very cool hitchers! These are slipper snails, a kind of limpet. They are harmless filter feeders and just stay in one place on the rocks, shells, etc all their lives. If they just happen to pick a snail shell, then all the better since they get to hitch a ride.
A cool lil bit is that the small ones are male and eventually become females as they age. As they make new snails, they stack up with the big female on the bottom, hermaphrodites in the middle, and males on top.
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