Common Hermit Crab

Common Hermit Crab

  • Scientific name: Pagurus bernhardus
  • Size: Up to 4cm in length
  • Distribution: Found in the cooler waters of Northern Europe.

While there are many species of hermit crab around the UK by far the most common is Pagurus bernhardus, the common hermit crab. This species does not have a hard carapace shell of its own and has to find and use the shell of another creature.

Description

Hermit crabs are usually seen inside a shell with their eyes, claws and legs protruding outwards. However, they will retreat entirely within the shell when they are threatened. If they are viewed out of a shell they have a soft, twisted body which has evolved in this way to allow them to fit inside of the many different sized shells they will come across. The colour is usually reddish orange, although they can be a darker brown and even purple.

Distribution and Habitat

The common hermit crab is generally found in the colder waters of northern Europe. They can be found in Nordic waters with their range extending southwards, making them common around all of the British Isles. They are found on the Atlantic coastlines of Spain, France and Portugal, but they are absent from the Mediterranean Sea and the coast of North Africa. They are common around rocky and mixed seabeds and can be found in some areas with only minimal rock cover.

Behaviour and Feeding

Unlike other crab species hermit crabs do not have their own carapace and must find the discarded shell of another sea creature (usually a whelk or a cockle) and make this their home. The hermit crab will then move itself around with the shell on its back and retreat into the shell when it is threatened by a predator, blocking the entrance with its claws.

Hermit Crab out of shell

A hermit crab pictured out of its shell.

As the hermit crab grows it must leave its original shell and search for a larger home. Hermit crab sometimes form a symbiotic relationship with a white ragworm. The worm will also live in the shell with the crab and eat any excess food matter that gets into the shell. Hermit crabs will crawl along the seabed and will eat any decomposing plant or animal matter that they come across. They also have the ability to filter feed and consume plankton and other forms of microscopic sea creatures. Generally, the smaller hermit crabs of a centimetre or so in length will be found around the intertidal zone. When hermit crabs are fully grown as three or four centimetres long they will move into deeper water and be found at depths down around to two hundred metres. Common hermit crabs have some use as a bait for sea anglers. More information can be found by clicking here.

Other Species of Hermit Crab in UK Waters

South Claw Hermit Crab and Leach's Hermit Crab

There are a number of other hermit crab species in British waters. Two of the most common are the south claw hermit crab (Diogenes pugilator) and Leach’s hermit crab (Pagurus prideauxi). The south claw hermit crab (pictured above, left) is easily identifiable by the left claw which is much larger than the right. This species only grows to a maximum of 2.5cm in length and is usually half this size. It is generally found around the south and west of the British Isles. Leach’s hermit crab (pictured above, right) is also easy to identify as it is always found with a cloak anemone (Adamsia palliata) attached to its shell – indeed these two creatures live in a symbiotic relationship to such an extend that they die if they are separated for any length of time. The two creatures both gain benefits from their relationship, the hermit crab gains extra protection from the anemone  and its stinging tentacles, while the anemone gains more opportunities to feed due to being attached to the crab’s shell.

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