>>104405188It's honestly very hard to determine what they are.
If mermaids were aquatic mammals, then they'd need to come up to the surface for air rather frequently the way dolphins and whales do.
However, if they were fish, they'd have gills. But Ariel style mermaids clearly have noses, and they clearly have the ability to breath in through their mouths as well, otherwise they wouldn't be able to speak underwater.
