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It is important to choose shells with the correct opening size for your hermit crabs. The opening size you choose should be 1/8" bigger than your hermit crab's big claw. You should measure the length of the largest claw, not how wide it is.

Hermit crabs use their biggest claw as a trap door. Their claw should be recessed just inside the opening. If their big claw is down inside, the shell may be big enough or possibly a little too big depending on how deep the claw is in the opening. If the claw is too big to recede within the opening, your crab definitely needs a larger shell.

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A good rule of thumb is to buy shells with an opening that's 1/8" larger than your hermit crab's big claw.