Pescaton

Pescatons were a technologically advanced race of shark-like humanoids.

Biology
They were significantly taller than humans, having the head, body and texture of a shark, very sharp teeth, clawed fins, and webbed feet. They were cunning, ravenous, and ferocious, attacking and feeding on anything that crossed their path. Although their steel-like skin made them invulnerable to conventional weapons, they could be killed by either long periods out of salt water or prolonged, high-pitched noise, after which their flesh disintegrated, leaving behind their vertebrae.

History
The Fourth Doctor first encountered the Pescatons on their dying home world in the 15th century. Their leader, Zor, had brought him there to help them find a new planet. The Doctor refused, defeated Zor in mental combat, and escaped back to the TARDIS.

In the late 20th century, the Pescatons invaded Earth, attacking and killing many people in London. The Doctor and Sarah Jane Smith discovered their weakness to high-pitched noise and set a trap for Zor, killing him. When he died, all the other Pescatons followed suit. (AG: Doctor Who and the Pescatons)

Behind the scenes
Unusually, the Doctor called the Pescatons "evil" on many occasions. This characterisation of another species is atypical for the Doctor. Indeed, it lays the foundation for another extraordinary feature of the story: the Doctor and Sarah devise and participate in the genocide of the Pescatons, without either character displaying even a tinge of remorse.