Biology of Fabia subquadrata — Pearce 
9 
the diminished rigidity of the exoskeleton in 
the posthard instars. 
The Stage III Females 
This is the first stage subsequent to the Stage 
I instar in which there is an increase in cara- 
pace width over that of the preceding instar, the 
Stage II form. The average width of 41 Stage III 
females was 5.4 mm, with a range of 4.0-5. 9 
mm. This instar is also the first in which the 
abdomen is more than one-third as wide as the 
carapace. The average abdomen width of the 
above Stage III crabs was 3.6 mm. The range 
was 3. 2-4.2 mm. 
Except for the relatively wider abdomen and 
larger overall body size the Stage III crab is, 
externally, morphologically similar to the Stage 
II form. The carapace is soft and membranous, 
the pereiopods are slender and subcylindrical 
and devoid of swimming hairs. The sternal 
groove, however, is shallower and the abdomen 
no longer lies within the confines of this de- 
pression. Rather, it extends both laterally and 
anteriorly beyond the borders of the groove. The 
pleopods are almost identical in both structure 
and setation with those of the Stage II instar. 
The Stage IV Females 
The average carapace width of 33 Stage IV 
crabs is 5.8 mm. They range in width from 5.3- 
6.1 mm. The average abdomen width of these 
crabs is 5.4 mm with a range of from 4.8-5 .9 
mm. This stage is not only larger than the Stage 
III instars but in addition obvious external 
changes indicate that it is sexually more mature 
than those stages which precede it. Ovaries con- 
taining large numbers of developing eggs were 
observed in 29% of the Stage IV crabs. Also, 
while no ovigerous Stage IV crabs have been 
noted, it is significant that at this time the pleo- 
pods undergo the greatest change since their 
initial appearance. These modifications in the 
pleopods involve changes in size, proportion, 
and setal decoration. This is in preparation for 
the deposition and attachment of eggs. Finally, 
the abdomen is now nearly as wide as the cara- 
pace and is more concave than in previous instars. 
The Stage V Females 
This is the definitive adult female crab and 
the stage most commonly found throughout the 
year. As in previous posthard forms the ex- 
oskeleton is membranous and, while the body 
shape is similar to the Stage IV crabs, the rela- 
tively large growth of the abdomen causes this 
instar to become very awkward in its move- 
ments, especially when compared with the earlier 
stages. The abdomen is as wide or wider than 
the carapace and normally it protrudes laterally 
beyond the coxopodites and anteriorly to the 
mouth parts. 
There is a great deal of variability in this 
stage, especially in the size and width of the 
abdomen relative to the carapace. From ob- 
servations made on moulting Stage V crabs it 
has been found that this stage consists of not 
just one instar, as is usually true in the previous 
stages, but rather of a series of growth instars, 
in which the general morphology remains the 
same but with each succeeding instar become 
somewhat larger than the one preceding. This 
results in a wide range of size within this one, 
arbitrarily designated stage. The smallest Stage 
V crab observed measured only 4 mm in cara- 
pace width, whereas several Stage V crabs were 
found to measure 14 mm. The average carapace 
width of all observed Stage V crabs (831) was 
9.5 mm, and the average abdomen width was 
10.3 mm. 
Christensen and McDermott (1958:162) dis- 
cuss the effect of the presence of P. ostreufn in 
slow-growing spat. They suggest that, while the 
growth of the crab is retarded in such host 
oysters, the development is not affected to a 
similar extent. The data gathered on the F. sub- 
quadrata^M. modiolus relationship would sug- 
gest that a similar situation prevails. The very 
small, below average in size, Stage V F. sub- 
quadrata are usually found in relatively smaller 
host mussels. In a more recent study Houghton 
(1963:254) reports a similar situation for P. 
pisum. 
In addition to a positive correlation between 
crab and host size it has been determined that 
there is a negative correlation between the size 
of the Stage V crabs and the depth of the water 
from which they were removed. Crabs reaching 
