Common Hermit Crab

• comportamientoIt has to change shells several times as it grows, and now and then a group will queue up in order of size to swap shells one after another.
• The shell is its limiting resource, which is why hermit crabs compete intensely for shells. An individual assesses an empty shell by inserting a claw, gauging the interior and the weight before deciding on the switch.
• And they form vacancy chains: when a large empty shell turns up, a queue of hermit crabs forms in order of size, and as soon as the largest moves in, each one takes the shell the one before has left. It is a well-documented collective behaviour, much cited in economics as an analogy for housing markets.
• It often carries passengers: anemones of the genus Calliactis settle on its shell and provide chemical protection in exchange for transport and food scraps. The crab itself moves them onto the new shell when it switches.
• Its abdomen is soft, asymmetrical and coiled, adapted to the spiral of the shell.
• It has been used in experiments on pain perception in invertebrates: studies show that it abandons good-quality shells when given shocks, and that this behaviour is not a simple reflex but one that weighs costs against benefits.

Skeleton type

Invertebrate - partly calcified exoskeleton, protected by borrowed gastropod shells (crustacean)

Habitat and environment

Rocky and sandy bottoms of the north-eastern Atlantic, from the Russian White Sea to Portugal, between the intertidal and 140 m

Birth weight

Tiny zoea larva at hatching

Size at birth

A planktonic zoea larva with a long development before it settles

Sexual maturity

Under a year (early breeding is documented, with females raising young in their first year)

Gestation / incubation

The egg develops over a period the sources do not put a figure on

Litter / clutch size

Fecundity varies with the female's size (no precise figure found)

Sources consulted: SeaLifeBase; TortoiseTurtle.com; iNaturalist; Cambridge Core (reproduction life history strategy study); MarLIN; Animal Diversity Web; Grokipedia; Reef Life Survey