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A hermit crab’s shell is his home and his protection from predators and dessication. Hermit crabs take up residence in discarded shells and can not make their own shell. When kept as pets it is important that you choose suitable shells for your hermit crab.
In semi-terrestrial hermit crabs a well-fitting shell is essential for maintaining low evaporation rates and carrying ample water. An appropriately sized shell in good condition allows invasion of inland environments offering more shade, food and fresh water for C. clypeatus studied on Curacao. Hermit crabs with broken, ill-fitting shells are restricted to the coast, must rely on drinking saltwater, and appear to be in relatively poor condition.
Terrestrial hermit crabs show “shell facilitation”; that is, larger populations of crab generate, through wear, larger numbers of shells suitable for adult crabs. 
Hermit crab’s should be allowed to choose the shell they prefer from a selection of different sizes and types of shells. Natural shells are the best option. Painted shells should be avoided. Shells should not have jagged edges or holes in them.
Most species of hermit crabs will prefer a shell with a round opening. Coenobita compressus (Ecquadorian) prefers a shell with a D shaped opening.
Shells should be cleaned and boiled before offering to your hermit crabs. If you collected shells from the beach be sure the shells are EMPTY before bringing them home.
Hermit crabs can be very stubborn about changing shells but do not attempt to force a crab from it’s shell.
Contrary to common belief, a molt does not mandate a shell change! If the existing shell is roomy enough to allow for growth during a molt, the hermit crab may feel no need to change shells. Additionally, you will find some hermit crabs are chronic shell shoppers, always trying on something new.
Hermit crab shells are sold online by the size of opening. To determine the correct opening size measure the opening as shown here:
Photos of common hermit crab shells:
Common Hermit Crab Shells
Are hermit crabs looking for lighter and larger shells?
The distribution, abundance and shell selection behavior of three species of hermit crabs
Land Hermit Crabs Use the Smell of Dead Conspecifics to Locate Shells
Coenobita violscens shell behavior
The Social Lives of Hermit Crabs
Check our collection of PDFs for more research documents about coenobita and sea shells.
1. Wilde, 1973
2. Abrams 1978