Sharks and whales look similar but they're different types of animals whales are mammals while sharks are fish which body part do sharks have that whales lack?

the only difference is sharks have gills and whales do not

Gills, sharks have gills that require a constant movement to filter the water through the gills and take the o2 out of the water as their oxygen supply, but it is a very small amount they need and require. Whales have a blow hole to blow out excess water and some air, they can breathe above water but still require water and the oxygen in it to survive.