How do those Shells Stay on, Anyway?

I can only properly imagine being a merman, inasmuch as one can properly imagine such a thing. Now, the question I have, as a merman and a hu-man, is,

How do those shells stay on?

This isn't just my question (human or mer). It's the question of all thirteen-to-eighteen-year-old merkids. I bet, just as a guy wonders about how the hell he's going to get that bra off Suzy Wearsabranow, merdudes wonder about shells. Or do they? Maybe it's really easy to take the shells off? Maybe they adhere only loosely to mermaid boobs by some property of deep sea physics?

That's unlikely, because mermaids come up for air, and their shells stay on; unless they're slutty mermaids, or sexually liberated mermaids (not the same thing), and they leave their shells at home. Or maybe they just forget their shells at home.

Or maybe our awareness of mermaids has been puritanically censored by the great Prudish Merkings, (Mermaids have kings, right? Or queens, whatever – they adhere to undersea despotism, that's the point), and instead of fig leaves they saw fit to affix sea shells, since figs don't grow underwater.

Merchicks clearly know the answers, and they're just not telling.