Biology of Fabia subquadrata — PEARCE 
29 
the dense population observed is actually a re- 
productive swarming. To the present author’s 
knowledge neither Miyadi or Thorson was able 
to study subsequently the respective areas, and 
hence it is unknown whether or not these popu- 
lations were maintained. 
The present investigations substantiate the 
finding of Christensen and McDermott (1958: 
174) that the soft-shelled, posthard females do 
not normally leave their host. As indicated 
earlier, however, at least the immediate post- 
hard instars of Fabia are able to leave their dying 
host, and a small Stage V Pinnotheres puget- 
tensis has been observed, and photographed, 
leaving its ascidian host, Halo cynthia igaboja. 
Another aspect which should be investigated 
further is the size differential between male and 
female Stage I crabs of at least two species, and 
the possibly related phenomenon in which males 
of this stage were observed to moult into soft, 
posthard forms as reported by the late Dr. At- 
kins ( 1958). She regarded these moults as hav- 
ing possible significance in the growth of male 
crabs. The results reported by both Christensen 
and McDermott (1958:153) and the present 
investigation indicate a somewhat larger size for 
the Stage I male than for the comparable female 
instar. The former found that the female P. 
ostreum ranges from 1. 3-2.7 mm in carapace 
width, while the male ranges from 1. 4-4.6 mm. 
Female F. subquadrata range from 1. 5-6.2 and 
average 3.5 mm (29 individuals), while the 
males range from 1. 3-6.8 and average 4.1 mm 
(54 individuals). Atkins (1958) stated that 
hard' Stage I males would moult into a soft stage 
which was frequently followed by two more 
moults. Thus in P. pisum, which Atkins studied, 
one to three thin-shelled instars intervened be- 
tween successive thick-shelled or hard forms. 
Since Christensen and McDermott (1958:164) 
were unable to keep alive the male Stage I P. 
ostreum under their laboratory conditions, they 
could not observe such moultings. 
Male Stage I F. subquadrata survive in the 
laboratory as long or longer than the same 
female instar. Such a crab was taken to the 
Zoology Department of the University of Wash- 
ington following the end of the normal summer 
session at the Friday Harbor Laboratories (Au- 
gust 30, 1959). This crab survived, in spite of 
no actual feeding or efforts to maintain a nor- 
mal environmental temperature, in the confine- 
ment of a finger bowl until the following spring 
(May I960). Similarly, many male Stage I crabs 
were held throughout the summer months at the 
Friday Harbor Laboratories. Of particular sig- 
nificance was the fact that eight such crabs did 
moult into soft, posthard forms. It is thought 
that the slightly greater size of the male Stage I 
crabs may be a reflection of a growth moulting 
which has previously been regarded as anoma- 
lous or as not actually occurring. 
With regard to the crab-host relationship a 
number of interesting conclusions can be drawn. 
There is no doubt that the relationship between 
F. subquadrata and M. modiolus is parasitic in 
nature, especially if the broad definition of Hop- 
kins (1957:413) is used. The extensive, almost 
ubiquitous, damage to the ctenidia as well as to 
the underlying mantle and palps cannot be con- 
strued as anything but a result of a parasitic re- 
lationship. As with some parasitic relationships 
(Allee et al., 1949) it seems to have developed 
with a degree of specificity. By this it is meant 
that many species of pinnotherid crabs, includ- 
ing Fabia, are primarily found, at least in the 
adult instar, in a single host species. There are, 
however, exceptions to this generalizations, both 
for Fabia and the other pinnotherid species. Al- 
though in the waters of Puget Sound adult 
female F. subquadrata almost invariably occur 
in the definitive or primary host, M. modiolus, 
Wells (1928:289) reports it with both Mytilus 
edulis and M. calif ornianus as well as in the 
branchial sac of the ascidian, Styela gibbsii. The 
crab found in the latter host was noted, how- 
ever, as being immature. During the present in- 
vestigation no crabs, of any stage, were found in 
either M. edulis or M. calif ornianus. In the more 
southern extensions of its range adult F. sub- 
quadrata has recently been found in M. cali- 
f ornianus. The status of the definitive host, M. , 
modiolus, in these waters is not known. 
The closely related pinnotherid, Pinnotheres 
pugettensis, which from present information 
may have a life history very similar to that of 
Fabia, was found only in the large ascidians, 
Halo cynthia igaboja and Ascidia paratropa. 
While Wells (1928:286) reports it only from 
Halo cynthia ( Fethyum ) aurantium collected by 
