Benthic and Pelagic Habitats of Red Crab — Boyd 
401 
6002. The specimens captured just south of 
Monterey represent the most northern intrusion 
of the animals during the period 1955-1960, 
and their occurrence in the plankton coincides 
with their stranding on the beach of Monterey 
in January I960 (Glynn, 1961). The period 
between 1957 and I960 was noted for the 
intensification of the countercurrent system and 
the intrusion of many southern forms into 
northern waters (Radovich, 1961). 
The pelagic distribution of P. planipes in 
the Gulf of California is less well documented 
because only occasional cruises have been made 
there. The available data indicate that the crabs 
sometimes occur in abundance; for instance, 
large shoals of crabs were sighted in the north- 
ern half of the Gulf during the Vermillion Sea 
Expedition of the Scripps Institution of Ocean- 
ography in 1959. The crabs may have been 
swept in from areas on the western side of the 
peninsula as a consequence of the sporadic 
exchange of water between the California Cur- 
rent and the Gulf of California (see charts by 
Cromwell and Bennett, 1959). The alternative 
explanation is that the crabs were from a per- 
manent population, even though the benthic 
habitat probably is very limited; the continental 
shelf is almost completely lacking in the Gulf, 
and the shore line drops off precipitously to 
depths of many hundreds of fathoms. 
P. planipes has been captured more fre- 
quently during night hours than during day- 
light hours in the plankton tows of the ccofi 
Cruises. In the monthly cruises 6001 to 6008 
(January to August I960), a total of 1,237 
plankton tows were made within the region 
where P. planipes might be expected to occur; 
601 were made at night and 636 in the day; 
197 nighttime stations and 94 daytime stations 
yielded crabs. When these data are analyzed 
by 2 X 2 contingency analysis, the probability 
(p < 0.01) indicates higher nighttime capture. 
This analysis compared only positive and nega- 
tive plankton tows and neglected the numbers 
of individuals caught. This information sub- 
stantiates the picture of greater nighttime cap- 
ture; during the March cruise (6003), 536 
crabs were caught at night and only 96 in the 
daytime (the number of stations occupied at 
night was 71, and by day, 64). A chi-square 
test of the numbers of crabs caught at the 
hourly intervals contrasted with the numbers 
expected at these hours if the crabs had oc- 
curred with the same frequency at all times 
indicates a highly significant difference in fre- 
quency (p < 0.001). This difference is due 
to the vertical migration of the crabs to the 
surface waters at night. The possible explana- 
tion that the crabs are avoiding the net during 
the day is rejected from analysis of a multiple 
linear regression (Boyd, 1967), indicating that 
larger crabs were caught during daylight hours; 
if the day-night abundance differences were 
due solely to avoidance, the larger animals pre- 
sumably would have been able to avoid the net 
more easily than the small animals. While there 
is a tendency toward upward nocturnal migra- 
tion it is by no means inviolate; crabs have 
often been sighted at the surface during the 
day. It is believed that in neritic waters the 
animals may settle to the bottom during the 
day when a suitable substrate is available. 
SWARMS AND BEACH STRANDINGS 
At times great numbers of red crabs are 
seen swimming at the surface of the ocean, 
particularly in the area along the western coast 
of Baja California. The late Bell M. Shimada 
of the Inter-American Tropical Tuna Commis- 
sion reported (personal communication) steam- 
ing through such numbers that the "ship 
seemed to crunch through them for at least 
ten miles.” Concentrations in excess of 100 
crabs/m 2 of water surface over broad areas 
have been seen and photographed, but concen- 
trations of 1-10 m 2 are more common. Reports 
by many individuals indicate that surface 
swarms may be sighted throughout the year, 
both night and day; the occurrence of the 
swarms does not seem to be associated with 
seasonal (winter) breeding cycles. 
Mass mortalities of the crabs result when 
the swarms are washed up on the beach; it was 
such a stranding at Monterey, California in the 
winter of 1858 that resulted in the description 
of the species by Stimpson (I860). Many 
strandings were noted in 1958 and 1959 in 
the San Diego area, where the crabs had not 
been sighted in many years, and strandings on 
the beaches south of Punta San Eugenio are 
common. 
