Cuttlebone

Actually, it's not really a bone. A cuttlebone is actually the shell of the cuttlefish. But then, the cuttlefish is not a fish. It is really a sea-going mollusk. And, if you are following all of this, you should also know that cuttlebones are mainly sold for use with pet birds.

Well, it doesn't really matter what it is called or typically used for, but it is an excellent source of needed calcium for your hermit crabs. Just break it up or grind into a fine powder. Since this is a natural product, size can vary. Typically around 4 - 5" in length.