NEW YORK SHELL CLUB NOTES No. 244 September 1978 Page 7 
(called seminal receptacles) on the ventral or bottom side of the 
female. Upon receiving the sperm in the seminal receptacles, the 
female leaves the water and again returns to the benthic environ- 
ment. Once more, the female crab seeks out a mussel host. The tiny 
crab, which may now measure 5mm in carapace width, enters a host 
mussel and again moults into a new instar. 
The instar following the hard, swimming instar is characterized 
again by having a soft exoskeleton and tiny cylindrical appendages. 
This instar passes through several other moults, each resulting in 
a crab similar in appearance to the previous instar but somewhat 
larger. Also, during this period the abdomen increases greatly in 
width. After four such moults the final, definite adult female crab 
is attained. At this time the female crab produces a large number 
of eggs, perhaps over a thousand. These eggs are fertilized as they 
are extruded from the female and pass by the openings to the seminal 
receptacles. Thus sperm that were transferred to the female during 
the swimming or swarming stage of the hard instar fertilize eggs 
that are extruded from her ovaries weeks or months after fertiliza- 
tion has occurred. 
It is thought that the swarming males which were found in the open 
water continue to seek out swarming female crabs to fertilize them. 
Because the males remain in the open waters much longer they are 
more subject to predation. Therefore, following the swarming acti- 
vities of both sexes the numbers of males are greatly reduced; very 
few males are to be found in host mussels following swarming. 
The cycle is thus completed when the eggs of the adult female hatch 
and once again zoae leaves the host mussel for open water. It can 
be seen from the foregoing that this crab has an interesting life 
history in which, at two distinct periods, the instars are to be 
found in the water column: one when the larvae leave the host and 
the second period during the swarming activities. 
It was earlier thought that the male crabs must swim or crawl about 
the bottom seeking out female crabs within host mussels. From the 
research I have done, it would appear that, in fact, this does not 
happen but rather both sexes engage in swarming activities when 
fertilization occurs. It is now known that this unique life history 
is characteristic of the West Coast mussel crab, Fabia subquadrata, 
and the Bast Coast mussel crab Pinnotheres maculatus. It ts also 
believed that the oyster crab, Ss ostreum, probably has a very sim- 
ilar life history. 
Pinnotherid crabs from the West Coast of North America include, per- 
haps, one dozen species. One genus, Pinnixa, has several species. 
The life history of these crabs differs considerably from the life 
history of crabs belonging to the genera Fabia and Pinnotheres. 
Crabs belonging to the genus Pinnixa are foun with a wide range of 
hosts including several bivalve moiluscs and certain gastropods as 
well as several polychaete worms. Work done with the West Coast 
mussel crab indicates that it is also found with Mytilus edulis as 
well as M, californianus. The East Coast muaeg anals pinnelhere? 
Maculatus, is principally found with the mussel, M. edulis. Ss 
crab Sear dawaver: ss eouad in other hosts, particularly in the 
Southern part of its range. Additional hosts include the pen shells, 
Atrina species, and scallops. 
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