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Hermit crab
Hermit crabs are found in terrestrial and aquatic environments, and seek out empty molluscan shells to use as a home. Unlike other crabs, they have soft, spirally-curved abdomens, rendering them defenseless without these shells.
Hermit crabs must regularly upgrade their shell to suit their growing bodies, and this leads to huge competition amongst the crabs. Interestingly, a unique vacancy chain behaviour has been observed, whereby an individual tries out a new shell and, upon finding it too big, returns to its original shell and waits by the vacant one. While the original crab waits, others gather and line up in ascending size order. Eventually, an individual claims the vacant shell, triggering a chain reaction in which all crabs present move up a size. Recent work has suggested hermit crabs suffer from entrapment in plastic debris, further emphasising the need to eliminate plastic waste from coastal environments.