A shell is moving around on the beach! Upon looking closer, you see a hermit crab hiding under the shell. As hermit crab has the name "crab", and it has a hard shell, a pair of claws and four pairs of ambulatory legs, just like a crab, some may think that it is a young crab. Yet they are not the same but are close relatives, in Arthropoda of Order Decapoda.
There are over a thousand species of hermit crabs known worldwide, classified into three families: Coenobitidae, Diogenidae and Paguridae. All their larvae live in the sea. Only those from the Family Coenobitidae live on land or on beaches upon maturing to adulthood. Adults of the Diogenidae and Paguridae stay in the sea or move to beaches.
As the name suggests, hermit crabs hide away most of the time, typically in shells that have been left vacant by molluscs. The shell offers protection for the crab's soft abdomen and the female's eggs, as well as against changes to water level, temperature and salinity of the surrounding environment. It also reduces the risk of the crab being eaten.
The shell is vital for a hermit crab and has to meet specific conditions. Firstly, as the abdomen of the crab is dextral, that is, with a clockwise spiral, it must choose a shell that is also dextral. And the shell must not be too large as the crab will carry it around. Too large a shell will waste energy and block its movement. However, if the shell is too small, the crab must soon change it as it grows.
Fighting for shells
The shells used by hermit crabs come from other shellfish. But hermit crabs don't kill other shellfish for the shells. Instead, they use shells left by dead shellfish. While most hermit crabs need shells for shelter, some instead live among rocks and sponges. Exceptions are Coconut Crabs (Birgus latro) of Family Coenobitidae, which need no shells as they become adults.
Sometimes, when there is not enough suitable empty shells, the hermit crabs will exchange shells voluntarily or through negotiations when two of them can benefit. In some cases, however, a larger hermit crab may attack another, to force it to give up the shell.
Not only do hermit crabs have to struggle for shells among themselves, but they also have to face the threat from humans. Many people like to collect shells on the beach for selling or other purposes, even just for fun. We are too often unaware that every shell that we pick up, is one less chance for a hermit crab to survive!