Biology of Pachygrapsus crassipes —HlATT 
145 
TABLE 1 
A Comparative Summary of the Number of Gills, Volume of Gills in Relation to Body 
Volume, and Habitats of Selected Littoral Brachyurans on the Coast of Central California. 
SPECIES 
NUMBER OF 
SPECIMENS 
TESTED 
NUMBER 
OF GILLS 
PERCENTAGE OF GILL 
VOLUME TO BODY 
VOLUME 
HABITAT 
Range 
Mean 
Cancer antennarius .. 
4 
18 
4.9-5.1 
5.0 
Low-tide zone and below; 
crevice dweller 
Cancer magister . . 
1 
18 
4.4 
Below low-tide mark; 
sandy substrate 
Pugettia productus ... 
4 
18 
3.1-4.0 
372 
On algae in tidal zone 
and off shore 
Hemigrapsus nudus . 
3 
16 
27-2.8 
2.77 
Rockweed belt; sand 
below rocks 
Hemigrapsus oregonensis.... 
2 
16 
23-2.5 
2.4 
Tidal zone; muddy shores 
Pachygrapsus crassipes . 
5 
18 
23-2.5 
2.36 
High-tide mark and 
above; rock crevices 
seashore where normal environmental condi¬ 
tions prevailed. Ten specimens of P. crassipes 
and 12 specimens of H. nudus, all in stage C„ 
were collected at Pacific Grove and placed in 
dry aquaria. The aquaria were concealed high 
on the rocks under a large boulder which ex¬ 
cluded the direct rays of the sun. Observations 
were made at 15-minute intervals from 4:00 
A.M. to 10:00 P.M. 
The results of these tests were comparable 
with those obtained in the laboratory, except 
that death did not ensue as rapidly. 
Under these seashore conditions, crabs of a 
size similar to those utilized in the laboratory 
lived approximately twice as long. A direct 
relationship between the size of the crab and 
the tolerance to desiccation was found; the 
smaller crabs became inactive and died sooner 
than the larger ones. Crabs of both species 
having a carapace width of approximately 40 
millimeters lived about 48 hours. One specimen 
of H. nudus having a carapace width of 57 milli¬ 
meters lived 70 hours. This apparent relation¬ 
ship between tolerance to desiccation and size 
seems to explain, in part at least, why smaller 
crabs of both species are invariably found in 
the most moist area of the habitat. These data 
demonstrate that the littoral horizon selected by 
each of the species concerned is not determined 
by its ability to withstand long periods of desic¬ 
cation. 
It is highly probable that some physical as 
well as physiological changes would occur in 
the respiratory mechanism of a crab which 
remained exposed to air for relatively extended 
periods. Further, this change should be cor¬ 
related with the amount of exposure endured; 
therefore, a proportionate decrease in gill vol¬ 
ume and surface to reduce desiccation would 
seem a reasonable supposition in view of the 
fact that ability to resist desiccation is generally 
dependent upon the surface area of external 
membranes. Pearse (1929^) has shown that 
as crabs become adjusted to a terrestrial life 
there is a gradual reduction in the number and 
volume of the gills. In order to ascertain the 
status of the gill-volume-body-volume ratio of 
P. crassipes in contrast to other Brachyura, a 
series of crabs was collected from habitats 
ranging from below low-tide level to above 
high-tide level. 
The proportionate volume of the gills com¬ 
pared to the volume of the body was secured 
in the following manner: The crabs were 
hardened 24 hours in 1 per cent chromic acid 
and subsequently rinsed with tap water; pereio- 
pods were removed at the fracture plane; the 
carapace was removed, and the gills were cut 
