Benthic and Pelagic Habitats of Red Crab — Boyd 
397 
Transect heading 2I0°T from 26° 25' N x 112° 30' W 
Transect length =85 nautical miles 
Station Station Station Station Station Shore 
Fig. 2. Vertical profile showing bathymetry, temperature structure, oxygen concentration and distribu- 
tion of Pleuroncodes plan ip es on the western coast of Baja California, Mexico, in December I960. 
P. planipes, a voracious omnivore, did not eat 
the holothurians when placed with them in 
shipboard aquaria (although the crabs would 
eat pieces of fish under similar circumstances). 
The other abundant invertebrate was a gastro- 
pod, Nassarius miser Dali, identified by Emery 
P. Chace of the San Diego Museum of Natural 
History. There were very few worm tubes or 
other obvious animals in the bottom sediment. 
It is possible that the constant sifting of the 
substrate by P. planipes in their search for food 
(Nicol, 1932) would reduce the numbers of any 
invertebrate animals having no defense against 
the crabs. 
No crabs larger than 26 mm in standard 
carapace length (as measured from the notch 
between the rostral and subrostral spine and 
the median posterior margin of the carapace) 
have been found in plankton collections, and 
it has been assumed that this was their maxi- 
mum size. However, at station 7 (depth about 
300 m, see Fig. 2) the otter trawl and the 
closing dredge brought up crabs with a mean 
standard carapace length of 27.9 mm, and a 
maximum standard carapace length of 32.0 mm. 
Juxtaposition of this size on a calculated growth 
curve (Boyd, 1967) suggests that these larger 
crabs living along the edge of the continental 
shelf constitute an older year class; they have 
probably completed their second year of life. 
Since no individuals of this size have been taken 
in the plankton, they are presumably exclu- 
sively benthic at this age. Station 7 was the 
deepest station from which crabs were dredged 
from the bottom. The mean length of the 
crabs caught from this deep station differed 
significantly (p < 0.05) from the mean lengths 
of crabs dredged from other stations, where 
the means did not differ from each other. 
It appears that P. planipes lives to some 
extent on the bottom in its first two years of 
life and is also found as a planktonic animal 
at this stage. The relative amount of time spent 
in these two environments is unknown, but 
data from plankton collections indicate that 
there is some diurnal exchange between them, 
with crabs occurring in the surface water at night 
and settling to greater depths and perhaps to 
the bottom during the daytime hours (when a 
suitable bottom is available). After their sec- 
