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PACIFIC SCIENCE, Vol. XXI, July 1967 
All the crabs involved in the strandings that 
author has observed have been in the upper 
50 cm of water in the surf zone. According 
to Inman and Quinn (1952) there is a net 
transport of this surface layer of water onto 
a beach with breaking waves. This onshore 
transport is balanced by water moving offshore 
in fast-moving rip currents and also along the 
bottom. The beach, then, is the ultimate desti- 
nation of any object floating in the near-shore 
surface waters. An onshore wind and a reced- 
ing tide hasten and intensify the stranding. 
The number of crabs involved in the strand- 
ings may be very large; one report (by George 
E. Lindsay, Museum of Natural History at 
San Diego, California, personal communica- 
tion) from the Gulf of California notes the 
crabs occurring in windrows up to 3 ft deep 
and 10 ft wide over a stretch of beach 3-4 
miles long. 
PREDATORS 
Presumably, when the crabs are in the pelagic 
phase they are preyed upon by large oceanic 
game fishes, notably albacore, yellowfin tuna, 
and skipjack tuna; these animals are not known 
to be bottom feeders. Alverson (1963) indi- 
cated that P. planipes constituted 78.1% of the 
volume of the yellowfin tuna’s stomach con- 
tents in the area along the western coast of 
Baja California (approximately the area shaded 
in Figure 4). Around Alijos Rocks (24°57'N, 
115°45'W) the percentage was as high as 97.5. 
P. planipes amounted to 34.1% of the volume 
of the stomach contents of all the yellowfin 
tuna caught and sampled in the entire eastern 
Pacific Ocean, and occurred in 39% of the 
stomachs which Alverson examined. He noted 
that P. planipes was also a significant food 
item for skipjack tuna as well as yellowfin 
tuna. McHugh (1952) found P. planipes com- 
prised about 11% of the total volume of the 
contents of the albacore stomachs examined in 
his study; the percentage was higher (13-43) 
for those albacore which had been caught in 
areas where crabs were more abundant. 
J. C. Quast (personal communication), in a 
survey of food habits of common kelp-bed 
game fishes along southern California, noted 
that these fishes were feeding on P. planipes 
during the time of his study (1959). Among 
those fishes were kelp bass ( Paralabrax clathra- 
tus ), sheeps-head ( Pimelometapon pulchrum ) , 
various rockfishes ( Sehastodes spp.), senorita 
( Oxyjulis calif ornica) , and sculpin ( Scorpaena 
guttata ). Yellowtail ( Seriola dorsalis ) and 
white sea bass (Cyno scion nohilis) have been 
found to feed on P. planipes at the Coronado 
Islands, Baja California, Mexico. Bottom-living 
fishes are also known to feed on P. planipes; 
boccacio ( Sehastodes paucispinis) , the barber- 
pole fish (5. ruhrivinctus) , lingcod ( Ophiodon 
elon gains') , and various other species which 
had been feeding on P. planipes have been 
caught off La Jolla, California, at a depth of 
330 ft. 
SUMMARY 
Pleuroncodes planipes has been found to 
exist as a benthic animal on the continental 
shelf of western Baja California, south of 
Punta San Eugenio. Two distinct populations 
seem to exist: a group of larger animals in 
their third year of life, or older, lives on the 
outer margin of the shelf ; inshore of that group 
lives a population of animals in their first and 
second years of life. The smaller animals oc- 
cupy the greater area, and exist in densities 
of 9-1 1/m 2 of bottom. Plankton tows made 
in this area and elsewhere indicate that the 
smaller crabs occur pelagically with a marked 
daily rhythm, being more abundant in the sur- 
face waters at night. When the crabs are in 
the plankton they are dispersed to the north 
by the coastal countercurrents and to the south- 
west by the California Current and Equatorial 
Current. When the crabs are at the surface they 
are fed upon by the tuna and albacore, and as 
the shoals of crabs are set on shore by wind 
and currents they are eaten by near-shore fishes 
or they may be stranded and die on the beaches. 
ACKNOWLEDGMENTS 
The help and encouragement of Dr. M. W. 
Johnson, Dr. E. W. Fager, Dr. C. L. Hubbs, 
and Dr. M. Blackburn, all of the Scripps Insti- 
tution of Oceanography, are gratefully acknowl- 
edged. This study was partly supported by the 
Scripps Tuna Oceanography Research Program 
