Biology of Fabia sub quadrat a — Pearce 
31 
although it apparently matures only in the latter 
secondary host, and here there is a noticeable 
reduction in survival and percentage of infesta- 
tion when compared to the primary host rela- 
tionship with Crass os trea virginica. In addition 
McDermott (1962^:2) has observed that there 
are annual fluctuations in the incidence of in- 
festation of the secondary host bivalves by P. 
ostreum, and these fluctuations should be studied 
to determine if there are any correlations with 
fluctuations or relative abundance of the primary 
host organism, in this case C. virginica. 
Finally we have the case of P. pisum which, 
according to evidence presented by Christensen 
(1958), almost always develops, in the Kattegat, 
first in Spisula solida and then undergoes an 
obligatory host change to M. modiolus. In a 
more recent paper, however, Christensen ( 1962: 
6) notes occasional exceptions to this general 
pattern: he has found an ovigerous female in a 
Spisula and, in addition, has found several new 
genera which can serve as the initial host. These 
are Glycimeris, Cardium, Laevicardium, and 
Mactra. 
Thus, it would appear that there is a tendency 
for certain species to fill several niches, at least 
during the juvenile stages. Whether, in an evolu- 
tionary sense, these species are progressing from 
an original intraspecific or single-host condition 
to one in which a number of host species are 
infested is not known. It might be suspected, 
however, that while the adults of many of the 
species thus far studied appear to be quite spe- 
cific in their use of hosts, a definite advantage 
would accrue to a species which was able to de- 
velop in more than one host or niche. This 
would be especially true of forms living in trop- 
ical waters, where a greater speciation has oc- 
curred but the total number of any one species, 
and hence of potential hosts, might not be so 
great as in temperate or boreal waters. Sakai 
(1939:589) reports one species of Pinnotheres 
as occurring in at least five different bivalves 
found in Japanese waters (although there is no 
statement as to the stages involved ) ; and the 
tropical species listed by Rathbun (1918) are 
frequently taken from several hosts. Again, how- 
ever, there are no statements as to the stage of 
the crabs. It is of interest that Pinnotheres macu- 
latus, which is distributed throughout a wide 
range of latitude, is found in association with 
many different types of hosts, including poly- 
chaete worms, mussels, oysters, and scallops. 
Even in temperate waters there is an advan- 
tage to the infestation of multiple host species, 
if only by the immature crabs. In the event 
of an epizootic involving the definitive host, 
those individual crabs which have infested the 
secondary hosts during the postplanktonic- 
hard stage of their development would still be 
available to reinfest the surviving, previously 
uninfested individuals of the primary host 
population. 
Intergradations of morphology have been 
found between comparable instars of the species 
so far studied. Both the first crab stage and Stage 
I instar of P. ostreum have rigid, well calcified 
exoskeletons and possess the rod-like structures 
which connect the carapace to the sternum 
(Christensen and McDermott, 1958:150). In 
Fabia the first crab stage is not hard, while the 
Stage I form is; the latter stage of this species 
does have the well developed columnar struc- 
tures linking the dorsal and ventral surfaces of 
the body. Similarly, in P. pisum the first crab 
stage is not hard, while the Stage I instar is. 
In addition to these variations in the hosts 
utilized, and in the morphology of the invasive 
instars, there are the differences already discussed 
in the reproductive biology of the various spe- 
cies. P. ostreum mates within the bivalve host of 
the female while, from all available evidence, 
both F. subquadrata and P. maculatus engage in 
a freeswimming swarming during which copula- 
tion occurs. While nothing definite is known 
about this aspect of the biology of P. pisum , col- 
lections made by Christensen (personal corre- 
spondence) indicate simultaneous occupancy of 
the host bivalves by pairs (male and female) 
of Stage I crabs. On the other hand, the author 
is in possession of a female, Stage I Pinnotheres 
taylori taken freeswimming in a midwater 
plankton trawl. 
Because of these intergradations between the 
various species it is extremely difficult, as sug- 
gested by Christensen and McDermott (1958: 
177), to generalize in any way with regard to 
the biology of the pinnotherid crabs. The fact 
that in many species the Stage I female is modi- 
fied for a freeswimming existence, and yet does 
