Biology of Pachygrapsus crassipes —Hi ATT 
155 
tacean in detail, the exuviation of the river 
crayfish ( Astacus fluviatilis). Later, Couch 
(1837) was amazed to find a perfect exuvia of 
the lobster ( Astacus etiropeus ) and was respon¬ 
sible for the initial inference that after a cer¬ 
tain size is reached the lobster discontinues the 
molting act. His description of exuviation ac¬ 
complished by the common edible crab, Cancer 
pagurus, in 1843 was the first account of ecdysis 
within the Brachyura. Salter (I860) read be¬ 
fore the Linnaean Society an account of ecdysis 
in the lobster and described in considerable 
detail each precasting and casting activity, and 
emphasized that the observation was indeed 
rare. That he was entitled to this assumption is 
indicated by my experience with P. crassipes 
(see p. 160). Several later descriptions of the 
molting act of the lobster (Packard, 1886; Her¬ 
rick, 1909; and Elmhirst, 1923), together with 
the description of ecdysis in M. squinado by 
Drach (1939), have served to provide a basis 
for comparison between the brachyuran and 
astacuran modes of exuviation. 
Exuviation 
Two distinct phases of ecdysis are apparent: 
(1) a passive phase (no significant muscular 
activity), initiated in P. crassipes approximately 
1 day prior to the molting act and manifest 
by water absorption which serves to increase 
body volume and which exerts sufficient pres¬ 
sure to separate the epimeral line or pleural 
groove on the branchiostegites; (2) an active 
phase, which is identified by actual muscular 
activity significant to the molting act. The 
active phase is initiated immediately after the 
split along the pleural groove, and its termina¬ 
tion is manifest by the complete exuviation. 
Crabs used in the study of ecdysis were reared 
in individual aquaria tilted to provide deep sea 
water at one end and none at the other, thus 
closely simulating the natural water-air rela¬ 
tionship of the normal environment. Many of 
the crabs resided in the aquaria in excess of 8 
months, and in that interval successfully com¬ 
pleted their fourth molt. The laboratory diet 
consisted of small pieces of fresh liver offered 
once daily, after which the water was changed. 
To facilitate observation of the molting act as 
well as other behavior traits, a shelf with a 
glass bottom was constructed directly above 
the author’s desk, thus providing a means of 
constant observation for most of the day. It was 
found that observation from the ventral aspect 
provided a maximum opportunity to view the 
details of ecdysis. 
Both behavioristic and morphological changes 
are manifest prior to the impending molt. For 
4 or 5 days preceding the molting act crabs 
above 10 millimeters in width cling to a corner 
of the aquarium and ordinarily refuse food, but 
exceptions were encountered in which a small 
quantity would be eaten until 2 days prior to 
the molt. This abstinence from feeding is con¬ 
spicuous because the animals are normally vora¬ 
cious, actually leaping to seize suspended food. 
Crabs below 10 millimeters in width regularly 
consume food until 2 days before ecdysis with¬ 
out exhibiting signs of inertness, a characteristic 
undoubtedly stemming from the shorter inter- 
molt interval undergone by small individuals. 
The abstinence from food and the general 
seclusiveness seem to be natural phenomena 
common to both astacuran and brachyuran 
types. Elmhirst (1923) and Herrick (1909) 
described identical reactions in the lobster, while 
Broekhuysen (1941) indicates similar habits for 
Cyclograpsus punctatus M. Edw. on the South 
African coast. Muscular activity in P. crassipes 
virtually ceases; the crab will move but slightly 
if disturbed the day preceding the molt. The 
diminished activity preceding the molt was like¬ 
wise noted by Hay (1905) for Callinectes 
sapidus, and undoubtedly results from the tran¬ 
sitory condition of muscle insertions as they 
change from the old integument to the new. 
It is apparent that movements by crabs in this 
condition would be decidedly ineffectual. 
A trio of morphological signs indicative of 
the impending molt are relatively precise in 
this species. The first, and most perceptible 
change, is that in the pigmentation, which, al- 
