The Biology of the Mussel Crab, Fabia subquadrata, from 
the Waters of the San Juan Archipelago, Washington 
Jack B. Pearce 1 
The mussel crab, Fabia subquadrata, described 
by Dana in 1851 from material collected in 
Puget Sound, is placed in the subfamily Pinno- 
therinae Milne-Edwards, one of the two sub- 
families of the Pinnotheridae indigenous to the 
Americas. 
Most species of pinnotherids live in associa- 
tion with a host organism. The relationships 
have been variously described as parasitism, 
commensalism, or mutualism. Although the 
mussel crab is usually found in association with 
the horse mussel, Modiolus modiolus, several 
other pelecypod hosts as well as a tunicate have 
been reported (Wells, 1928:289). The present 
research has revealed additional bivalve host 
species. It is frequently reported from both spe- 
cies of Mytilus in waters south of Puget Sound, 
although in the latter waters it does not appear 
to frequent these hosts. 
The life cycle of F. subquadrata is complex 
and, as with many pinnotherids, includes several 
developmental forms subsequent to the typical 
decapod larval stages (i.e., the zoea and mega- 
lops) and before the definitive adult stage is 
reached. As in most marine decapods, the early 
pinnotherid zoea and megalops are planktonic. 
Upon moulting from the megalops into the first 
true crab stage the animal, it is thought, leaves 
the plankton and becomes associated with its 
host. 
Postplanktonic developmental stages of a pin- 
notherid were first described by Atkins (1926: 
475) for Pinnotheres pisum, which is common 
to the coast of the British Isles. Later Stauber 
( 1945:269) found that the developmental cycle 
of the North American east coast pinnotherid, 
Pinnotheres ostreum, was very much the same as 
that previously described for P. pisum. The re- 
1 Department of Zoology, University of Washing- 
ton, Seattle 5. Present address: Marine Laboratory, 
Humboldt State College, Areata, California 95521. 
Manuscript received June 25, 1964. 
suits of the present study indicate that F. sub- 
quadrata passes through stages comparable to 
those reported for P. pisum by Atkins and P. 
ostreum by Stauber and by Christensen and Mc- 
Dermott (1958:150). There are, however, im- 
portant differences in the developmental cycle 
of P. subquadrata. 
The present known range of the mussel crab 
is from the coast of Alaska to southern Cali- 
fornia. Rathbun (1918:102) noted it in waters 
250 m deep and Wells (1940:47) found it in 
mussels dredged at a depth of 220 m. Hart 
(personal communication) has found it in M. 
modiolus taken inter tidally near Victoria, Van- 
couver Island, British Columbia. 
While it is true that pinnotherid crabs have 
been known from ancient times, only recently 
have there been any comprehensive studies of 
any member of this family. Atkin’s early ob- 
servations ( 1926) on the moulting stages of 
P. pisum laid the groundwork for future studies. 
This was followed by Hart’s investigations 
(1935) in which she reported success in hatch- 
ing the eggs of Pinnotheres taylori and rearing 
them through the first true crab stage. Sandoz 
and Hopkins (1947:250) were able to rear 
P. ostreum to this same stage. These investiga- 
tions extended the earlier work of Atkins, in 
which the hard and posthard stages subsequent 
to the first crab stage had been described. At- 
kins ( 1955) later raised two species of Brit- 
ish pinnotherids, P. pisum and P. pinnotheres, 
through the megalops stage. 
Most of the workers cited above were con- 
cerned largely with the early development of the 
crabs rather than with their ecology or associa- 
tion with the hosts. Wells’ studies ( 1928, 1940) 
were among the first published papers concerned 
with the biology of American species of pinno- 
therids. Later Stauber ( 1945 ) investigated the 
postlarval development and habits of the oyster 
crab, P. ostreum. This work was followed by that 
of Christensen and McDermott (1958) which 
»i tew 
3 
