Zaralok

A razor-toothed blade fin was a creature, resembling a shark, which was encountered by Amy Pond and The Doctor on Poseidon 8. (VG: TARDIS, Shadows of the Vashta Nerada)

Biology
The razor-toothed blade fin had some qualaties resembling a Great White shark but was much larger and had two main back fins, a feature that no other sharks have. They also seem to have no effect in salt water. (VG: Shadows of the Vashta Nerada)