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PACIFIC SCIENCE, Vol. XXI, July 1967 
shelf rather than at abyssal depths. The bottom 
fauna on the continental shelf of Baja Cali- 
fornia has been essentially unsampled. 
METHODS 
A cruise was made to the shelf area of the 
west coast of Baja California in the region of 
26°N in November and December I960. The 
following sampling and measuring devices 
were used at most of the stations occupied: 
(1) a dredge with a mouth opening 0.5 ft by 
2.0 ft; this device was designed to remain 
closed while being raised or lowered (to avoid 
catching planktonic specimens), but to open 
upon contact with the bottom; (2) an otter 
trawl with a mouth 10 ft wide; (3) a series 
of plankton nets 1 meter in diameter which 
were towed horizontally, with one net close to 
the bottom, another intermediate in depth, and 
the third at the surface; the two subsurface 
nets were rigged with the Leavitt opening- 
closing device, and were open only while being 
towed at their specific depths; (4) a 900-ft 
range bathythermograph; and (5) seven Nan- 
sen bottles spaced evenly through the water 
column; the water from these samplers was 
titrated by the standard Winkler method for 
dissolved oxygen content. 
THE BENTHIC HABITAT 
On November 30, I960 at the first station 
(26°02'N, 112°58'W) , with a water depth 
of 58 fathoms, a 15-minute tow of the dredge 
collected 23 specimens of P. planipes mired 
in a ball of gray mud. The 10-ft otter trawl, 
towed on the bottom for 25 minutes at the 
same location, caught about 30,000 crabs. A 
10-minute tow with the series of plankton nets 
caught one crab at the surface and none at the 
two lower depths. It seemed that P. planipes 
was abundant in the benthos. 
During the cruise 19 one-meter net tows 
(each of a single net) were made at different 
depths at various times of day and night; 13 
stations were occupied. After sampling at the 
first station the time of the plankton tows was 
lengthened to 20 minutes; each net strained 
about 800 m 3 of water. A total of seven crabs 
were caught in these plankton tows. The otter 
trawl, towed for 25 minutes at each station, 
probably strained no more than five times as 
much water as the meter net, but caught hun- 
dreds of crabs. The difference in catch between 
the plankton net and the trawl was so great 
it seems likely that all of the crabs caught in 
the otter trawl were benthic, even though the 
trawl remained open while it was being raised 
and lowered. This was substantiated by results 
obtained using the open-closing dredge which, 
with an opening only 12% of that of a meter 
net, caught several times as many crabs. 
A transect of 5 stations was made across 
the continental shelf from a point 26°25'N, 
112°30'W, along a course 210° true; the 
western-most station was 85 nautical miles 
from shore. The substrate on the shelf ranged 
from gray muddy sand to gray mud, and gave 
way to naked rocks with solitary corals and 
crinoids on the continental slope. Crabs were 
found abundantly on the bottom between 
depths of 75 and 300 m. Figure 2 presents 
the distribution of crabs and hydrographic data 
along this transect. Other stations occupied 
during the cruise supported the general picture. 
No crabs were caught on Uncle Sam Bank, 
which is rough and rocky, nor were any caught 
by trawls on subsequent cruises at the base of 
the continental slope in that area, at depths of 
1,700 fathoms, where the sediment is again 
fine. 
Other samples were taken on the shelf from 
25°N to 31°N, but no crabs were found north 
of Punta San Eugenio (27°50'N). Trawls 
towed on a subsequent cruise in April 1961 
showed the crabs to be present on the shelf at 
24 °N. It is probable that they occur southward 
to the tip of the Baja California peninsula. The 
benthic distribution in the Gulf of California 
is completely unknown, but the crabs have 
been seen at the surface in the Gulf and also 
have been washed ashore there in great num- 
bers. It is possible that a benthic population 
exists in areas of the Gulf where the proper 
depth and substrate occur. 
Considerable numbers of two other species 
of invertebrates were collected with P. planipes. 
One of these was the holothurian (2 cm long), 
Cucumaria chilensis Ludwig, identified by 
Elisabeth Deichmann of the Museum of Com- 
parative Zoology, Cambridge, Massachusetts. 
