From Encyclopaedia Britannica
MERMAN, a fabled marine creature with the head and upper body of a human being and the tail of a fish. Similar divine or semidivine beings appear in ancient mythologies (e.g., the Chaldean sea god Ea, or Oannes). In European folklore, mermaids (sometimes called sirens) and mermen were natural beings who, like fairies, had magical and prophetic powers. They loved music and often sang. Though very long-lived, they were mortal and had no souls.
Many folktales record marriages between mermaids (who might assume human form) and men. In most, the man steals the mermaid's cap or belt, her comb or mirror. While the objects are hidden she lives with him; if she finds them she returns at once to the sea. In some variants the marriage lasts while certain agreed-upon conditions are fulfilled, and it ends when the conditions are broken.