As a hermit crab grows in size, it must abandon its old shell and exchange it for a larger, more accommodating one. It is therefore important that you always keep additional shells in your hermit crab enclosure to help facilitate these transitions.As a hermit crab grows in size, it must abandon its old shell and exchange it for a larger, more accommodating one. It is therefore important that you always keep additional shells in your hermit crab enclosure to help facilitate these transitions.