505.3 
9\n 
v. 14 
\®\l' Reproductible Life History, and Larval Ecology of the 
California Xanthidae, the Pebble Crabs 
Jens W. Knudsen 1 
This study is concerned with the reproduction, 
life history, and larval ecology of the California 
Xanthidae, the pebble crabs. The Brachyura of 
the west coast of North America are not well 
known with reference to their behavior and 
ecology. Because of its economic importance, the 
edible crab, Cancer magister Dana, has been 
made the subject of a series of studies by Wey- 
mouth (1917), Weymouth and MacKay (1934, 
1936), and MacKay (1942). The only studies 
of economically unimportant species are those of 
Wells ( 1940) on the pinnotherid crabs of Puget 
Sound, and Hiatt (1948) on the lined shore 
crab, Pachygrapsus crassipes Randall. The latter 
study is concerned primarily with ecology and 
behavior, and its thoroughness makes it a pattern 
for similar studies. The present paper is the 
partial result of a study on the family Xanthidae 
made by the writer while at the Allan Hancock 
Foundation, University of Southern California. 
The writer is grateful to Dr. John S. Garth and 
Dr. Norman T. Mattox for their guidance in 
this work. 
The xanthid crabs of California are common 
but seldom seen members of the lower intertidal- 
zone fauna, found along protected rocky beaches. 
Their superficial resemblance to beach pebbles 
and their ability to "play dead” make them an 
interesting group to study. Because of their 
similarity, the scope of this paper has been 
broadened to cover the four major species found 
locally, rather than just one species. Thus, Loph- 
opanopeus leucomanus leucomanus (Locking- 
ton), Lophopanopeus bellus diegensis Rathbun, 
Paraxanthias taylori (Stimpson), and Cyclo- 
. xanthops novemdentatus (Lockington) are dis- 
cussed collectively as the major California Xan- 
thidae. Information concerning the reproduction 
and life history of this group is almost totally 
1 Assistant Professor, Department of Biology, Pacific 
Lutheran College, Parkland, Washington. Manuscript 
received September 12, 1958. 
lacking. This is also true, with regard to larval 
ecology, for all Brachyura of western North 
America. 
REPRODUCTION 
Sexual Dimorphism and Maturity 
The only distinct, sexually dimorphic charac- 
teristics of significance in the California Xanthi- 
dae are found in the abdomen of mature and 
submature specimens. The chelipeds of some 
male crabs are said to be larger than those of 
the female of the same species. There is con- 
siderable variation of length-width ratios of 
chelae among the local xanthid crabs, but there 
is no strongly marked trend towards dimor- 
phism. It is generally true that the largest speci- 
mens are males. However, if the life span of the 
male and female crab is approximately the same, 
then the size difference is easily explained by 
the work of Hiatt (1948: 165) with Pachy- 
grapsus crassipes. Hiatt demonstrated that the 
normal molt cycle of female crabs is interrupted 
by periods of egg bearing, and that a longer time 
is thus required for the female to reach a size 
equal to a male. If this applies to the Xanthidae 
as well, then the larger size of the male is not 
sexual dimorphism, but is due to a normal in- 
terruption of the female molt cycle. The abdo- 
men, however, shows definite dimorphic trends, 
and at the time of maturity differs distinctly in 
male and female specimens. 
In taxonomic work, as well as in natural his- 
tory, it is often very useful to know if a par- 
ticular specimen or size-class is mature. Gonad 
study is impractical because (1) the specimen 
is destroyed, (2) the sex organs may be dor- 
mant at the time of year which the specimen is 
collected, making determination impossible, and 
(3 ) the individual would be of no further use 
in a natural history study. Therefore, it is use- 
ful to have certain external signs with which 
mature specimens may be separated from im- 
3 
