Lives in a portable home...
If you go out to the rocks exposed at low tide and look into one of the pools you may see something puzzling: a shell apparently scuttling along the bottom under its on steam! On closer inspection you will see that the shell has long, jointed legs. It is in fact a crab with a shell on its back - a hermit crab.
Hermit Crabs have soft abdomens, unprotected by a carapace. The shells they live in, which may be straight or coiled, are an excellent substitute for this tough covering. The right pincer claw is always larger than the left and is used not only for capturing food but also for blocking the entrance to the shell. There are a great number of different species of hermit crab, the biggest being the famous coconut or robber crab (really a hermit crab which has given up living inside shells because it cannot find any big enough; ha!) that has developed an extra tough carapace to compensate for the lack of shell.
Hermit crabs start searching for a shell of suitable size very early in life. Once inside, no amount of prodding will induce a hermit crab to come out against its will. Growth takes place in a series of molts and the bigger crab must look for a roomier shell. Hermit crabs depend for their survival on the gastropod mollusks - whose shells they appropriate.
A hermit crab changes shells very quickly, so that it is exposed and defenseless for only a very short amount of time (yeah, I'd be in a hurry, too!). It finds a new shell before leaving the old one, of course, as that would be really risky to just drop your shell and wander around aimlessly, wouldn't ya say? After carefully inspecting a potential new shell with its pincers to make sure it is acceptable, it sidles smartly into the new shelter/shell.
As pets, several marine species of hermit crabs are common in the marine aquarium trade. Personally, I prefer the decorative, scenic hobby of freshwater aquariums, but anyway... Of the approximately 15 terrestrial species in the world, the following are commonly kept as pets: Caribbean hermit crab, Australian land hermit crab, and the Ecuadorian hermit crab. Other species are less common but growing in availability & popularity as pets.
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 the Caribbean hermit crab (Purple Pincher or PP) have a 23-year lifespan if properly treated, and some have lived longer than 32 years.
Image Credit: http://en.wikipedia.org/wiki/File:Calliactis_and_Dardanus_001.JPG
---End of Post "Hermit Crab"