Snails don't change shells
Collecting marine life in FL is not hard to do legally
SW fishing liscence
You do need to be careful about what you take. A random small snail could be a protected conch that would get you a very nice fine.
The other thing to watch for is that some snails are fish or coral eaters.
Putting that aside he is not eating the shell... Maybe eating what is in or on it. Or laying eggs inside
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