A shark was a type of predatory cartilaginous fish. Sharks were native to Earth, though there were other planets which had creatures similar to sharks.
In Atlantis, the Second Doctor, Ben, Polly and Jamie were nearly fed to sharks as a sacrifice to Amdo. (TV: The Underwater Menace)
Sharks were known to live in the Atlantic Ocean, as they were mentioned on the Fancy by Henry Avery. (TV: The Curse of the Black Spot)
During World War II, Alfred Beadle and his crewmates were eaten by sharks when a U-boat sank their ship. (PROSE: Nightshade)
In the 1970s, the Australian branch of UNIT discovered a fragment of Brokk's spaceship in the belly of a shark. (PROSE: The Eye of the Giant)
Sam Jones was afraid of sharks after watching the film Jaws 2. (PROSE: The Taint)
On an Earth-like planet, the First Doctor, John and Gillian helped a race of intelligent frogs catch a shark which was trying to eat them. (COMIC: Shark Bait)