If you're collecting shells on the beach, be sure to place them in some salt water and observe them for a while to be sure they're empty. My wife collected some empty shells and one of them wasn't really empty. We held them up to the light, shook them, etc. Didn't matter. We didn't realize there was a hermit in one of them until it started to smell.
All you really need to do is boil them in tap water for about ten minutes. Any bacteria in it will be killed and any stuck-on crud will have loosened up.