Most famous among these are the hermit crabs. There is hardly any tropical sea shore where hermit crabs wouldn't give a scary surprise to young shell collectors who expect to gather a nice empty shell. Abandoned snail shells on sea shores rarely remain empty for a long time. They are well-demanded by a number of different-sized hermit crabs, which crawl into snail shells to protect their soft body parts from potential predators.
Having its soft back part protected, its large claws in front of the body are an effective protection for the hermit crab against hungry predators. Without any shell, a hermit crab would certainly make an easy prey for birds or other crab consumers, thus the protecting house is vital for each individual crab.
The only inconvenience of using empty shells as a crab housing seems that the newly inhabited snail shell doesn't grow when the crab is growing. Therefore, the growing hermit crabs are constantly searching for larger shells that fit better, and intensify this search the more uncomfortable the former shell becomes. The crabs need to "change room" several times during their lifespan, and the larger they get, the harder it is for them to find suitable shells.
The biggest hermit crab species, the so-called robber crab (Birgus latro), is endangered and listed in the IUCN Red Data Book. This large crab (reaching sizes of up to 50cm in length when stretched) is capable of climbing coconut trees in order to cut the coconuts down with its powerful claws. It then feeds on the grounded coconuts.
Snail shells of its size class do not seem to be widespread (or even available) on shores, so the robber crab lives without any protecting shell. Therefore, it has a harder back part than his smaller relatives, the shell-dwelling hermit crabs. By far, crabs aren't the only animals interested in abandoned snail shells. In Lake Tanganjika in eastern Africa, some small cichlid fish species learned to use empty snail shells as a shelter for their offspring. The designer version of a shelter made of snail shells is carried by some caddysfly larvae. These larvae, which live in freshwater habitats worldwide, usually build a case with different hard material, among it also small snail shells. Sometimes, an unlucky tiny snail is built into the case as well while alive.
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