The undersea kingdom of Atlantis is an established fact in DC canon; it's where Arthur Curry [Aquaman] hails from, and that origin is the reason for his powers. However, traditional "merpeople", with tails and fins, also exist; one of Clark Kent's [Superman] most notable romances is with a girl named Lori Lemarus, who happens to be a full on mermaid. Lori mentioned there was some type of connection in an old [Post-crisis] comic, but my memory of it was vagie.
Did they evolve differently from regular Atlanteans?
Why do they still have tails?
and are their abilities different in other ways?