Most hermit crabs have long, spirally curved abdomens, which are soft. The vulnerable abdomen is protected from predators by a salvaged empty seashell carried by the hermit crab, into which its whole body can retract.
Most hermit crabs will use the shells of sea snails (even hollow pieces of wood and stone) to occupy. The tip of the hermit crab's abdomen is adapted to clasp strongly onto the columella of the snail shell. As the hermit crab grows in size, it has to find a larger shell and abandon the previous one. This is often present vigorous competition among hermit crabs for shells.
The availability of empty shells at any given place, depends on the relative abundance of gastropods and hermit crabs, matched for size. Hermit crabs that are kept together, may fight or kill a competitor to gain access to the shell they favor. However, if the crabs vary significantly in size, the occurrence of fights over empty shells will decrease or remain non-existent. Several hermit crab species, both terrestrial and marine, use "vacancy" chains” to find new shells: when a new, bigger shell becomes available, hermit crabs gather around it and form a kind of queue from largest to smallest. When the largest crab moves into the new shell, the second biggest crab moves into the newly vacated shell, thereby making its previous shell available to the third crab, and so on.
A hermit crab with a shell that is too small cannot grow as fast as those with well-fitting shells, and is more likely to be eaten if it cannot retract completely into the shell.