Biology of Fakia subquadrata— PEARCE 
Observations on the percentage of infestation 
of the various size groups in the mussel popu- 
lation tend to confirm this speculation. From 
Figure 4 it may be seen that the average adult 
size ( 8 mm ) of the crabs on this date is reached 
in the mussel length group of 5 1-60 mm. This 
same average size is also typical of the crabs 
found in the larger mussel length groups, i.e., 
larger than 60 mm, although the total number of 
adult crabs found in each mussel length group 
decreases above the 61-70 mm range (Fig. 5). 
This decrease in the number of crabs per group 
is probably due to the postreproductive mortality 
following the adult and ovigerous stages. It can 
also be noted that the greatest number of mus- 
sels is found in the 71-80 mm length group 
(Fig. 6) ; and furthermore, while there are almost 
as many mussels to be found in the 81-90 mm 
group as in the 61-70 mm range, the number of 
infested mussels in the former group is only 
one-third that in the latter. Since this is gen- 
erally true of the mussel populations at all the 
collecting sites, it seems to indicate that the crabs 
are outlived by their hosts which, usually, are 
not subsequently reinfested. 
Closely related to the problem of the size re- 
lationship existing between F. sub quadrat a and 
its host is the problem of how permanent is the 
relationship between the crab and its individual 
host mussel once this has been established. Does 
the crab remain within the host after having 
initially infested it, or is the relationship transi- 
tory, with the crab at some period leaving the 
host? 
Rathbun ( 1918:62 ), Orton ( 1921 : 533 ), and 
Berner (1952:345) all suggested that the vari- 
ous hard stage male pinnotherids with which 
they worked were freeliving. Christensen and 
McDermott (1958:175) doubt this for P. os- 
treum and P pisum, and they report that the 
male leaves the host only temporarily during 
the copulatory period to seek a mate. They state 
also that this migration is only a phase in the 
ordinarily commensal or parasitic life of the 
crab. More recently, Sastry and Menzel (1962: 
390), in their discussion of the chemotactic re- 
sponses of the pinnotherid, Pinnotheres macu- 
latus, to its bivalve hosts, quote Rathbun (1918: 
76) to the effect that the early stages of the 
male are freeliving whereas the later adult males 
23 
are commensal in habit. A current investigation 
of this species in the temperate waters off Cape 
Cod indicates that, as with P ostreum and F. 
suhqnadrata, all the stages, with the exception of 
the invasive first true crab stage and the swarm- 
ing Stage I instar, are normally symbiotic. 
It has been observed that Stage V crabs and 
posthard forms can and do vacate a host mussel 
which is moribund. This is, however, the only 
time that these stages have been found outside a 
host organism under laboratory conditions. 
During August 1959' several immature crabs 
were found in bivalves not previously recorded 
as hosts for F. subquadrata . The new host species 
include Astarte compacta, Cardita ventricosa, 
Crenella columbiana , and Kellia sp. All these 
species are quite small. None, according to Old- 
royd (1928), reaches a length greater than 25 
mm. One of them, C. columbiana , rarely exceeds 
16 mm. All the crabs found in these hosts were 
either prehard or Stage I crabs. Because none of 
these bivalves normally attains a size sufficient 
to contain an adult F. subquadrata it is thought 
that the Stage I crabs, after leaving these small 
initial host species to take part in the copulatory 
swarming, do not return to the smaller bivalve 
species but rather secondarily infest a larger host, 
usually M. modiolus. If the smaller initial host 
species are reinfested by postswarming crabs it 
is quite likely that the definitive adult crab stage 
is not attained. Examination of 262 individuals 
of these small bivalve species has not revealed 
the presence of a single adult crab. 
A comparable host change has been described 
for P. pisum by Christensen (1958:3). Fie 
found that on the west coast of Sweden the first 
crab stage initially infests the lamellibranch 
Spisula solida and that later, upon reaching the 
Stage I instar, it leaves S. solida and secondarily 
infests M. modiolus. The major difference be- 
tween F. subquadrata and P. pisum , in this re- 
gard, is that in the latter the host change be-* 
tween lamellibranch species seems to be regular 
or obligate, whereas F. subquadrata appears only 
occasionally to undergo interspecific change, the 
usual situation being intraspecific. In a more re- 
cent report, Christensen (1962:6) notes the 
occurrence of an ovigerous P. pisum in a pri- 
mary host, S. solida, collected at Frederikshavn, 
Denmark, in the summer of I960. This indicates 
