Biology of Pachygrapsus crassipes — HlATT 
167 
in an attempt to identify and separate the homo¬ 
logous groups in this series of size frequencies. 
The method employed was that developed by 
O. E. Sette and used by Brock (1943) in his 
studies on the Oregon albacore fishery. Although 
the plotted data had trimodal characteristics, the 
differences in height between the modal and 
adjacent intermodal classes were slight and 
variable. It was impossible to fit any set of 
normal curves to the data. 
Factors responsible for the absence of clearly 
defined age groups are undoubtedly associated 
with (1) the frequent exuvial periodicity for 
all age classes; (2) the variability in post-exu- 
vial size increments; (3) the extensive breed¬ 
ing season. 
Inasmuch as the number of molts and their 
corresponding intermolt intervals are approxi¬ 
mately known, it was possible to designate 
graphically the size-age relationship. Since there 
is a period from November to March during 
which ecdysis is unlikely to occur, and since 
breeding females apparently lose one molt each 
year because of the extended ovigerous period, 
these modifications were incorporated into the 
size-age scale presented in Figure 10. The fre¬ 
quency with which ovigerous females were col¬ 
lected during each month of the year shows 
that June represents the mid-point in the breed¬ 
ing season; consequently, the curves for size 
and age stem from that period. 
Several factors pertinent to growth and age 
are exhibited in Figure 10. First, three distinct 
age groups are represented, with crabs of both 
sexes attaining the supposed C 4 T stage during 
the third year. Measurements of several thou¬ 
sand individuals collected during August, 1940, 
showed that the frequency of large male crabs 
fell abruptly at 40 millimeters width with the 
largest specimen taken measuring 47.0 milli¬ 
meters, while the frequency of large female 
crabs descended rapidly from the 30-millimeter 
size with the largest specimen measuring 44 
millimeters. Second, the crabs of both sexes 
about 13 millimeters in width may be con¬ 
sidered to represent maximal size for first-year 
individuals; crabs between this size and 30 
millimeters represent second-year individuals; 
AGE IN DAYS 
Fig. 10. Size and age curves for P. crassipes. 
