Common hermit crab  (Pagurus bernhardus)

In Britain, small hermit crabs are a common feature of the shore, where they frequently adopt the shells of edible periwinkles (Littorina littorea), flat periwinkles (Littorina obtusata) and dog whelks (Nucella lapidus). In deeper water, the shells of the whelk (Buccinum) are often occupied. Occasionally, if two hermit crabs meet, one will attempt to 'steal' the other's shell by forcibly evicting the current owner.

The common hermit crab is an omnivorous scavenger, and can also obtain food by filtering organic particles from the water (4). Reproductive activity tends to peak in January and February in populations dwelling on the shore, but in those living in deeper water, females have been found carrying eggs throughout the year (4). The female carries the eggs for around two months, after which time the pelagic larvae persist for a number of weeks. Maturity is typically reached before one year of age (4).

A number of animals are associated with this hermit crab, particularly the sea anemone Calliactis parasitica, which lives on the shell of the hermit crab, and provides increased protection against predators, receiving improved food collection in return (4). This is known as a symbiotic relationship, as both parties benefit from the association (4). A parasitic barnacle (Peltogaster paguri) is often seen under the abdomen of the hermit crab as a yellowish mass, which is often confused with the crab's eggs (4).