Mermaids are a sapient species native to the oceans of Earth. They appear like Humans from the waist up, but are fish-like from the waist down. They have a set of inner eyelids and can only survive on land for limited periods of time. However, some rare members of this species have mastered the ability to transform their tail into a pair of legs, allowing them to operate on land.
The Mermaids build coral-based cities on the ocean floor, and their houses have roofs of mussel shell that open and close with the flow of the water.
- Doctor Who comic stories - "Guests of King Neptune" (1966)
- Big Finish Doctor Who audio stories - "Cryptobiosis" (2005)
- When the Sixth Doctor encounters Mermaids in "Cryptobiosis", he claims that he previously thought they were just a myth. This suggests that the audio stories happen in a different continuity from the comic strip, in which the First Doctor meets King Neptune and his Mermaid servants.