I did my master's thesis on behavioral interactions between
lobsters and crabs (Jonah and sand crabs, Cancer borealis and Cancer
irroratus) and how they affect trapping of these species. What I found
was that if a lobster got into a trap first, crabs would not enter the
'parlor' or holding section. This was because if a crab started up the
entry to the parlor, the lobster made aggressive displays which made the
crab back down. Crabs did not keep other crabs out, nor lobsters, so if a
crab got in first, lobsters would still come in. In general, these two
species of crabs are less aggressive than lobsters.
These are "pea" crabs. They live, often in pairs, inside the oyster
shell, eating food collected on mucous strands in the oyster. Because
they do cause damage to oyster mantle and gills, the crabs are
considered parasites. Pea crabs are not harmful to man.
As so often happens, common names are used loosely and
inconsistently in the shrimp family. The "prawn" of Great Britain and
other countries is essentially the same animal as the shrimp of the
United States, the only biological difference being that prawns have
their second abdominal flap (counting from the head towards the tail)
overlapping the first and the third. In this country, the term "shrimp"
applies to all crustaceans of the Natantia group, regardless of size.
"Crayfish" or "crawfish" are names given to both a common freshwater
crustacean and to the saltwater spiny lobster.