SCUBAHUB with two or three . ‘ Parasitic ’ is a misnomer because the anemone ’ s stinging cells provide valuable additional defence for the hermit in return for transport and food benefits , and the slander is even repeated in the anemone ’ s scientific name Calliactis parasitica ! Despite mutual advantage , the relationship is quite loose in that both hermit crab and anemone often live independently from one another .
In contrast , and their names gives the game away , the anemone hermit crab ( Pagurus prideaux ) and cloak anemone ( Adamsia palliata ) share a much closer bond , hardly ever being found living apart . The cloak anemone ’ s base wraps around the hermit ’ s shell and the anemone even builds a porch-like extension as the crab grows , thus negating its need to trade up to a larger shell . Oddly , the easiest way of identifying this species of hermit can be to look for the tell-tale pink spots on the anemone ’ s base . The spots aren ’ t always easy to see but , if the anemone gets nudged as the crab runs across the seabed , it reveals its presence in a very obvious way . It releases long sticky white threads ( acontia ) that carry additional stinging cells as a defence mechanism .
There are also species of hydroids that specialise in living on shells occupied by hermit crabs , where they form an attractive pale pink ‘ fur ’. The hydroid colony consists of different types of polyps and their development and relative position on the shell is thought to be affected by the crab ’ s breathing currents .
Last but not least of the best-known hermit crab ‘ friends ’ is the commensal ragworm ( Neanthes fucata ) that lodges in the hermit crab ’ s shell within a mucus tube . These worms live with a good proportion of large hermit crabs but I have only had the treat of seeing one dash out to share its host ’ s meal on a couple of occasions . It is sometimes said that the hermit crab derives no benefit from the relationship , but it appears that the worm may have a useful role in clearing the crab ’ s faeces out from the back of its shell . Wouldn ’ t you share meals with a friend who was willing to do that ? �
Fire ! A hermit crab ’ s cloak anemone releases its defensive threads
The small female of this hermit crab couple has a good covering of the specialised hydroids
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