6 
Stage II moult is observed. Stages following it 
can be readily distinguished, however, on the 
basis of the differential growth of the abdomen 
and increased complexity of the pleopods. 
Since Atkins ( 1926 : 475 ) recognized only the 
Stage I-Stage V crabs and did not describe the 
prehard series, the nomenclature originally ap- 
plied by her to the pinnotherid developmental 
instars is no longer adequate. However, as all the 
prehard stages have not been described for any 
pinnotherid crab it would be difficult to rename 
or renumber these forms at this time. For this 
reason her original terminology, with some 
modifications made by Christensen and Mc- 
Dermott ( 1958 ), has been retained in this 
investigation. 
Invasive and Prehard Stages 
The carapace of the invasive first crab stage 
of F. subquadrata is more square in outline than 
are the later prehard stages, which tend to be 
ovoid in shape. The eyestalks and pereiopods of 
the first crab stage are also proportionately 
larger in relation to the rest of the body than 
are those of succeeding prehard instars. The 
pereiopods of this instar are covered with swim- 
ming hairs or setae. The distributional pattern 
of these hairs is different, however, from that of 
the Stage I or hard crab, which also has similar 
setae. The pereiopods of the first crab stage oc- 
cur with the hairs distributed over much of the 
surface, giving the appendage a bottle-brush ap- 
pearance. The hairs of this stage are also much 
more sparse and the entire structure does not 
appear to be as efficient an arrangement for 
swimming as that of the pereiopods of the Stage 
I crab. Since the first stage crab apparently seeks 
out or in some manner becomes associated with 
a host immediately after moulting from the 
megalops, appendages well adapted to extended 
swimming activities are not necessary. This in- 
star is able to swim, however, as is demonstrated 
by its activities in the laboratory. 
As an individual crab progresses through the 
series of prehard moults the swimming hairs 
found on the pereiopods, as well as the swim- 
ming abilities and activity, are lost until the 
Stage I or hard instar is attained. At this point 
the swimming hairs, as well as the general mor- 
PACIFIC SCIENCE, Vol. XX, January 1966 
phology, become highly modified. The signifi- 
cance of this sudden transformation is discussed 
in a later portion of this paper. 
The pleopods of the first crab stage and the 
first few prehard instars subsequent to it are 
merely small knobs protruding from the ven- 
tral surface of the abdomen. At this time there 
is no differentiation into endo- or exopodites. 
In the later prehard instars immediately preced- 
ing the Stage I or hard instar, the pleopods 
become very conspicuous and show clear differ- 
entiation into endo- and exopodite portions. 
The smallest P. subquadrata found within a 
mussel measured 0.85 mm in carapace width. 
This crab is somewhat larger than the first crab 
stage of this species reared by Hart (personal 
communication), which had a carapace width of 
0.76 mm. This difference in size may be ac- 
counted for by assuming that the formerly 
planktonic first crab stage undergoes a moult 
very soon after entering, the host mussel. Con- 
sequently it would be difficult to find a true first 
stage crab in a host mussel. 
However, comparison of the supposed first 
crab stages removed from mussels with the 
known first crab stage raised by Hart indicates 
that morphologically they are very similar or 
identical. 
As earlier noted, Hart has reared F. sub- 
quadrata through five prehard Instars, the largest 
of these still being somewhat smaller than the 
smallest Stage I instar yet observed ( 1.3 mm). 
For this reason it may be suspected that several 
instars intervene between the aforementioned 
forms. Christensen and McDermott ( 1958 : 150 ) 
found that the smallest P. ostreum collected 
measured 0.59 mm. They suggested that at least 
four moults occur before a crab would moult 
into the Stage I instar. The smallest Stage I 
instar in their collection also measures 1.3 mm. 
Since, however, the Stage I P. subquadrata is 
normally somewhat larger, it is suspected that 
at least seven moults occur between the invasive 
first crab stage and the average Stage I instar. 
When the method used by Hiatt ( 1948:165 ) to 
extrapolate the number of intermoult periods in 
Pachygrapsus crassipes was applied to F. sub- 
quadrata it was confirmed that approximately 
seven to eight moults occurred between the first 
crab stage and the average size Stage I instar. 
