1. a person who has withdrawn to a solitary place for a life of religious seclusion.
2. any person living in seclusion; recluse.
3. Zoology . an animal of solitary habits.
#1 is irrelevant; as far as I can tell, hermit crabs are not religious. #2 refers to humans, but the idea of seclusion is there. #3 would be the definition applied. But hermit crabs are anything but reclusive; "solitary habits" do not suit their lifestyle. They're gregarious, love company, and do not do well alone. One website even mentions that they appreciate a mirror for added company; I will have to supply mine with one.
When we find them on the beach, it is usually in a crowd, sometimes of a hundred or more, including several distinct species; they do not seem to have any tribal leanings.
To stock my tank, I brought home several handfuls of hermits; now there are about 16 hermits, hairy hermits, grainy hand hermits, and greenmarks**, living in the aquarium. (There may be more; every now and then a shell like a grain of rice starts walking about; it's been borrowed by a hermit so small I didn't realize it was there.)
These are four of the larger ones, hanging out together. When I first saw them, all three smaller ones were piled on top of the weight lifter in the big whelk shell. He is a grainy hand hermit, recognizable by the red antennae, and the bluish legs. The other three are hairies (dotted-line antennae, possibly blue knees); greenmarks have red antennae and orange stripes on the legs, and are much smaller.
The shore crabs, now; they are the true hermits. Each one lives alone in his little hideaway; coming face to face with another, they raise their pincers in a challenge: "Back off!"
* A collective noun for a group of crabs is "cast"; a cast of crabs. I couldn't find one for hermits, so I've invented one.
**Hairy hermits are Pagurus hirsutiusculus; Grainy hands, Pagurus granosimanus; Greenmarks, Pagurus caurinus.