How can mermaids hold their breathe for?

Well, it's a matter of opinion. I think mermaids have gills on their neck or sides, or maybe they breath air, and can hold it for hours like dolphins and whales.

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I reckon the latter option, but given no-one has seen a mermaid it's hard to know..... ;-)

like whales and dolphins they can just hold their breath for a really long time, because their lungs are on the human half of their body they cant breath underwater