Found all around the coast of Britain, the Cloak Anemone, Adamsia carciniopados lives almost exclusively in a symbiotic relationship with the hermit crab, Pagurus prideaux. The anemone envelops the hermit crab's shell and secretes a tough membrane as the crab grows, effectively increasing the size of the host shell. This is a huge benefit to the crab as it therefore never needs to find a larger shell as it grows. (Changing shells is a moment of serious danger to hermit crabs which are soft and vulnerable without the host shells protection). Not only does the Cloak Anemone provide shelter but if something is tempted to attack the crab, the anemone will eject stinging threads known as acontia.
The anemone is always positioned on the crab with its tentacles on the underside of the crab and in this way, is able to feed on the particles of food left behind by the crab as it moves forward.
In my experience these crabs are very quarrelsome and are often seen fighting and dragging one another around, probably in an attempt to evict the protagonist. See the following picture showing where this has happened.
St. Abbs Marine Reserve, Berwickshire. Scotland