Shoal (The Underwater War)

The Shoal or Mermin were a fish species who communicated by colours of the skin as a sign of emotions. They lived in underwater cities. The individuals, formerly called Shoalies, were divided between the "low swimmers", the "medium swimmers" and the "high swimmers". The low swimmers had a white globe, which they could use to stun other Shoals or even kill them. The fish creatures had also fashioned a forked weapon as protection.

Their home planet was Hydron and the capital city of their planet was Reef. They used underwater roads to travel above the water beds. (PROSE:  The Underwater War)

Centuries later, the species were called the Mermin by the Eleventh Doctor, who had become a legendary saviour named Duktur, and by themselves, including when talking about an individual of their people. (COMIC: Without a Paddle)