Biology of Fabia subquadrata — PEARCE 
eter to the nearest 0.01 mm. The greatest width 
of both carapace and abdomen were noted. 
The crabs were then placed in standard house- 
hold polyethylene ice cube trays. Each tray con- 
sists of 14 cubicles and 1 crab was held in each 
of these. A "vaporite” pen was used to number 
each cubicle with the crab’s respective catalog 
number. In this manner several hundred crabs 
could be retained, facilitating observations on 
their behavior, ecdysis, and subsequent changes. 
The crabs were kept at temperatures approxi- 
mating those of their natural environment, and 
either a flow of water from the sea water system 
or several daily changes were used to maintain 
adequate environmental conditions. 
DEVELOPMENTAL STAGES IN F. subquadrata 
As previously noted the life cycle of F. sub- 
quadrata is similar to that described for P. pisum 
(Atkins, 1926:475) and P. ostreum (Stauber, 
1945:272; Christensen and McDermott, 1958: 
150). The typical planktonic zoeal and megalo- 
pal stages are followed by a series of true crab 
instars. The first of these is the invasive crab 
(Christensen and McDermott, 1958:150). Fol- 
lowing the invasion of the host organism several 
instars occur which are collectively designated 
as prehard stages . These prehard crabs have a 
soft, membranous exoskeleton. With the excep- 
tion of the initial invasive stage there is little 
setal ornamentation on the pereiopods, which 
are cylindrical in shape. 
While the prehard instars were thought to oc- 
cur they were not described for any pinnotherid 
until the investigation of P. ostreum by Chris- 
tensen and McDermott (1958:147). The total 
number of prehard instars is still not known for 
any species, although Hart (personal commu- 
nication) has found up to five prehard instars 
in F. subquadrata. Since the terminal one of 
these is smaller than the smallest Stage I crabs 
of this species it is certain that at least seven 
prehard instars normally occur between the 
megalops and the Stage I crab. Because there is 
a considerable variation in the size of the termi- 
nal prehard instar it may be assumed that the 
total number of prehard instars also varies 
somewhat. 
Subsequent to the series of prehard crab 
5 
stages a very anomalous instar appears. First de- 
scribed by Atkins (1926:478) for P. pisum as 
the Stage I crab, this instar is, in its morphology 
and behavior, entirely different from any of the 
stages preceding or following it. The exoskele- 
ton is well calcified and very hard. It is, in many 
species, highly pigmented with definite patterns 
on the carapace. Above all it is highly modified 
for a temporary, freeswimming planktonic ex- 
istence. The setal ornamentation found on the 
pereiopods is extensive and, in addition, these 
appendages are broad and flattened in contrast 
to the cylindrical condition noted in the pre- 
hard instars. They thus serve as very effective 
swimming appendages. It has been reported for 
P. pisum (Atkins, 1926:475) and P. ostreum 
(Stauber, 1945:272; Christensen and McDer- 
mott, 1958:152) that at this stage of develop- 
ment the males leave their host to seek out 
females, copulating with them in their host. 
To this point of development the male and 
female crabs have paralleled each other. The ex- 
ternal morphology of both sexes is very similar 
throughout the prehard series and the Stage I 
instar. Only by the examination of the external 
genitalia can the two sexes be distinguished. 
Following this stage a dichotomy occurs in the 
developmental cycle of the two sexes. The male 
is thought to remain in the hard stage, dying 
after breeding. The female, however, moults 
soon after copulation and the new posthard in- 
star is soft, with a membranous exoskeleton 
comparable to that of the earlier prehard stages. 
The first posthard stage is referred to as the 
Stage II female. It is followed, both in P. pisum 
and P. ostreum, by Stages III, IV, and V. These 
stages are characterized by an overall increase in 
size, greater complexity of the pleopods, and an 
increase in the width of the abdomen relative 
to the carapace width. The Stage V crab is the 
terminal adult female. Although subsequent 
moults may occur, they result largely in an in- 
crease in size and there is little morphological 
change. 
The Stage II crab is very similar in appearance 
to the terminal prehard instar. In F. subquadrata 
there is little increase in carapace or abdomen 
width during the terminal prehard-Stage I and 
Stage I-Stage II moults. This stage ( II ) is very 
difficult to identify unless the actual Stage I- 
