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Hermit crabs are decapod crustaceans of the superfamily Paguroidea. Most of the 1100 species possess an asymmetrical abdomen which is concealed in an empty gastropod shell that is carried around by the hermit crab.
As the hermit crab grows in size, it has to find a larger shell and abandon the previous one. This habit of living in a second hand shell gives rise to the popular name "hermit crab", by analogy to a hermit who lives alone. 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 kind 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. Hermit crabs that are kept together may fight or kill a competitor to gain access to the shell they favor. However, if the crabs vary significantly in size, the occurrence of fights over empty shells will decrease or remain non-existent.
A hermit crab with a shell that is too small cannot grow as fast as those with well-fitting shells, and is more likely to be eaten if it cannot retract completely into the shell.
For some larger marine species, supporting one or more sea anemones on the shell can scare away predators. The sea anemone benefits, because it is in position to consume fragments of the hermit crab's meals.
The young develop in stages, with the first two (the nauplius and protozoea) occurring inside the egg. Most hermit crab larvae hatch at the third stage, the zoea. This is a larval stage wherein the crab has several long spines, a long narrow abdomen, and large fringed antennae. After several zoeal moults, this is followed by the final larval stage, the megalopa stage.
Six families are recognised in the superfamily Paguroidea, containing around 1100 species in total in 120 genera.
These omnivorous or herbivorous species can be useful in the household aquarium as scavengers, because they eat algae and debris.
Hermit crabs are often seen as a "throwaway pet" that would live only a few months, but species such as Coenobita clypeatus have a 23 year lifespan if properly treated, and some have lived longer than 32 years.
In general, and despite their moniker, hermit crabs are social animals that do best in groups. In the wild they can be found in colonies of a hundred or more. Therefore, many sellers encourage the purchase of more than one crab. They also require a temperature and humidity-controlled environment, and adequate substrate that is deep enough to allow them to completely bury themselves while moulting.
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