Sky shark

The Sky Sharks were a type of large, predatory sky fish.

Biology
Sky sharks looked much like the sharks of Earth, several metres long. Like other sky fish, they could fly through the air by using electrical currents, which reacted with crystalline elements in the fog. They needed this fog to survive and would die if removed from it.

Sky sharks were attracted to sounds in the fog, which resonated the crystals and created delta waves in their brains. They were attracted to the sound given off by a sonic screwdriver, going so far as to eat it. On the other hand, Human singing would calm the sky sharks. (DW: A Christmas Carol)



History
The sky sharks lived in the fog around Sardicktown. They were feared by the Human colonists; indeed, Elliot Sardick planned to use his weather-controlling machine to keep the Humans in line, using their fear to control them.

When Kazran Sardick was a child, he wanted to learn about the sky fish. When the Doctor used his sonic screwdriver to attract the fish, he accidentally attracted a sky shark, which bit the screwdriver in half and tried to eat Kazran and the Doctor. The Doctor was able to get half the screwdriver and stun the shark, but it was actually weakening from being removed from the fog. The Doctor planned to return the shark to sky, keeping it in a cryogenic chamber to preserve it. When they went to get the chamber, the shark revived and followed them. Luckily, the chamber they choose to keep the shark in previously held Abigail Pettigrew, who was able to pacify the shark with her singing. The shark was then successfully returned to the clouds.

The shark was later used by the Doctor, Kazran and Abigail. On a later Christmas eve, they used it to pull a carriage, allowing them to fly through the air. Many years later, the sonic screwdriver in the shark's stomach was used to amplify and redirect the singing of Abigail, allowing the fog to be controlled. Finally, Kazran and Abigail use the shark to pull their carriage on a final night together. (DW: A Christmas Carol)