Aridian

The Aridians were a piscine humanoid race native to Aridius.

Profile
Aridians possessed silvery skin, crested heads and fins along their spines. They originally lived in an underwater city, but the oceans slowly dried up and turned into deserts. The Aridians had to not only adapt to this new climate, but also contend with the Mire Beasts and a brief Dalek invasion when they landed their DARDIS on Aridius in search of the Doctor and his companions. (DW: The Chase)

The chose to colloborate with the Daleks and surrender the Doctor and Barbara.

Society
Little was known about their society, aside from the fact that they appeared to be led by a ruling council of elders, whose orders could not be challenged by more junior Aridians.

At the time the Doctor encountered the culture, it was clearly in decline, having all but lost the evolutionary war on Aridius to the Mire Beasts. The Mire Beasts had managed virtually uncontrolled growth, which had pushed the Aridians to a tiny hidden stronghold.