Biology of Fabia subquadrata—VFARCE 
carapace width of the mussel crabs and the 
length of their host mussels: by grouping shell 
lengths and plotting them against the mean cara- 
pace width of all the crabs found in each shell 
length group, he could obtain a curve indicating 
that the carapace width was, in general, propor- 
tional to the shell length of the host. Atkins 
(1926:482) was able to find a size correlation 
between 34 P. pisum and their bivalve hosts. 
More recently Christensen and McDermott 
(1958:160) established a similar relationship 
between all stages of P. ostreum , as well as for 
only the Stage V instars of that species, and their 
host, the American oyster, Crassosirea virginica. 
The latter proposed that external factors, with 
the amount of food probably being the most im- 
portant, act upon both the bivalve and its sym- 
biont crab so as to regulate the growth of the 
crab in such a manner that it "fits” the host. 
19 
They further note that while smaller crabs may 
occasionally be found in proportionately larger 
bivalves, the converse seldom occurs, i.e., large 
crabs are rarely found in proportionately smaller 
oysters. 
The present work extends that of Wells, who 
used a relatively small sample of 111 unstaged 
crabs in his study. During the present investiga- 
tion examination of 305 crab-mussel associa- 
tions has resulted in the curve presented as 
Figure 4. This curve, derived from material col- 
lected at one station during the period July 8 
to November 11, 1959, is indicative of this 
correlation. 
After examination of the host mussels for the 
presence of the pinnotherid crabs two rather 
interesting facts emerge. One was that immature 
crabs are almost always found in the propor- 
tionately smaller host mussels. This is true even 
NO. OF CRABS 
29 30 32 19 22 43 62 41 23 4 
LENGTH GROUPS OF MUSSELS -MM 
Fig. 4. Curve showing positive correlation between carapace width of all stages of F. subquadrata and length 
of host mussels from which they were removed. Both mean width and size range are given for the crabs. 
