Biology of Fabia subquadrata — PEARCE 
Abnormal In stars 
Although the sequence of developmental in- 
stars, as already described, represents the normal 
situation, investigations preceding this one have 
revealed occasional deviations from this general 
pattern by other pinnotherid species. Orton 
(1921:533) described a single male P. pisum 
which was morphologically similar to a soft, 
posthard female. Stauber (1945:280) discussed 
a second stage, posthard male P. ostreum which 
appeared externally to resemble the Stage II or 
III females. He notes that they were found in 
"appreciable” numbers and that their size distri- 
bution was somewhat greater than that of the 
typical Stage I males. He suggested that these 
atypical males might be ". . . the result of some 
sort of parasitism as Mercier and Poisson (1929) 
have reported for P. pisum .” Stauber further sug- 
gested that these posthard male forms were 
copulatory partners for the larger posthard fe- 
males. Christensen and McDermott (1958:152) 
suggest that the abnormal P. ostreum referred to 
by Stauber were actually prehards and that the 
greater size range of Stauber’s second stage, post- 
hard male over his Stage I series of male crabs 
was probably due to a sampling error. They do 
make the reservation that a hard Stage I male 
may, "now and then,” moult to a soft-shelled 
form. Atkins (1958) presents evidence that, at 
least in P. pisum, the hard or Stage I males do 
quite frequently undergo a metamorphic moult 
into a soft posthard form. She has repeatedly ob- 
served the same crab change from one form to 
another with usually two or three soft forms in- 
tervening between hard instars. These soft post- 
hard males are usually found during the summer 
months, June-August inclusive, in southwest 
England. It is during this period that the males 
moult and young crabs are found in mussels. 
Because of this she suggested that the soft post- 
hard males occur during the periods of rapid 
growth. 
A similar situation has been found with re- 
gard to F. subquadrata. During the summer of 
1959 eight Stage I males were observed to moult 
into soft posthard forms. The latter are similar 
in body shape to the Stage I instars but are soft 
and membranous. The pereiopods of these soft 
posthard males are, as those of posthard females, 
11 
subcylindrical and with few swimming hairs. 
Such moults are not accompanied by significant 
growth. In no case has a posthard male been 
observed to undergo further moulting, as was 
observed by Atkins in P. pisum. 
These posthard male F. subquadrata were ob- 
served only during the summer months of July 
and August. This does not necessarily mean that 
they do not occur at other times, since they could 
easily have been mistaken for prehard forms had 
they not been observed moulting from the Stage 
I instar. 
Apparent abnormalities are found not only in 
the males but also in the morphology of the 
Stage I females. In these cases Stage I females 
are noted whose abdomens are precociously 
widened. This increase in width over that of the 
normal individuals is quite large, the abdomen- 
carapace ratio approaching that found in the 
Stage III females. Other morphological aspects 
of these individuals tend to be normal, although 
these forms are invariably larger than the aver- 
age female Stage I instar. Of 183 Stage I females 
examined, 5 were of this anomalous type. 
Christensen and McDermott (1958:152) re- 
port similar anomalies in the Stage I females of 
P. ostreum. In two cases they found individuals 
considerably larger than the normal Stage I 
females. Both these crabs had abnormally formed 
pleopods. However, they do not mention any 
extraordinary increase in the relative abdomen 
width of these crabs. It was their opinion that 
they had been retarded in their development, 
ECDYSIS IN F. subquadrata 
Ecdysis is one of the most significant events 
in the life history of any crustacean. In a few 
crustaceans it has evolved to be primarily a 
mechanism allowing an increase in size to occur. 
This is true both in the freshwater decapods, 
the Potamonidae, crabs which hatch from the 
egg as a replica of the adult (Rathbun, 1918: 
11), and in a species of the Oxyrhynca or spider 
crabs, Nacioides serpulifera (Rathbun, 1914: 
653). In most marine crustaceans, however, 
moulting is accompanied not only by increased 
size but also by considerable morphological 
change. In no group is this more true than 
in the family Pinnotheridae. Certainly other 
