Photo copyright Tim Nicholson.
Hermit crab, taken on Horagali Faru (Raa Atoll) Maldives
Hermit crabs scuttle about the sea-floor using someone else's shell for a home. They always use empty shells and never kill the original occupant.
When the crab becomes to large for its shell, it looks for another. When it finds a likely looking one, it will try it on. If the shell doesn't fit, or is too heavy, the crab returns to its old shell and continues it search.
Where there is a large population of hermit crabs and a shortage of shells, the crab will accept a sub-standard home: maybe a cracked or uncomfortable shell. But in good conditions it will be very particular about the new shell it chooses.
Two hermit crabs will fight for the possession of an empty shell, or even a shell inhabited by one of the combitants.
Atlas of the Maldives: A Reference for Travellers, Divers and Sailors, by Tim Godfrey,(2007)
The Underwater Photographer: Digital and Traditional Techniques by Martin Edge, Paperback, 536 pages (2009)