Hermit crabs depend on a properly fitting snail shell for protection from predators. Crabs in shells that are too small grow more slowly, are less tolerant of air exposure and drying out, and are more likely to be eaten by a predator than those that can withdraw completely into their shell. When a crab has outgrown its snail shell, it has to look for a bigger one that fits properly. They locate shells by smell, either when the preceding owner (the snail) dies and begins to decay or by detecting calcium, the major component of snail shells. The crabs do not generally kill snails to find new shells, although they may fight with other hermit crabs over empty shells, especially when empty shells are scarce. After locating a new shell, the crab investigates its surface and internal size by rolling it over and exploring it with its claws and walking legs. If it looks like a good fit, the crab withdraws its abdomen from its old shell and inserts it into the new one so quickly that it can be difficult to see. Switching shells is risky-the crab could be attacked by a predator or lose one or both shells to other hermit crabs.

zCrabs rarely abandon their current shells without a new home lined up. However, if a crab has been buried in the sand during a storm or flooding event, it is likely to abandon its shell to return to the surface Although abandoning its shell increases its chances of surviving such a burial, this behavior also increases the risk of predation and of being buried or injured in another flooding event.

Particular species of hermits tend to prefer certain species of snail shells, but size is the critical factor. Occupying larger and heavier shells gives them greater protection, but it takes more energy to lug a large house around. Shells often become covered with other organisms, such as barnacles or algae, that make them heavier and more difficult to carry. Some hermit species live in shells that get entirely covered by bryozoan colonies that eventually dissolve the snail shell, leaving the crab with a house composed entirely of the bryozoans. As the bryozoan colony grows, so too do the crabs, so these crabs have no need to change their homes.

Hermit crabs with poor-fitting shells are chemically attracted to dying gastropods (snails and related animals) and other hermit crabs where a shell may become available. In the Mediterranean species Clibanarius erythropus, gastropod predation sites attract dozens of hermit crabs. Researcher Elena Tricarico and colleagues observed that these aggregations function as shell exchange markets: the first crab to arrive takes the empty shell, and a chain of shell exchanges among the crabs follows. They found that simulated snail predation sites quickly attracted a larger number of hermit crabs than other types of sites; therefore, aggregation is the most efficient tactic for this species to acquire new shells. In Belize, the land hermit Coenobita clypeatus has an even more organized shell exchange in what is called a "synchronous vacancy chain" by Randi Rotjan and colleagues, who discovered this unique behavior. When a large, empty shell becomes available, many crabs gather around it, which can take hours. As they gather, the crabs arrange themselves into a line of decreasing size, starting with the largest crab holding onto the empty shell. As though choreographed, the crabs begin shell swapping, one after the other, a smaller crab climbing into a new shell right after it is vacated by the slightly larger crab ahead of it. What makes the synchronous chain possible is that smaller crabs linger near a too-large shell, perhaps attracting others, waiting until a bigger crab comes along, which increases their chances of getting a good-fitting hand-me-down.

Most hermit crabs and most snails are right-handed and spiral clockwise. Some species, however, are left-handed and spiral counterclockwise, and they need to find snail shells that are coiled that way. Since the majority of snail species are right-handed, left-handed hermits may have trouble finding left-handed snails. Adults of the lefthanded Pefrochirus diogenes, the largest hermit crab in the Caribbean, are often found in shells of the queen conch Eustrombus gigas. As an exception to the general rule, they will attack and eat the conch, thus obtaining both a meal and a shell.