Hermit crabs can live either on land or in water. They're not true crabs, though.
In fact, they're more like squat lobsters than true crabs. True crabs have abdomens protected by hard shells.
To protect themselves, hermit crabs search for abandoned shells — usually sea snail shells. When they find one that fits, they tuck themselves inside it for protection and carry it with them wherever they go.
This habit of living in a borrowed shell gave rise to the hermit crab's name. Since hermit crabs often retreat completely into their borrowed shells for protection, some people think they act like hermits.
Hermits are people who live alone and do not interact often with others. Hermit crabs aren't really hermits, though.
They tend to be quite social animals that enjoy living in groups. In the wild, they can often be found in large groups of 100 or more.
As hermit crabs grow, they often outgrow their borrowed shell. When this happens, they have to “shop" for a new one.
Hermit crabs can be very picky about the shell they choose. They want a shell that they can fit into completely in case they need to hide for protection.
Sometimes hermit crabs that live together help each other find new shells. When a new, large shell becomes available, hermit crabs have been known to form a line by it from largest to smallest.
The largest crab moves into the new shell. The next largest crab moves into the shell just vacated by the last crab and so on.
Sea anemones are poisonous, so other creatures will usually avoid them and leave the hermit crabs alone.
Most hermit crabs are fairly small. However, one land-dwelling hermit crab can get quite large. The coconut crab can grow to be up to three feet long and weigh nine pounds!
The coconut crab is unique in another way, too. When it outgrows the largest shell it can find, it grows a shell of its own.