Biology of Fabia subquadrata — PEARCE 
The abdomen width of the smallest F. sub- 
quadrata removed from a mussel was 0.26 mm 
or approximately one-third the carapace width. 
This is a ratio that is approximated in all de- 
velopmental instars through the Stage II post- 
hard. With the exception of a few abnormal 
females it is true for the hard Stage I form. 
Apparently it is rare for a male Stage I F. sub- 
quadrata to moult into a posthard, soft carapace 
crab. As will be discussed later, however, such 
males do occasionally occur and, in fact, may be 
more common than suspected. In order to obtain 
an approximation of the size of the terminal 
prehard instars, both males and females, each 
individual collected was measured. If, within a 
week, the crab moulted into a Stage I instar the 
previous dimensions were recorded as those of a 
terminal prehard. 
The carapace width of 19 male prehard crabs 
varied from 3. 0-5. 3 mm, with a mean width 
of 4.3 mm. The abdomen width ranged from 
1. 1-2.1 mm and averaged 1.7 mm. It should be 
kept in mind, however, that males may occa- 
sionally moult into a soft instar from the hard 
Stage I form. Furthermore, as will be discussed 
later, this soft instar may subsequently revert to 
the hard form. Such a moulting sequence may 
thus invalidate these measurements. 
Since the female regularly moults from a 
terminal prehard form into the Stage I instar 
any dimensions of these forms can be accepted 
as valid. Thirteen such moults were observed 
and the individuals involved ranged from 2.7- 
5.1 mm in carapace width prior to the moult. 
The average carapace width of these terminal 
prehard females was 4.1 mm. 
The Stage 1 (Hard) Crabs 
This is one of the stages originally described 
for P. pisum by Atkins (1926:478) and sub- 
sequently applied to the comparable instar of 
P. ostreum by Stauber (1945:272-276). The 
latter suggested that it was during this stage 
that P. ostreum invaded its oyster host. 
The Stage I instar of F. subquadrata is in 
many ways morphologically similar to the Stage 
I form in both P. pisum and P. ostreum. In all 
three species the exoskeleton is well calcified 
and very hard. The pereiopods are flattened 
7 
and well ornamented with functional swimming 
hairs; in F. subquadrata and P. ostreum only the 
second and third pereiopods bear the long plu- 
mose swimming hairs, whereas they are present 
on all the walking legs of P. pisum ( Christensen 
and McDermott, 1958:152). While Darbishire 
( 1900) is quoted (Christensen and McDermott, 
1958:152) as stating that the Stage I P. pisum 
uses the third and fourth pereiopods for swim- 
ming, in contrast with P. ostreum which uses 
the second and third, recent observations by 
Christensen (personal communication) confirm 
that P. pisum uses primarily the second and 
third pereiopods, as does F . subquadrata. 
The carapace of the Stage I F. subquadrata, 
like that of P. pisum, is quite convex. The sur- 
face of this structure has a distinct pattern of 
bright orange markings (see Maerz and Paul, 
1930, plate 10, E-12 ) . This pattern is very 
constant and is found in almost all Stage I crabs. 
The background is a brilliant white. The orange 
pattern tends to fade to a dull brown (Maerz 
and Paul, plate 13, G-ll) some weeks after be- 
ing removed from the host mussel. Other stages 
of this species, both pre- and posthard, do not 
present any indication of this pigmentation. 
Macroscopically the exoskeleton in these latter 
forms appears colorless, although microscopic 
examination reveals isolated black and red 
chromatophores. Finally, as reported for the 
comparable stage of P. ostreum by Christensen 
and McDermott (1958:152), the Stage I F. 
subquadrata was noted to have two cylindrical 
rods connecting the dorsal and ventral sides of 
the body. These structures, along with the al- 
ready discussed exoskeletal rigidity, may be 
modifications for a freeswimming existence. 
Finally, in addition to these differences, the 
Stage I F. subquadrata varies from the other 
stages in having a heavy pubescence along ’the 
antero-lateral margins of the carapace. This 
pubescence appears somewhat heavier in the 
male, but such differences are hard to quantitate. 
The average carapace width of 54 male Stage 
I crabs, selected at random from collections 
made on July 29, 1959, is 4.1 mm, with a range 
of from 1. 3-6.8 mm. The mean of 29 female 
Stage I crabs collected on the same date is 3.5 
mm, with a range of 1. 5-6.2 mm. This does not 
