16 
dence, and if copulation were consummated in 
the host mussel, it would be expected that oc- 
casional pairs of males and females would be 
found in the large numbers of mussels examined 
throughout the year. Such double infestations 
have not been found, however. Furthermore, the 
finding that Stage I males and females swarm 
together in open water gives evidence that the 
copulatory act takes place outside the host. 
This is contrary to what other investigators 
have found to be true in related pinnotherids 
(Thompson, 1835; Orton, 1921:533; and Chris- 
tensen and McDermott, 1958:166). 
More recent evidence, involving the attrac- 
tion of swimming Stage I crabs to a "night light” 
used at the Friday Harbor Laboratories, indi- 
cates that the swarming behavior is probably 
restricted to late May and early July. Of eight 
crabs taken in this manner during the spring 
and summer of 1961 only one was obtained later 
than mid- June and none earlier than the 4th of 
May. 
It is of interest that the average sizes of the 
swarming Stage I males and females, taken in 
the mid -water trawl and at the "night light,” are 
approximately the same. The average carapace 
width of 32 males is 3.51 mm, while the same 
number of females averaged 3-58 mm. 
Collection data show that swarming was not 
restricted to only one station or limited area. 
Rather, it was found to occur at several widely 
separated stations in the San Juan Archipelago 
and Puget Sound. Since the host mussel is widely 
distributed in these waters, it would be ex- 
pected that the crab symbiont is found equally 
dispersed. 
A current investigation of Pinnotheres macu- 
latus as part of the benthic community in the 
Woods Hole area indicates that this species also 
engages in a copulatory swarming. In 1963 this 
swarming reached its peak during the last two 
weeks of October. A more detailed account of 
this and other aspects of the biology of P. macu- - 
latus will appear in a later paper. 
When all the material collected from the San 
Juan Archipelago area in 1958-59 is considered, 
it is clear that the greatest number of ovigerous 
females appears during the winter months. Large 
numbers are initially found in early November 
and form a significant portion of the population 
PACIFIC SCIENCE, Vol. XX, January 1966 
through the middle of February, with a peak at 
the last of January. As many as 75 (60%) of 
the total adult population of 126 Stage Y fe- 
males collected off Point Caution on January 28, 
1959 were ovigerous. Collections made during 
the winter of 1959-60 indicate that the ovigers 
were more numerous earlier in the season during 
this particular year. A sample of 22 Stage V 
crabs collected off Mineral Point and examined 
on November 23, 1959 contained 19 ovigerous 
crabs ( 87 % ) . Large percentages of ovigers were 
found at the other collecting sites during this 
period. 
Since the swarming females copulate in late 
May, an interval of some 21 to 26 weeks would 
ensue before the start of egg deposition in 
November and December. During this period 
the precociously inseminated female must un- 
dergo the series of growth and metamorphic 
moults which have already been described. Be- 
cause there is no overwintering of immatures as 
reported for P. ostreum by Christensen and 
McDermott (1958:158), the number of imma- 
tures, both male and female, present in the 
population is low during the winter months of 
November, December, and January. 
The immature crabs are found in greatest 
numbers during the early- and mid-summer 
months. On July 21, 1959 they constituted 56% 
of a total sample of 119 crabs collected off Point 
Caution. They formed a comparable percentage 
of the population at the other stations during 
the period of June 15” August 1. The Stage I 
crabs were particularly prevalent in the samples 
taken during May. The collection taken at Min- 
eral Point on May 4, 1959 included 37 Stage I 
instars. These 37 crabs constituted 31% of the 
total population. The remainder were mostly 
prehard stages which would moult into the Stage 
I form before the month was over. The early 
posthard forms (the Stage II and III instars) 
were more in evidence during June and early 
July, with the later posthard forms (the Stage 
IV and V instars ) becoming prevalent in August 
and September. 
From the discussion above and Figure 3 it is 
obvious that the ovigerous females occur pre- 
dominantly during the months of November, 
December, and January. During February the 
eggs begin to hatch and the new larvae spend 
