I have just listened to a fascinating programme on BBC4extra put out last Sunday 9th August and still available called 'Spirals in Time, the Secret Life and Afterlife of Seashells'.
Dr Helen Scales tells some amazing facts you didn't know (well I didn't) on the subject. I particularly liked hearing her tell how hermit crabs when they are all getting too big for their shells arrange themselves in a queue, with the largest at the front and smallest at the back. The largest vacates his shell for a larger newly discovered 'empty' and they all change, consecutively forward in to the vacated one in front! Astonishing and worth a listen.