From Nookipedia, the Animal Crossing wiki
Hermit crabs (ヤドカリ, Yadokari), a type of crab, are a bug that can be caught in Animal Crossing series. This bug can be caught in the Japanese-exclusive Dōbutsu no Mori e+ and in Animal Crossing: New Leaf on the Island.
 Donating to the Museum
 In New Leaf
"Hermit crabs live in empty shells they can fit their bodies into. As they grow, they find bigger shells. They're quite picky about each new shell and first measure the opening with their claws. The shells they choose usually fit well, and they'll sometimes fight one another over ownership rights."
After being donated, the hermit crab can be found in the first room of the bug exhibit next to the pond.