12 
PACIFIC SCIENCE, Vol. XX, January 1966 
crustacean groups have representatives which 
undergo extensive changes through ecdysis, but 
few others, particularly among the braehyuran 
families, have fitted into the postlarval (post- 
planktonic) portion of their life cycle such 
complex morphological changes as accompany 
ecdysis in the pinnotherids. 
While previous investigators have described 
ecdysis and accompanying phenomena in other 
brachyurans (Drach, 1939; Hiatt, 1948; Guysel- 
man, 1953; and Knudsen, 1957), little informa- 
tion is available concerning these processes in 
the Pinnotheridae. For this reason careful notes 
were made of any moulting activities of F. sub- 
quadrata during the period of this study. Sub- 
sequent studies of ecdysis in F. subquadrata as 
well as other West Coast pinnotherids have been 
made (Pearce, \962b) . These studies involved 
the use of both light and electron microscopes in 
•determining tissue changes which occur during 
ecdysis. These data will be included in a separate 
paper, the present work noting only the macro- 
scopic aspects of ecdysis in F. subquadrata. 
Two distinct phases of ecdysis can be recog- 
nized in all brachyurans. The first is preparatory 
and, to all outward appearances, is passive in 
nature although there can be no doubt that 
physiologically the animal is very active. The 
second, or active phase, involves the actual ex- 
uviation of the cast. This phase is characterized 
by a great deal of movement by the crab. 
Most observations concerning the moulting of 
F. subquadrata were made on animals recently 
removed from a host mussel. A total of 134 
moults were recorded. In 61 of these the dimen- 
sions were noted both before and after ecdysis. 
Prehard and posthard crabs that are about to 
moult can be easily detected. One to two days 
prior to exuviation animals in this state become 
"creamy” and very opaque in appearance. Unlike 
other species which have been studied (Hiatt, 
1948:155), they do remain quite active. All 
stages of Fabia have moulted under laboratory 
conditions — some after being held as long as six 
weeks. Christensen and McDermott (1958:150) 
found it difficult to obtain moulting P. ostreum 
under the laboratory conditions in which they 
worked. Unless crabs were "obviously ready to 
moult on arrival to the laboratory” no moulting 
occurred in the Petri dishes in which they were 
held. These authors, therefore, had to resort to 
other techniques in order to obtain moulting 
specimens. 
No external change in color or opacity her- 
alds approaching exuviation in the hard Stage I 
crabs. Only the somewhat more flexible nature 
of the exoskeleton and the appearance of a crack 
along the postero-lateral margins of the carapace 
indicates that ecdysis is under way. The cara- 
pace of the Stage I form does not become as soft 
or decalcified as is indicated for some other 
braehyuran species (Hiatt, 1948:156); however, 
a recent paper by Knudsen (1957:134) states 
that in the California Xanthidae the exoskele- 
ton does not become fragile prior to ecdysis. It 
may be that, at least in the case of F. sub- 
quadrata, since the following posthard instars 
are not heavily calcified the hardening salts re- 
main in the exuviae of the Stage I instar rather 
than being retained in the crab’s tissues to be 
subsequently redeposited in the new exoskeleton. 
About one day after the onset of the opaque 
appearance in the pre- and posthard crabs a 
crack appears along the epimeral line, and at 
this time the active phase begins. The body now 
expands due to the uptake of water (Drach, 
1939; Guyselman, 1953:129). This in effect 
lifts and frees the posterior portions of the cara- 
pace. In the pre- and. posthard stages the old 
integument being shed has the consistency of 
heavy, wet cellophane. Further, because of its 
supple nature, it is never lifted to the extent of 
a 30° angle as was noted in Pachygrapsus eras - 
sipes by Hiatt ( 1948: 157 ) or as is found in the 
Stage I mussel crabs. Rather, the old integument 
lies free upon the dorsal surface of the new 
integument of the carapace. 
As is noted by Knudsen (1957:136) for the 
xanthid crabs, it is evident that muscular move- 
ments occur during this period since the new 
integument can be observed to be pulled inward, 
forming surface depressions. 
Following the freeing of the posterior por- 
tions of the carapace the last pair of thoracic 
appendages, the fourth pereiopods, and the ab- 
domen are simultaneously freed from the old 
integument. This is a procedure intermediate 
between that observed by Knudsen (1957:136), 
who insists that the abdomen is freed first in 
