Megalodon is the largest marine predator and was the featured animal In the Jaws of Death.
The Megalodon is a predocessor of the Great White Shark. It is similar to it in many ways however it was more than double it's size. Due to it's size it was capable of killing and attacking it's own kind.
There were two Megalodon's in the episode. A young one that was the size of a Great White. The other was an adult. It tried numerous times to eat Nigel but ultimately swims away.