What effects the sierns to mermaids?

Sirens are mythical creatures from Greek mythology who were half-human and half-fish and were capable of enticing sailors with their beautiful singing voices. Mermaids, on the other hand, are mythological sea creatures from European folklore who are typically portrayed as having the upper body of a woman and the tail of a fish. Therefore, since they don't coexist in the same mythology they do not transform into each other.