Why is the shark's mouth on the underside?

Just a conjecture here; I don't know for sure. Some sharks such as nurse sharks, and shark cousins such as skates and rays, are fundamentally bottom-dwellers that prey on animals that lie exposed on the substrate or slightly buried in it—flatfish, crabs, worms, molluscs, etc. read more

Some sharks such as nurse sharks, and shark cousins such as skates and rays, are fundamentally bottom-dwellers that prey on animals that lie exposed on the substrate or slightly buried in it—flatfish, crabs, worms, molluscs, etc. For these predators, obviously the underside of the head is the optimal place for the mouth to be. read more

Sharks do have noses, but they only use them for smelling, not for breathing. You'll find their pair of nostrils on the underside of their snouts. Some species, like the Nurse Shark, even have some extra smellers called nasal barbels, which stick out near the nostrils and mouth. Ampullae of Lorenzini. These are electroreceptive organs. read more

A shark's only real weapon is its mouth. Like its fins and sense organs, the shark's mouth is a highly effective physiological adaptation, perfectly suited to its job. There are two elements that make the mouth so effective: the teeth and the jaws. Shark teeth are something like the teeth of land predators. read more