168 
PACIFIC SCIENCE, Vol. II, July, 1948 
and crabs above this width seem to be third- or 
perhaps fourth-year specimens and show no 
evidence of impending ecdysis. Third, the 
curves indicate that size-age dimorphism first 
becomes apparent in crabs with a carapace 
width between 20 and 25 millimeters. Further, 
the data set forth in Figure 11 present evidence 
to show that this dimorphism occurs just sub¬ 
sequent to the onset of sexual maturity in 
females. These data were secured from 80 
ovigerous females collected from August 3 to 
10, 1940. In addition to establishing corrobora¬ 
tive evidence with respect to size dimorphism, 
the data presented in Figure 11 show that the 
size span for ovigerous females is too extensive 
to fall within any single age group, but does 
coincide well with the data presented in Figure 
10. Fourth, it is apparent that the size diver¬ 
gence between sexes, after sexual maturity in 
females, is associated with a successive propor¬ 
tional decrease in post-exuvial size increment. 
Notwithstanding all the variables attendant 
upon the growth of crabs, the life span and 
growth characteristics set forth above are, in 
general, characteristic of this species. 
Comparative information on age investiga¬ 
tions of other species of Brachyura is particu¬ 
larly significant because virtually all workers 
have followed different avenues of approach, 
yet nearly all obtained comparable results. Aside 
from the comparatively lengthy life span of 8 
to 10 years for Cancer magister (MacKay, 
1942), those known are very similar to that 
of P. erassipes. Hay (1905) and Churchill 
>0 
9 
e 
16 16 20 22 24 26 - 28 30 32 34 36 33 40 42 44 
WIDTH IN MM. 
Fig. 11. Size frequency of ovigerous P. crassipes 
collected during August, 1940. 
(1918) found a 3-year life span for Callinectes 
sapidus with mating and spawning initiated in 
the second year, and Broekhuysen (1936, 1942) 
discovered a similar span for Carcinides maenas 
and Cyclograpsus punctatus. 
GENERAL HABITS AND BEHAVIOR 
Visual, Chemical, and Tactile Senses 
Observations and simple experiments per¬ 
formed with both captive and wild crabs indi¬ 
cate that the visual and tactile senses of P. cras¬ 
sipes are well developed, while the chemical 
sense seems to operate on a somewhat lower 
functional grade. The complex integration and 
mutual cooperation of all three senses are of 
such nature that an estimation of the degree of 
participation in an observed reaction can only 
be arbitrary at best. 
Although visual acuteness among the Crus¬ 
tacea is doubted by many investigators, an 
observer of P. crassipes could scarcely fail to 
note their alertness. The stalked eyes are capable 
of considerable movement. At rest the normal, 
undisturbed animal will frequently move the 
eyes forward, backward, and laterally. These 
movements suggest a critical surveillance of 
the environment in an attempt to achieve the 
most acute images possible. When frightened, 
the animals lower the eyestalks laterally into a 
groove for their reception between the orbital 
and suborbital areas of the carapace. The faceted 
surface of the lowered eye receives protection 
from the orbital spine. 
It is generally believed that the sharpness of 
any image recorded by this type of eye will 
depend upon the number of facets which per¬ 
ceive the object. Inasmuch as the number of 
facets engaged by an object varies inversely 
as the square of the distance, P. crassipes must 
be somewhat myopic. If the number of facets 
engaged by the object provides the lone cri¬ 
terion for visual acuteness, the rapid reaction 
of P. crassipes to persons walking at a distance 
of several yards is somewhat enigmatic. It 
would seem, therefore, that a moving object 
