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Differences Between the Sexes
It's difficult for a layperson to determine the sex of a hermit crab. They have five pairs of legs and their sex organs are on the third set of legs of the female and the fifth set of legs of the male. Ordinarily, females have three tiny pleopods (false legs) on their abdomens and males do not. Since the crabs travel using another animal's shell for protection, the sexual areas of both male and female are ordinarily covered. Removing the crab from its shell to identify it can seriously injure it.
How Hermit Crabs Mate and Reproduce
It's unclear to scientists whether hermit crab males find a mate by touch or smell. There is very little aggression between males when seeking mates. In order to mate, a male must rotate his shell and position his body so that his fifth pair of legs are against the female's third pair of legs where their genitalia are located. When mating, the crabs must both come part way out of their shells, which makes them vulnerable to injury and predators. Sperm enters the female's belly where it fertilizes eggs that come from the base of its third pair of legs. The eggs proceed to the female's abdomen where they hang on leg-like processes and certain hairs. She tilts her body into salt water to deposit the eggs, which then hatch and larvae emerge. After several stages, the baby crabs go to shore to find their initial shells. They mate and lay their first eggs when they are about two years old.
Size and Type of Shells
Hermit crabs depend upon their borrowed hard shells to protect and hydrate their delicate soft-shelled abdomens and shelter them from predators. Frequent molting is part of the life cycle of hermit crabs and they must find a larger shell as they grow. They cannot survive without a suitable shell that fits properly. Because of the rather complex mating dynamic, older males, who have the largest shells, are at a disadvantage when trying to find a mate. The male must maneuver his shell in a synchronous fashion with the female, so the shell size and shape must be compatible for these animals to mate.
Synchronous Shell Swapping
Social shell swapping is biologically adaptive for hermit crabs because of their extreme dependence on their shell "real estate." Their periodic need for a larger shell is reason to cooperate with one another to find their next shell home. Mating can be limited in areas where there aren't enough shells. As scientists from Tufts University and the New England Aquarium have observed, hermit crabs take part in synchronous shell swapping where they form a queue from the largest to smallest. When a slightly larger shell becomes available for the largest crab, the others swap shells one after the other down the line. This adaptive behavior protects the species from harm as well as ensuring mating. Males will usually mate with females whose shells are most compatible to allow them to maneuver properly.