Could anyone who is geeky enough to try and come up with an answer or explanation be it real or a headcanon explain something for me.. Why is Cloyster part ice type?

I never understand it. I get other Pokemon being Ice type. Dewgong swims in cold waters of the North. Mamoswine based on extinct mammoths that lived in frozen tundras. But why is Cloyster an ice type? It's just a large clam or oyster and they don't live in cold waters. So why?