What's the difference between a mermaid and a siren in your au?
Ooo thank you for the question!
You've unlocked my trap card: Mythos Gushing lolol
So, in my au (and in general for my merfolk aus), mermaids and sirens are both similar and very different from each other. Kind of like two sides of the same coin.
Mermaids can have all kinds of tails (fish, mammal, crab, etc.) and are all technically within the same "merfolk" umbrella. Mermaids do have magic, but they're more, I guess you could say, "real" than sirens are. Like, closer to a human in a way because of that.
On the other hand, sirens are pretty much ocean spirits. They are much more magical than merfolk are as such. (Sea witches in this au tend to have siren blood somewhere in their lines.) Sirens are also much more temperamental than merfolk, luring both humans and merfolk alike to their deaths. Because of that, they're also more elusive and aren't seen as often as merfolk are.
So because of the above, a lot of merfolk warn their kids about sirens or use sirens as warnings to make them behave. (Which is why Nami, Usopp, and Zoro low-key freak out upon meeting Sanji. They were half-convinced sirens weren't real.)
