Amazingly, when a new empty usable shell is added a hermit crab population, it will move from crab to crab, and in only a few days will be found by the right sized host! (Reese, 1969). How does this sizing happen when each crab is trying to work selfishly towards only larger shells? In other words what is the mechanism for finding optimally-sized shell?

Finding the Perfect Fit

The 'sufficient system' is due to the extreme shell competition between hermit crabs. For example, if a small crab finds a large roomy shell, he will inhabit that shell until challenged by a larger crab. When the larger challenging crab identifies a competitor it sends a number of visible dominance ques. Thus each crab ends up with a shell that is closer to its body size. Although the larger crab has the advantage, the smaller crab continues to have a shell, and must grow until he can challenge another for a larger shell.

The Good, the Bad, the Ugly (shells): How do Crabs Know?

Crab’s gain information about another shell by drumming against its side with their claws during shell competition, this way they can gain information about shell size as well as any shell damage. Certain crabs have been shown to prefer certain types of snail species shells. Larger shells are advantageous and have been shown to be connected to a larger number of eggs in females (Reese, 1969), which is directly linked to the reproductive success of the crabs. However, it has also been shown that shells with narrow openings decrease predation. There are obviously tradeoffs between long term survival and immediate reproductive success (Bertness, 1981).  Additionally if there is a threat from a predator, crabs are more likely to choose non-holed shells over more sturdy shells with holes (Rotjan, 2004).

However, although there is obvious information exchange and certain environmental pressures for shell choices, a large part of switching shells is random. Crab’s have been shown to move into another crab’s shell, only to decide to move back into its old one (Diaz, 1995). Thus there are both random elements as well as choice in hermit crab shell competition.

Social Dynamics of Shell Selection

Hermit crabs exhibit shell searching behavior through both finding empty shells and through displays of dominance and size, creating a dynamic and efficient system where shells travel throughout the population.

Watch the skirmish of the two hermit crabs in the movie below and the subsequent eviction. Notice how quickly the shell switch takes place.