Common hermit crab (Pagurus bernhardus)
|Size||Carapace length: up to 35 cm (2)|
Common and widespread (2).
Despite the common name, hermit crabs are related more closely to lobsters than to crabs. They lack a hard carapace, and adopt the empty shells of gastropod molluscs (such as whelks), carrying them around and swapping them for a larger shell as they grow (3). When seen out of a shell, hermit crabs have a bizarre appearance; the soft abdomen is twisted, which allows it to fit into the coils of the gastropod shell (4). The common hermit crab is typically reddish or brownish in colour, and has two pincers on the first pair of walking legs. The right pincer is larger than the left, and both have a rough, granular surface (4).
Common and widespread in north-west Europe, and found around all of the coasts of Britain (2).
Inhabits both rocky and sandy areas from the shore to depths of 140m (4).
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).
Not currently threatened.
Specific conservation action has not been targeted at this species.
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- Abdomen: in arthropods (crustaceans, insects and arachnids) the abdomen is the hind region of the body, which is usually segmented to a degree. In crustacea (e.g. crabs) the limbs attach to the abdomen; in insects the limbs are attached to the thorax (the part of the body nearest to the head) and not the abdomen. In vertebrates the abdomen is the part of the body that contains the internal organs (except the heart and lungs).
- Carapace: the top shell of a turtle. In arthropods (insects, crabs etc), the fused head and thorax (the part of the body located near the head) also known as ‘cephalothorax’.
- Larvae: stage in an animal’s lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.
- Omnivorous: of an organism that feeds on both plants and animals.
- Parasite: an organism that derives its food from, and lives in or on, another living organism at the host's expense.
- Pelagic: inhabits the open oceans.
National Biodiversity Network Species Dictionary (January, 2003)
Wilson, E. 2002. Pagurus bernhardus. Hermit crab. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. (November, 2002)
- Buczacki, S. (2002) Fauna Britannica. Hamlyn, London.
- Fish, J.D. and Fish, S. (1996) A student's guide to the seashore. Second Edition. Cambridge University Press, Cambridge.