Biology of Pachygrapsus crassipes —HIATT 
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convergent in the region of Japan. This isother¬ 
mic pattern seems to offer a distinct clue as to 
why the latitudinal distribution of this species 
is restricted in the Orient. However, more crit¬ 
ical examination shows that the tolerable tem¬ 
perature range for this species as exhibited on 
the Pacific coast of America is almost nowhere 
fully occupied in the Oriental region. 
That this species could, but does not, possess 
a greater latitudinal distribution in the Orient 
indicates that either (1) control of its distribu¬ 
tion is accomplished by factors (food supply, 
parasites, etc) which have not been taken into 
consideration because of insufficient knowledge 
of the Oriental region, or (2) the presence of 
this species in the Orient has been of relatively 
recent occurrence. The latter hypothesis seems 
unwarranted, however, for reasons cited below. 
Extremely rapid invasions of new areas are 
known to have been accomplished by this and 
other species of Crustacea. Stebbing (1888, 
1893, 1906), Alcock (1899, 1900), Fulton 
and Grant (1901), Chilton (1911), and Pan¬ 
ning (1939) have recorded numerous observa¬ 
tions of Crustacea which, by means of trans¬ 
oceanic shipping, have attained new frontiers. 
Some of these transportations have been suc¬ 
cessful, e.g., Eriocheir japonicus as described by 
Panning (1939). It is certain that at present, 
and for a long time past, the dispersal of P. 
crassipes over its geographical range could not 
have occurred by a littoral migration along the 
north Pacific shores, presumably because of the 
unfavorable temperatures. The low winter tem¬ 
perature along the Oregon coast probably de¬ 
limits the northern extent of this species; other 
ecological requirements seem satisfactory farther 
northward. If a littoral migration of this type 
had taken place when the climate was temperate 
along the north Pacific shores, one would expect 
to find some manifestation of morphological 
change accompanying the long isolation of the 
populations. However, the American and Jap¬ 
anese types are apparently identical. 
Although the currents in the Pacific are 
favorable for the transportation of pelagic fauna 
between American and Japanese coasts, it is 
highly improbable that this species was dis- 
