This is a classic myth that has been around for centuries. Authors as far back in time as Homer and Ovid have written about it.
The mystery of the woman with a human torso and head and the tail of a fish (and sometimes the legs and wings of a bird, which were depicted as such in the iconography) has always sparked the imagination and fascinated humankind.
This character can be traced throughout the history of literature. Ovid tells us that they were companions of Proserpina (Persephone for the Greeks), who threw themselves into the sea desperate not to find her when she was abducted by Pluto (Hades for the Greeks). The gods, pitying them and not wanting them to die, transformed them into a being with the upper body of a woman, the lower body of a fish (and for some, feet of a hen and wings of a bird). Homer describes them as birds with the face of a woman.
But let’s focus on the Spanish myth. In Spain we find them in numerous songs, romances and graphic representations of all kinds. They get angry when they hear the sailors whistling because they think it is a mockery of them, in which case they form a circle swimming around the ship to scare the crew.
In the Cantabrian coast, there are several versions. One of them, the most popular among the people, which has passed from mouth to mouth over generations and which is spoken to children to keep them away from the dangers of the sea, tells of a beautiful young woman with blue eyes, who spent the day on the cliffs and seafood in dangerous areas. Her mother punished her, and one day, tired, she said to her “God forbid that you should become a fish”. Said and done, that very day she fell into the sea and became the so-called “sirenuca“.
Another version tells us that Lantaron, king of the sea, grants sailors who capture a mermaid in his nets the possibility of marrying her, as long as they comply with a series of rules. The first is to kiss her immediately so that her tail becomes legs. The second is to hide the mother-of-pearl mirror that she will give him in a place where she cannot find it, because if he does so, her legs will turn back into tails.
Sources:
Mercedes Cano Herrera. (2007). Entre anjanas y duendes : mitología tradicional ibérica. Castilla Ediciones.