Biology of Fabia s ubquadr at a—. Pearce 
15 
middle August, During this- period as much as 
13% of the observed population was undergo- 
ing ecdysis. Just as a correlation was noted by 
Hiatt (1948:161) for P, eras sipes, so was a 
correlation noted between monthly temperature 
means and the percentage of ecdysis for F. sub- 
quadrata ( Fig, 2 ) . The temperatures are surface 
water recordings made by the U. S. Coast and 
Geodetic Survey in 1958-59' for the waters of 
the San Juan Archipelago, and closely approxi- 
mate the means recorded over a five-year period 
by Phifer and Thompson (1937), While these 
temperatures are taken at the surface, it is as- 
sumed that they would be valid at the depths 
from which the mussels were removed since 
Phifer and Thompson ( 1937:34) note that the 
waters of San Juan Channel are very homogene- 
ous to a depth of 100 m. 
It was noted chat the percentage of Stage V 
females in ecdysis was greatest during August, 
regardless of the depth from which they were 
removed. Figure 2 represents only Stage V 
crabs and does not include those immature crabs 
observed undergoing developmental moults 
throughout the year. 
While increased temperature may not in itself 
be the direct cause for the onset of ecdysis, it 
may be an indirect factor. Certainly the moult- 
ing sequence is correlated in a number of ways 
with reproduction, the success of which in turn 
depends upon factors favoring the survival of 
the zoea. Since it is known that phytoplankton 
standing crops vary with water temperatures and 
in turn are important for the development and 
survival of many zooplankton larval types, in- 
cluding zoea and megalops, so temperature may 
be correlated with the periodicity of ecdysis. 
THE DEVELOPMENTAL CYCLE 
Early investigators (Thompson, 1835) as- 
sumed that the male pinnotherid sought out his 
copulatory partner by moving from host to host 
during the reproductive period. Later Orton 
(1921:533) tended to substantiate these views 
by his discovery of individual male P. pisum 
caught between the valves of the host mussel, 
Mytilus ednlis. The implication was that the 
males were trapped while seeking to gain en- 
trance to the host of a female. In addition, Or- 
ton found a Stage I female with its spermathecae 
filled with viable sperm. This latter discovery 
was repeated by Atkins (1926:478) and was 
indicative of a precocious copulation in P. 
pisum. 
Examination of mid-water plankton trawls 
made by members of the Department of Ocean- 
ography of the University of Washington in 
the southern waters of Puget Sound during May, 
1957 resulted in the finding of 56 Stage I F. 
subquadrata of both sexes. Examination of ma- 
terials collected in a similar manner prior to 
and subsequent to this period revealed a paucity 
of these crabs, only 2 being found. Microscopic 
examination of the spermathecae of the females 
taken in the trawls, as well as those removed 
from host mussels, revealed that F. subquadrata , 
like P. pisum and P. ostreum (Christensen and 
McDermott, 1958), copulates precociously. 
Since the sample taken from these plankton 
tows represents the only time that female and 
male F. subquadrata have been found together, 
it is probable that copulation in this species oc- 
curs during a period when both the male and 
female Stage I crabs leave their host bivalves 
and assume a temporary freeliving existence. 2 
Of the total of 56 Stage I F. subquadrata taken 
in these tows, 29 were males and 27 females. 
Conversely, in only 3 out of 2,088 total observed 
infestations were double infestations of the host 
mussel ever found. In 2 of these cases 2 male 
Stage I crabs were found together, and in the 
remaining example a Stage I female was found 
together with an unsexable prehard crab. 
If copulation occurs while the female is in the 
Stage I form, as seems likely from present evi- 
2 It has only recently come to my attention that 
Sakai (1939:604) has reported a similar swarming 
in the pinnotherid, Tritodynamia horvathi, which is 
found in Japanese and Korean waters. He notes that 
both males and females swarm together in large num- 
bers. The swarming or migrations occur in Japanese 
waters "... from the middle of autumn to the begin- 
ning of winter.” In Tinkai Bay, Korea, however, Ka- 
mita (1935:36) reports it as occurring in June and 
July. The crabs are often found in such dense num- 
bers that they are harvested and used for fertilizer. 
In addition, large schools of fish follow the moving 
crabs, obviously feeding upon them. No ecological 
significance was attached to the swarming, and Sakai 
does not distinguish at what stage of the life cycle 
the swarming occurs. 
