The Dax were an underwater species from the destroyed planet Daxus.
The Dax resembled mermaids, with half their body a fish and half a humanoid torso. They also had black membranes on their torsos and webbed hands.
At an unknown future date, some Dax were living in an abyss near the Corinthian Project. They remained hidden, until discovered by the Tenth Doctor in a submarine. The project was about to blow up, but the Dax helped to save it. (NOTVALID: The Corinthian Project)