trade usually involves two crabs, they engage in a sort of physical fight probing, rocking and knocking each other's shells around. This type of behaviour gives the crabs the chance to check out the quality and size of one another's shells and actually usually results in both crabs benefitting from the trade. Large numbers of crabs can trade shells at once and it can take place in a very short period of time. Once they have traded it will not take the crab long to decide whether or not the shell is suitable. References Hermit crabs Marine crustaceans of New Zealand Crustaceans described in 1852