32 
not leave the host during this phase of the de- 
velopment, seems incongruous. It may be that 
the modified morphology of the Stage I female 
of P. ostreum is an adaptation for the possible 
host changes which do occasionally occur in this 
species. Christensen and McDermott (1958: 
175) point out that when several female P. 
ostreum invade the same host the excessive crabs 
must either "perish or migrate to other oysters.” 
According to Christensen’s (1958) information 
on P. pisum, it may be assumed that this is 
definitely the case in this species. It seems more 
reasonable, however, to regard the anomalous 
Stage I instar, which seems common to many 
pinnotherids, as a remnant of an earlier an- 
cestral life cycle in which both the male and 
female were hard and occasionally freeliving, 
and both simultaneously infested the host or- 
ganism. Later, during the evolution of the group, 
when the symbiotic existence was definitely as- 
sumed, the female started to moult into the 
large soft, posthard stages capable of producing 
the greater number of eggs necessary to the suc- 
cess and survival of a cryptic, parasitic form. At 
the same time the male became less necessary, 
and today, in at least some species, seems to be 
eliminated following copulation. It is suggested 
that this is a more reasonable hypothesis regard- 
ing the evolution of these forms than that pro- 
posed by Christensen and McDermott. They 
state (1958:176) that posthard males, com- 
parable to the existing female stages, probably 
existed somewhere in the line of evolution. Since 
to this author’s knowledge no known males of 
any freeliving brachyuran species assumes the 
soft habitus of the posthard pinnotherid females, 
it seems unlikely that such males were present 
in the early evolutionary history of the Pinno- 
theridae. 
Finally, as was mentioned earlier, a definite 
correlation has been noted between the depth 
from which the host mussels are collected and 
the percentage of infestation, as well as between 
depth and the size of the symbiont crabs. Pos- 
sible reasons for the correlation between the 
depth of water in which the mussels are found 
and the amount of infestation have been dis- 
cussed previously. It is suggested that the smaller 
size of the crabs taken from mussels removed 
from relatively deeper waters (Fig. 1 ) is the re- 
PACIFIC SCIENCE, Vol. XX, January 1966 
suit of similar influences. A comparative quanti- 
tative study of the digestive tract contents of 
host mussels and their symbiont crabs might be 
of value in determining whether the limiting 
factors include total available food. 
From the foregoing discussion it is obvious 
that the investigation of the family Pinnotheri- 
dae is still as desirable today as it was several 
decades ago, when Rathbun (1918:10) made 
her admonishment concerning the family. It is 
thought that, particularly from a systematic 
point of view, a more intensive comparative 
study of the biology of various species will pro- 
duce an unsuspected amount of information. 
Not only will this information be of interest in 
itself, but it might be profitably applied to the 
study of benthic and pelagic communities. The 
effects of the symbiotic crabs on their host or- 
ganisms would undoubtedly influence the role 
of the latter within the community. This is espe- 
cially true when the host is one of the dominants 
within a community. In fact, M. modiolus has 
been considered a dominant organism in the 
Modiolus faciation of the Strongylocentrotus - 
Argobuccinum biome (Shelford, 1935:287) 
which is typical of the San Juan Channel. Pinno- 
theres ostreum has been investigated with regard 
to its effects on the economically important 
oyster beds of the east coast of the United States 
(Stauber, 1945; Haven, 1958). Hancock (1962) 
has reported that infestation by P. pisum reduces 
the average volume of the meats of the edible 
mussel, Mytilus edulis. 
More recently a study has been initiated to 
ascertain the role which P. maculatus plays in 
the benthic mussel communities found in the 
waters surrounding Cape Cod and the Elizabeth 
Islands. Some of the preliminary results of this 
study are contained in this paper. 
SUMMARY 
1. The postplanktonic stages which succeed 
the megalopal instar are described and their 
dimensions given. 
2. Like Pinnotheres ostreum, Fabia subqua- 
drata is shown to invade the host organism dur- 
ing the first true crab stage. 
3. "Abnormal” instars of both sexes are de- 
scribed. 
