Biology of Pachygrapsus crassipes —Hi ATT 
Variation in Post-exuvial Size Increment 
Considerable variation in the size increment 
after ecdysis seems apparent both between dif¬ 
ferent species of crabs and between different 
size groups within a species. In addition, a 
slight differential variation in size increments 
is apparent between the sexes. Williamson 
(1903) and Pearson (1908) investigated the 
post-exuvial size increment in C. pagurus and 
presented statements which implied that the 
size increment at each successive molting period 
was constant. When the data presented by the 
above authors were plotted by Olmstead and 
Baumberger (1923) it was shown that their 
conclusions were erroneous. Further, the latter 
workers demonstrated that the smaller and pre¬ 
sumably younger specimens increased to a 
relatively greater extent than did the older and 
larger specimens. Broekhuysen (1941) and 
MacKay (1942) found a similar condition in 
C. punctatus and C. magister, respectively, while 
Gray and Newcombe (1938) showed the con¬ 
verse to be true for C. sapidus, but to a lesser 
extent in males than in females. 
Individual variability, both with respect to 
animals of similar size and animals of different 
sizes, has been conspicuous throughout the 
present investigation of the molting charac¬ 
teristics of P. crassipes. An examination of 
Figure 8 will show that variability in post- 
exuvial size increment for both wild and cap¬ 
tive crabs above 10 millimeters in carapace 
breadth may reach as high as 200 per cent. 
Smaller captive animals exhibit a variation in 
excess of 400 per cent. It is also apparent that 
there is a proportionate decrease in the per¬ 
centage of post-exuvial width increments with 
age. However, when actual width increments 
instead of ratios were plotted, it was seen that 
young crabs (below 15 millimeters in breadth 
of the carapace) increase in size at ecdysis up 
to a maximum of 2 millimeters; middle-sized 
crabs (between 15 and 35 millimeters in 
breadth of the carapace) show increases up to 
4 millimeters following the molt; while very 
large crabs (above 35 millimeters in breadth 
163 
Fig. 8. Scatter diagram comparing the variation in 
percentage of the post-exuvial width increment in 
wild and captive crabs above 10 mm. in breadth of 
the carapace. The regression lines were fitted by the 
method of least squares. 
of the carapace) show much less growth at the 
molt and seldom increase in size more than 2 
millimeters in breadth of the carapace. 
Of great significance is the demonstrated fact 
(Fig. 8) that the laboratory environment, in 
spite of the fact that it was meticulously regu¬ 
lated, is dissimilar to the natural environment: 
this is shown by the generally smaller growth 
increment (size for size) for captive crabs as 
compared with wild crabs. The mean growth 
increments for all captive crabs and for all wild 
crabs above a carapace breadth of 10 millimeters 
were computed and the standard "t” test was 
applied to the two sets of data. The difference 
between the means of the two sets of data was 
found to be highly significant statistically ("t” 
= 7.27: V for 0.01 =2.8). The regression 
lines shown in Figure 8 illustrate this point 
graphically. The great similarity in structure 
and response of the various species of the higher 
Crustacea suggests a strong possibility that the 
debilitating effects of laboratory life will affect 
them all. It is of great importance, then, that 
experimentalists treat results obtained in the 
laboratory with reserve and caution before the 
precise effects of confinement are well known. 
