest part 
of the mussel. The crab’s abdomen is always 
placed against a pair of demibranchs, with the 
carapace facing the center of the mantle cavity 
and the frontal region oriented ventrally with 
respect to the mussel. Such a position insures 
that the chela and mouth parts of the crab are 
in a position which facilitates feeding. The crab 
maintains this position by inserting the dactyls 
of the pereiopods into the gill filaments and/or 
mantle tissues. 
In general the feeding is, as described by 
Orton ( 1921 ) for P. pisum, a matter of picking 
the mucous food strings from the food grooves 
of the ctenidium upon which the crab is sitting. 
The chelae are used initially to catch the strings 
which are passed to the mouth parts. The an- 
terior pair of pereiopods are observed sometimes 
to play a part in the manipulation of the food 
strings. 
Prolonged feeding in this manner ultimately 
results in extensive ctenidial erosion, as was re- 
ported as occurring in Crass os trea virginica 
(Stauber, 1945:284) and Mytilus edulis (Mc- 
Dermott, 1962^:163) due to the presence of 
P. ostreum. Ctenidial or gill erosion caused by 
adult crabs involves the entire portion of the 
ctenidium underlying the crab. This portion of 
the gill is eventually destroyed. Figure 7 reveals 
this damage and shows the difference between 
PACIFIC SCIENCE, Vol. XX, January 1966 
the eroded and undamaged ctenidia of a mussel. 
The damage seems to be caused by the constant 
contact of the chela against the edge of the 
ctenidium. It is also noted that the crab "nips” 
at the food string with the chela, and very likely 
the gill margins will also be pinched and cut as 
a result of this action. Damage to the palps often 
accompanies ctenidial erosion, and they may be 
much reduced in length as well as malformed 
due to the presence of a pinnotherid crab. 
Besides the effects of the chelae during feed- 
ing, the dactyls of the pereiopods, used to sup- 
port the crab, also contribute to the gill erosions. 
By repeatedly inserting the dactyls into the gill 
lamellae the crab causes progressive erosion dor- 
sally from the point of the initial damage. Once 
the entire underlying gill has been eroded away 
the crab maintains its position by inserting the 
dactyls into the mantle. This results in a patho- 
logical condition in which the constant irritation 
by the dactyls causes a blister or cyst-like forma- 
tion. This anomaly was found to be present in 
5 5 % of the mussels infested with Stage V crabs. 
A similar condition has also been reported in 
Anomia simplex infested by Pinnotheres (Mc- 
Dermott, 1962^: 163 ). As the erosion progresses 
dorsally toward the suspension of the ctenidium, 
the food groove is continually reconstituted. 
Without such a continuous regeneration it is 
doubtful whether the crab-mussel relationship 
could long endure, since the food groove is 
necessary to the feeding process of both organ- 
isms. Atkins ( 1931 ) has reported a similar 
regeneration of the food groove following de- 
liberate mechanical damage to the gills of My- 
tilus edulis. 
As reported by Stauber (1945:284) for P. 
ostreum, the ctenidial damage inflicted by the 
immature F. subquadrata, especially the Stage I 
forms, differs markedly from that described for 
the adult crabs. Because the Stage I crab is 
considerably smaller, and much flatter dorso- 
ventrally, than the Stage V crab, it is able to 
move about more extensively within the con- 
fines of the host. As a result of this movement 
the gill erosion is not restricted to the area be- 
neath and in immediate contact with the crab, 
but is found along the entire margin of the 
ctenidium. Moreover, erosion of both ctenidia is 
common. These erosions quite often cause the 
