NEW YORK SHELL CLUB NOTES No, 244 September 1978 Page 5 
PARASITIC CRABS AND MOLLUSKS 
Jack B. Pearce 
Sandy Hook Laboratory 
Northeast Fisheries Center, National Marine Fishery Service 
Highlands, New Jersey 07732 
Tiny crabs, referred to as commensal or pea crabs, are found living 
inside a wide range of hosts. These crabs belong to the family 
Pinnotheridae. In the past the crabs have been regarded as being 
commensals in terms of their relation to the host. MThe first inten- 
sive study done on these crabs was conducted by Christensen and 
McDermott (1958) who described the life history and biology of the 
oyster crab, Pinnotheres ostreum. This species is found in oysters 
collected along the eastern seaboard; many connoisseurs of oysters 
consider the tiny crabs to be an extra delicacy that can be found 
when eating oysters. 
These crabs are now regarded as being parasitic. They cause defi- 
nite harm to their bivalve hosts. In the process of moving about 
over the gills to collect food, damage is caused to the gill tissues. 
This damage has been shown (Haven, 1958) to affect the actual bio- 
mass or weight of tissues. The body mass of mussels and oysters 
which contain crabs often weigh less than those which are free of 
the parasites. In some cases damage can even be caused to the host's 
shells. I have found that mussels which support a parasitic crab 
are often less resistant to environmental stress and when there is 
a shortage of food the presence of the crab can result in starvation 
of the host bivalve, as well as the crab itself; the host simply 
cannot feed intensively enough to provide nutrients to both itself 
and the parasite. 
In recent studies done in the heavily polluted New York Bight and 
adjacent embayments, such as Raritan Bay, Jamaica Bay, and Newark 
Bay, I have noted that mussels are usually free of parasitic crabs. 
It therefore appears that the presence or absence of crabs might pro- 
vide an index as to water quality in our coastal, oceanic and estua- 
rine waters. Because of the difficulty of collecting large numbers 
of crabs over a wide area, I am asking that conchologists and shell 
collectors interested in the environmental quality of marine waters 
notify me when they collect living mussels that are found to contain 
parasitic crabs. Generally only one adult crab will be found in 
each mussel and it can usually be spotted easily because of its move- 
ment. As a knife or razor blade is passed through the opening be- 
tween the two valves and the valves are separated or laid flat, the 
movement of the crab can be detected. It would be greatly appreci- 
ated if, when individuals are in the field and can collect 20 to 50 
mussels at a time, these could be gently opened and observed for the 
presence of the crab. Information can be sent to me at the Sandy 
Hook Laboratory, Highlands, New Jersey 07732. I am interested in 
the presence of crabs in mussels collected anywhere in the New York 
Metropolitan area, and extending East through Long Island Sound and 
along the south side of Long Island to Montauk Point and Narragansett 
Bay. I am also interested in the presence of crabs in mussels col- 
lected from the New York area south to Lewes, Delaware. If shell 
collectors have available to them small vials, any crabs that might 
be found inside mussels could be placed individually in the vials 
which could be filled with plain rubbing or isopropyl alcohol. It 
