A shark is a fish, and like other fish, it has a backbone, lives in water, and breathes through gills. Unlike other fish, a shark has a skeleton made of cartilage, the same material that makes up the end of your nose.
Cartilage is strong, flexible, and lightweight.
Worldwide, there are over 350 species of sharks and 400 species of rays. Despite this diversity, you know when you're looking at a shark or a ray because their distinct body forms. Some adaptations enable sharks and rays to specialize in new ways of getting around, hunting, and feeding. These kinds of changes can increase the odds of survival, reproduction, and passing on those traits to the next generation, as well as enable species to use resources within a habitat differently, reducing competition. Their inner body temperature matches the temperature of the water.