hermit

erakko A religious recluse; someone who lives alone for religious reasons; an eremite.
A recluse; someone who lives alone and shuns human companionship.
A spiced cookie made with molasses, raisins, and nuts.
A hermit crab.
2016, Vicki Judah, ‎Kathy Nuttall, Exotic Animal Care and Management (page 279)

Because hermits are decapods and do not live within their own shells, they are not considered to be true crabs.