Biology of Pachygrapsus crassipes —HIATT 
205 
TABLE 11 
Summary of the Gross Growth Characteristics of Two Captive Male Specimens of P. crassipes 
Reared from the Megalops Stage. 
STAGE 
CARAPACE WIDTH 
(MM.) 
PRIOR TO ECDYSIS 
CARAPACE WIDTH 
(MM.) 
AFTER ECDYSIS 
SIZE INCREMENT 
IN MM. 
PERCENTAGE 
SIZE INCREMENT 
DAYS BE¬ 
TWEEN MOLTS 
No. 1 
No. 2 
No. 1 
No. 2 
No. 1 
No. 2 
No. 1 
No. 2 
No. 1 
No. 2 
First crab.......—.. 
3.7 
3.6 
? 
? 
Second crab........ 
37 
3~6 
4.5 
4.4 
65 
6*8 
21.7 
22.2 
14 
20 
Third crab.......... 
4.5 
4.4 
5.5 
5.3 
1.0 
0.9 
22.2 
20.4 
14 
17 
Fourth crab........ 
5.5 
5.3 
6.4 
6.1 
0.9 
0.8 
16.4 
15.1 
20 
16 
Fifth crab.. 
6.4 
6.1 
7.9 
7.2 
1.5 
1.1 
23.4 
18.0 
25 
19 
Sixth crab. 
7.9 
7.2 
10.1 
8.4 
2.2 
1.2 
27.8 
16.7 
26 
31 
Totals . 
6.7 
4.8 
111.5 
92.4 
99 
103 
The first crab stage of P. crassipes is likewise 
much larger than that of either H. nudus or 
H. oregonensis. It is nearly twice the width and 
length of H. nudus and more than twice the 
width and length of H. oregonensis. A descrip¬ 
tion of the first crab stage of P. crassipes follows: 
carapace length, 3.7 millimeters; carapace width, 
3.2 millimeters; front produced in two shallow 
lobes, unlike the straight frontal margin of the 
adult; the eyes proportionately very large, ex¬ 
tending past the sides of the carapace; the two 
lateral teeth of the carapace distinct, and the 
thoracic striae characteristic of the species faintly 
indicated; the pereiopods provided with fine 
setae, both plumose and simple, with bands of 
dark and light areas along the ambulatory legs; 
the antennae and antennules proportionately 
larger and dissimilar to subsequent crab stages; 
the dorsal aspect of the animal containing min¬ 
ute chromatophores which impart a stippled 
effect. 
The growth pattern of P. crassipes , from the 
megalops stage up to and including the sixth 
crab stage, was secured from observations on 
two captive crabs collected during the megalops 
condition. Both crabs were reared in large 
refrigerator dishes containing sea water from 
the locality in which they were collected. The 
mean monthly temperatures during the rearing 
period were: March, 14.8° C; April, 15.3° C.; 
May, 15.8° C; and June, 16.2° G Food in the 
form of minute polychaete worms ( Mercierella 
enigmatica Fauvel) was plentifully supplied. 
The data are summarized in Table 11. A re¬ 
markable constancy in total elapsed time with 
respect to the attainment of certain molt stages 
is immediately apparent. However, the varia¬ 
bility in size increment (19.1 per cent) signifi¬ 
cantly contributes additional evidence concern¬ 
ing individual variation in growth discussed at 
some length on page 163. 
THE TRANSITIONAL POSITION OF P. crassipes 
BETWEEN A LITTORAL AND TERRESTRIAL 
EXISTENCE 
Specific and distinct habitats occur in all 
parts of the earth; but nowhere are they more 
apparent than along the seashore, where most 
littoral animals occur in horizontal zones within 
a vertically thin intertidal area. The high littoral 
zone occupied by P. crassipes is significant in 
this respect because its upper limit on the 
strand is adjacent to true terrestrial conditions; 
few species of the Brachyura range higher. 
Therefore, P. crassipes is a species which has 
almost acquired a terrestrial existence, and it 
occupies a position on the strand higher than 
that of most littoral crabs. Certain morphological 
transformations in this species, which are cor¬ 
related with the attainment of a terrestrial 
habitat, have been studied by Pearse (1931). A 
physiological study by Jones (1941) on the 
osmo-regulatory features of this species has 
contributed additional evidence of its transi¬ 
tional character. The present investigation is 
concerned with several additional features which 
