166 
PACIFIC SCIENCE, Vol. II, July, 1948 
TABLE 3 
Summary of the Estimated Number of Molts, the Estimated Number of Days in the Inter- 
molt Interval between Each Successive Exuviation, and the Size Increments at Each 
Exuviation for Both Sexes of P . crassipes from Hatching to the Attainment of Maximum Size. 
INTERMOLT 
INTERVAL 
MOLT 
NUMBER 
FEMALES 
MALES 
Number 
of Days 
Size 
Increment* 
Number 
of Days 
Size 
Increment* 
Larval stagesf—.... 
35 
35 
1st to 2nd....... 
1 
18 
3.4- 4.4 
18 
3.4- 4.4 
2nd to 3rd... 
2 
19 
4.4- 5.5 
19 
4.4- 5.5 
3rd to 4th..... 
3 
21 
5.5- 6.8 
21 
5.5- 6.8 
4 th to 5 th. 
4 
22 
6.8- 8.2 
22 
6.8- 8.2 
5th to 6th.. 
5 
23 
8.2- 9.7 
23 
8.2- 9.7 
6th to 7 th..... 
6 
24 
9.7-11.4 
24 
9.7-11.4 
7 th to 8 th. 
7 
25 
11.4-13.2 
25 
11.4-13.2 
8th to 9th... 
8 
26 
13.2-15.1 
27 
13.2-15.1 
9th to 10th..... 
9 
27 
15.1-17.2 
29 
15.1-17.2 
10th to 11th... 
10 
27 
17.2-19.3 
31 
17.2-19.3 
11th to 12 th.. 
11 
28 
19.3-22.0 
33 
19.3-22.0 
12th to 13th. 
12 
29 
22.0-25.0 
35 
22.0-25.2 
13th to 14th.. 
13 
30 
25.0-27.8 
37 
25.2-28.2 
14th to 15th. 
14 
31 
27.8-30.3 
40 
28.2-31.3 
15 th to 16th. 
15 
32 
30.3-32.7 
44 
31.3-34.7 
16th to 17 th....... 
16 
33 
32.7-34.8 
45 
34.7-38.5 
17 th to 18th... 
17 
36 
34.8-36.7 
47 
38.5-42.5 
18th to 19th. 
18 
39 
36.7-38.4 
50 
42.5-46.0 
19th to 20th.... 
19 
40 
38.4-40.0 
20th to 21st. 
20 
41 
40.0-41.5 
21st to 22nd.......... 
21 
45 
41.5-42.8 
Total...... 
652 
652 
* The size increments are based upon the data set forth in Figure 9. All measurements represent the greatest carapace width 
in millimeters. 
t The intervals shown for larval stages are based on those found by Hart (1935). 
Hart (1935) found that the time interval 
from hatching to the first crab stage in H. nudus 
and H. oregonensis spanned 4 to 5 weeks. Each 
successive zoeal stage required more time than 
the one which preceded it. The present data, 
set forth in Table 11, indicate that the inter- 
molt interval between the first crab stage and 
the second crab stage varies between 14 and 20 
days, with a progressive increase in the inter- 
molt interval with age. Captive crabs larger 
than 7 millimeters in width were reared under 
conditions slightly unfavorable to optimum 
growth; hence the intermolt interval of captive 
specimens spans a somewhat longer period than 
that characteristic of wild crabs. The chief 
value of the figures obtained from captive ani¬ 
mals resides in the fact that they are maximal; 
consequently, they aid materially in computing 
intermolt intervals. Significant assistance was 
derived from the information on intermolt 
intervals of P. crassipes secured from wild speci¬ 
mens by Olmstead and Baumberger (1923). 
When the intermolt span for captive crabs was 
compared with that furnished by the above 
investigators for crabs of identical widths, it 
was found that the time involved for captive 
specimens was increased approximately one- 
third. A scale of intermolt intervals has been 
synthesized for both males and females and is 
presented in Table 3. 
The usual method of plotting size-frequency 
data, to ascertain the age groups of which a 
population is composed, was employed on a 
collection of several hundred crabs taken during 
a 1-week period in August, and was found to 
yield confusing results. Histograms were drawn 
for each distribution, and the modal groups 
comprising the distributions were superimposed 
with a number of normal curves having dif¬ 
ferent standard deviations and different areas, 
