Biology of Pachygrapsus crassipes —HIATT 
161 
Fig. 6. Typical post-exuvial expansion for three 
distinct sizes of crabs. See text for details. 
The impregnation of carbonates in the exo¬ 
skeleton of P. crassipes begins at the tip of the 
chelae and becomes apparent about 3 6 hours 
after ecdysis. However, the insufficient scleroti- 
zation proximal to the tips prevents the average¬ 
sized crabs (32.0 millimeters) from employing 
these structures defensively until the third or 
fourth day after the molt. The pinch is ineffec¬ 
tive on the fourth day but becomes progressively 
more effective until normalcy is nearly achieved 
on the twelfth day. Inasmuch as the carpus and 
merus of the ambulatory legs are the final por¬ 
tions of the integument to become sclerotized 
(Table 2), the ability of P. crassipes to employ 
these pereiopods effectively before the broad¬ 
ened sides have attained rigidity is attributable 
to their shape and direction of movement. The 
edges of these flattened podomeres are oriented 
approximately perpendicular to the substrate, 
and the long axis of the appendages assumes a 
lateral position which forms a right angle with 
the main axis of the animal. This orientation 
is significant because the direction of locomotion 
is likewise lateral (see p. 177). The legs are 
incapable of bending on their edges, but flex 
readily on their lateral surfaces prior to total 
sclerotization. Locomotion in the early post- 
molt interval is possible and effective because 
of the shape and orientation of the ambulatory 
appendages. 
Seasonal Exuvial Periodicity 
The recorded data indicate that ecdysis in 
Crustacea inhabiting temperate areas occurs 
more frequently during the warmer period of 
the year. Herrick (1909), Churchill (1918), 
and Broekhuysen (1936, 1941) concur in this 
belief; the former two authors further stated 
that most of the molts occur on the Atlantic 
coast of America from June to October, while 
the latter worker graphically illustrated a more 
extended molting season for the South African 
C. punctatus. The molting incidence of P. cras¬ 
sipes simulates the more extensive seasonal type 
illustrated by C. punctatus. 
The seasonal exuvial periodicity of P. cras¬ 
sipes under normal environmental conditions 
was ascertained through collections of crabs 
which were subsequently examined for their 
intermolt stage. Although sex was disregarded 
in the above analysis, it must be pointed out 
here that slight abnormalities in the molting 
incidence are exhibited by ovigerous females 
(see p. 154). These deviations from the normal 
cycle of successive molts are, however, insuf¬ 
ficient to alter the general chain of events. 
Ecdysis occurs most frequently during August, 
September, and October; but considerable exu¬ 
viation is apparent in all except the winter 
months, November through February. The exu¬ 
vial periodicity presented here is particularly 
apparent to the frequent intertidal-pool visitor. 
The early morning receding tide discloses a 
far greater number of exuviae in the pools dur¬ 
ing the warmer months of the year than during 
the cooler winter season. The rather lengthy 
span of relatively high molting frequency is 
probably associated with the small range of 
temperature characteristic of the geographical 
range of this species. 
The comparatively significant role of tempera¬ 
ture fluctuations in the physiology of poikilo- 
thermal species, together with the fact that 
temperature is one of the most widely fluctuat¬ 
ing physical characteristics in the environment 
of these littoral animals, provides the basis for 
a comparison between thermal fluctuation and 
seasonal molting periodicity. The temperature 
of the surface water was selected as that most 
appropriate for this purpose. The figures pre¬ 
sented were secured from the U. S. Coast and 
