When does a hermit crab change its shell?

Hermit crab - changes its shell when it has grown large enough to need a bigger home. This type of crab has a soft abdomen or "tail" which is folded up under the body but it is not protected by a shell of its own as are most other crabs. Instead, the hermit uses empty snail shells as portable shelters often having to fight with another crab for possession of an attractive home. Sometimes the homeseeker pulls out the original occupier eats it and then takes over the shell. One of the claws of the hermit is larger than the other. The crab uses this to stop up the entrance after withdrawing into the shell. The last two legs on its abdomen have roughened pads which grip the inside of the shell and hold the body in position. The crab has a spiral shaped abdomen and moves in and out of its shell with a spiral movement. One kind of hermit occupies a sponge which conveniently grows at the same pace as the crab. Sometimes sea anemones enter into partnership with hermit crabs and take up residence on top of the shells. The crab provides the sea anemone with transport and in return receives an extra shield against attack.