8 
appear to be as large a size difference between 
sexes as was found for P. ostreum by Christen- 
sen and McDermott (1958). In addition to a 
sexual dimorphism in size the Stage I crabs have 
other sexual differences. The abdomen of the 
female is different in shape from that of the 
male; the lateral margins of the males abdomen 
are concave, whereas those of the female are 
straight. It has also been noted that an occasional 
Stage I female will have an abdomen which is 
relatively wider than the 1 : 3 abdomen-carapace 
ratio which is characteristic of most of the Stage 
I crabs, both male and female. Finally, the ab- 
domen of the female bears four well developed 
pairs of pleopods which contrast markedly with 
the two pairs of highly modified reproductive 
appendages borne by the male. 
Both the male and female Stage I crabs have 
much stouter chelipeds than either the pre- or 
posthard growth forms. The merus and carpus 
are heavier and both fingers of the chela are 
swollen. 
As observed by Stauber (1945:274) in P. 
ostreum, the Stage I F. subquadrata possesses a 
locking mechanism whereby the abdomen may 
be secured in the sternal groove. On the fifth 
thoracic segments of the sternal groove there 
are pairs of antero-ventrally directed knobs. 
These knobs hook under shelves found on the 
opposing ventral surfaces of the abdomen in 
such a manner as to become securely locked 
when any attempt is made to lift forcibly 
the abdomen of the living crab. Consequently, 
whereas it is easy to displace the abdomen of the 
pre- and posthard instars it is very difficult to 
free the abdomen from the sternal groove in the 
Stage I crabs. 
The reproductive appendages of the male 
Stage I mussel crab are very similar to those de- 
scribed for P. ostreum by Stauber (1945:276), 
and quite dissimilar from the reproductive ap- 
pendages of P. pisum as described by Atkins 
(1926:476). Atkins described the first copula- 
tory appendage of P. pisum as blade-like and 
hairy. Recent examination of preserved P. pisum 
material by the present author verified a con- 
siderable difference. While the appendages of 
P. pisum are broad with almost parallel margins, 
except for the distal one-fourth of its length 
PACIFIC SCIENCE, Vol. XX, January 1966 
where tapering occurs, they are, in P. ostreum 
and F. subquadrata, slender and lanceolate. 
The Stage II Females 
There is no apparent increase in body size of 
this instar over the Stage I female. The average 
carapace width of seven Stage II female crabs 
which were observed to moult from the Stage I 
instar is 3.4 mm, with a range of 2. 9-3. 9 mm. 
The average abdomen width is 1.1 mm, with a 
range of 0.9— 1-3 mm. 
The exoskeleton of the Stage II crabs is soft 
and membranous, as is that of the prehard in- 
stars. There are few swimming setae or hairs 
to be found on the pereiopods, nor is there 
any pubescence along the anterolateral carapace 
margins. 
The appendages are subcylindrical, not flat- 
tened as in the Stage I instar. The carapace is 
ovoid; the angles of the subpentagonal carapace 
of the Stage I form have become rounded. It is 
during this stage that the lateral carapace sulci 
(one of the definitive characters of the genus; 
Rathbun, 1918:101) become pronounced. They 
appear faintly in the prehard stages and are 
hardly present at all in the Stage I instar. Wells 
(1928:289) notes that these sulci are present 
in the newly moulted Stage I crab but are lost 
with subsequent hardening. As previously noted, 
the typical pigmentation of the Stage I instars 
is lost in the Stage II forms. 
Stauber (1945:275) indicates large differ- 
ences between the pleopods of Stages I, II, and 
III in P. ostreum. Christensen and McDermott 
(1958:152) suggest that Stauber s series of 
Stage II crabs may have included some prehard 
individuals. At any rate, no such marked dif- 
ferences could be found between the pleopods 
of terminal prehards and Stages I, II, and III of 
F. subquadrata . 
There is little or no widening of the abdo- 
men relative to the carapace in the Stage II crab. 
The ratio between the two is approximately the 
same as that of the Stage I forms. The sternal 
groove remains deep and is only as wide as the 
abdomen. The locking mechanism which was 
present and functional in the Stage I crabs no 
longer operates. As was surmised by Stauber 
( 1945:278) for P. ostreum, this may be due to 
