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 a 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 it's contrast against the background. The hermit crab then seizes the shell with its walking legs and climbs on it, monitoring it's size. If the size is right, the crab investigates it's shape and texture by rolling it over between it's walking legs and running its claws over the surface.
Once the shell's opening has been located, the crab uses it's 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 abdomens as the crab repeatedly enters and withdraws. When completely satisfied with it's new mobile home, the hermit crab will emerge one last time, turn the shell over an make a final entrance.
According to the passage, a hermit crab changes shells when it _________.
outgrows the one it has
hunts for food
becomes aggressive
locates any vacant shell