Hermit crabs occupy the empty shell

Hermit crabs occupy the empty shells of dead sea snails for protection while still retaining their mobility. They are capable of discriminating among a selection of shells of various sizes and species, and they choose the one that fits the body most closely. Hermit crabs change shells as they grow, although in some marine environments a large enough variety of shells may not be available and the hermit crab may be forced to occupy a smaller-than-ideal “house.” When shell becomes too small for the hermit crab to occupy, it will sometimes become aggressive and fight other hermit crabs to gain a larger shell.
Hermit crabs may encounter empty shells in the course of their daily activity, but the vacant shell is usually spotted by sight. The hermit crab’s visual response increases with the size of an object and its contrast against the background. The hermit crab then seizes the shell with its walking legs and shape and texture by rolling it over between its walking legs and running its claws to remove any foreign material before preparing to enter. The crab rises above the opening, flexes its abdomen, and enters the shell backward. The shell interior is monitored by the abdomen as the crab repeatedly enters and withdraws. When completely satisfied with its new mobile home, the hermit crab will emerge one last time, turn the shell over and make a final entrance.