Hermit crabs are decapod crustaceans of the superfamily Paguroidea. Most species have long, curled abdomens, which are soft, unlike the hard abdomens seen in related crustaceans. The abdomen is protected from predators by a salvaged empty seashell carried by the hermit crab, into which its whole body can fit. The tip of the hermit crab's abdomen is adapted to clasp strongly onto the shell.
As the hermit crab grows in size, it has to find a bigger shell and abandon the previous one. Several hermit crab species, both terrestrial and marine, use "vacancy chains" to find new shells - when a new, bigger shell becomes available, hermit crabs gather around it and form a sort of queue from largest to smallest. When the largest crab moves into the new shell, the second biggest crab moves into the newly vacated shell, thereby making its previous shell available to the third crab, and so on.
Most species are aquatic and live in varying depths of saltwater, from shallow reefs and shorelines to deep sea bottoms. Tropical areas host some terrestrial species, though even those have aquatic larvae and therefore need access to water for reproduction. Most are nocturnal.
A few species do not use a "mobile home" and inhabit immobile structures left by polychaete worms, vermetid gastropods, corals and sponges
* There are over 600 species of hermit crabs in the world.
* Hermit crabs can live as long as 15 years although the majority of them live to around 6-8 years.
* Hermit crabs are not really classed as crabs due to the fact that they do not own their own shell.
* The Coconut crab or Birgus Latro is the largest hermit crab in the world with a size of up to 40 cms and a weight of up to 4.1 kgs!
* The shape of the shell that a baby hermit crab chooses determines the shape of its abdomen.
* Hermit crabs are fussy about the type of shells that they choose and are drawn to ones with mother of pearl linings.
* Hermit crabs drink by dipping their claws in water and depositing the water into their gills and mouth.
* Its nearest relatives are spiders and lobsters.
* Hermit crabs are not fussy eaters and will eat such things as fresh fruit and vegetables as well as small amounts of meat and fish.
* The Ecuadorian land crab is one of the smallest species of hermit crab at around 12mm in length
* Of all the species of hermit crab, only a few are generally classed as land or terrestrial and even these are born in the sea or ocean
* Hermit crabs moult as they grow, which means that they shed their exoskeleton and grow a new one. They will often consume their old skin for its nutrients.
* The abdomen of a hermit crab can change shape to fit the body of the shell that it is taking over. This is very good especially if the shell is of a spiral nature!
* If a land based hermit crab was left underwater for too long it would drown, even though it was born under water!
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