30 
PACIFIC SCIENCE, Vol. XX, January 1966 
Prof. Kincaid in the Friday Harbor region, the 
present author was unable to find it in the 
limited number of specimens which were avail- 
able for examination. All three of these tuni- 
cates are large and quite similar in their basic 
morphology. 
A report by McDermott (1962^:163) also 
contains information which indicates that, as in 
the Fabia-M odiolus relationship, the oyster crab, 
P. ostreum, may develop through the hard Stage 
I instar in a secondary host, M. edulis. After at- 
taining the hard stage ". . . both sexes leave the 
hosts and seek some other molluscs (oysters or 
jingle shells) in which the females may grow to 
maturity.” McDermott suggests, therefore, that 
it is possible for P. ostreum occasionally to 
utilize two hosts in completing its life cycle. 
In the same paper he reported that both P. 
ostreum and P. maculatus are able to develop to 
maturity in the jingle shell, Anomia simplex. 
This is, as he notes, another new record for 
P. maculatus , and emphasizes further ”... its 
profound lack of host specificity.” This pinno- 
therid has previously been reported from a wide 
range of hosts (Rathbun, 1918:76), and since 
Grey ( 1961 : 357 ) has reported it from the tubes 
of Chaetopterus it must be assumed to be quite 
widely distributed. 
While McDermott finds mature P. ostreum in 
Anomia he emphasizes that "the incidence and 
survival of P. ostreum in Anomia is not com- 
parable to what we have seen in the oyster. The 
incidence is much lower as is its survival to 
maturity.” 
Representatives of the subfamily Pinnothere- 
liinae, which includes the several species of pin- 
nixids found in Puget Sound, appear to be 
equally specific in the selection of, or survival in, 
their hosts. On the tidal flats of False Bay, San 
Juan Island, two species of lugworm occur and 
each is associated with a particular species of 
pinnotherid crab. Abarenicola pacifica , a worm 
living in muddy sand, is almost invariably found 
with Pinnixa schmitti; while A. vagabunda, a 
species dwelling in the clean sandy bars protect- 
ing the entrance to False Bay, is usually found 
with Pinnixa eburna (Healy and Wells, 1959: 
325 ) . However, Healy reports in the same paper 
an instance in which A. pacifica was found in the 
clean sand substrate favored by the A. vaga- 
bunda. With three of the former worms he found 
P. eburna. Thus there is an indication that the 
substratum, not the worm, determines the pres- 
ence of the crab. During the present investiga- 
tion, however, several hundred worms of both 
species were subsequently checked and no anom- 
alies were found in the crab-worm association, 
i.e., P. eburna was always found in association 
with A. vagabunda and P. schmitti with A. pa- 
cifica. One final example of the specificity of the 
crab-host relationship is the relationship of Pin- 
nixa fava and P. littoralis to the large lamelli- 
branch host, Schizothaerus capax. As with F. 
subquadrata, the juveniles of these two pinnixids 
are found with a wide range of small bivalve 
species, although interestingly enough these are 
never the same species in which the immature 
Fabia occur. The adult P. faba and P. littoralis, 
however, are found only in association with S. 
capax. Even though closely related, and similar 
in size, Schizothaerus nuttali never contains the 
adult pinnixids and rarely the immatures. The 
reason for this specificity is detailed in a recent 
paper (Pearce, 1962^:48). 
Thus, while many pinnotherid species may 
occasionally be found in other than the primary 
host during their early postplanktonic stages, 
the adults of most species so far studied appear 
typically to be found in a definitive or primary 
host species or type. The reasons for this spe- 
cificity undoubtedly center in the fact that the 
crabs have evolved in many ways to fit the 
environment provided by a specific host or- 
ganism. Apparent exceptions to the general rule, 
such as P. maculatus, must be more thoroughly 
investigated. 
Although at the present time there is not a 
great body of evidence from which to extra- 
polate, there are indications that the pinno- 
therids, as a group, are actively involved in a 
process of adaptation. For instance, within the 
subfamily Pinnotherinae there is considerable 
variation in the use of secondary hosts. F. sub- 
quadrata utilizes a wide range of bivalve species, 
of several families, for secondary hosts although, 
as earlier pointed out, in the waters of Puget 
Sound the adults have almost invariably been 
found in the definitive host, M. modiolus. 
P. ostreum, upon occasion, will infest M. 
edulis and A. simplex (McDermott, 1962^), 
