What Makes a Shark a Shark?
Sharks seem to have it all figured out, evolution-wise. Fossils of prehistoric sharks go all the way back to 450 million years ago, and sharks like the ones we know today emerged about 200 million years ago. This means that they survived the mass extinction that took out the dinosaurs and lived long before early human ancestors evolved less than two million years ago.
So, what makes a shark a shark? Here are just a few of its unique physical features:
Unlike fishes with bony skeletons, a shark’s skeleton is made out of cartilage. This is a flexible but strong connective tissue that’s also found throughout the human body, in places like the nose, ears, and in joints between bones. Sharks, rays, skates, and chimaeras (also know as rat fishes) all have cartilaginous skeletons. Cartilage is less dense than bone, allowing sharks to move quickly through the water without using too much energy.
While skates and rays also have cartilaginous skeletons, sharks have very different body shapes. A ray has flattened, enlarged pectoral fins that make up its disc-shaped body. Pectoral fins on sharks are located on both sides of its body, behind the gill slits. They help create lift as sharks move through the water, similar to an airplane’s wings. Sharks that linger near the ocean floor, like nurse sharks, may even use these fins to prop themselves up: they’ll make the space below their bodies look like fake caves to draw in prey like crabs.
While bony fishes have one gill opening on each side of its body, sharks have five to seven. The gills of bony fishes are also hidden behind flaps that open and close, whereas sharks have exposed gill slits. And the belief that sharks have to keep swimming constantly in order to breathe isn’t entirely true. Some species of shark use “ram ventilation,” which means they get oxygen to their gills by swimming rapidly with their mouths open for the water to flow through. Other sharks are less active bottom-dwellers that have spiracles, or openings behind the eyes, to pull water in and past the gills.
Bony fishes have bladders filled with air that help them stay afloat. A shark, however, has a very large liver that could make up 25 percent of its entire weight. The liver is filled with oil that helps the shark stay buoyant. Sharks store fat in their liver, which is important for migrating long distances. In fact, a well-fed shark stays afloat more easily than a lean one: more fat in the liver means more buoyancy.
These tiny scales got their name because they look a lot like sharp teeth: dermal denticle actually means “skin tooth.” Covering a shark’s skin, dermal denticles are part of what makes this predator such a stealthy hunter. They are useful for reducing drag in the water and increasing speed—shortfin mako sharks can swim at speeds over 30 miles per hour! Denticles even inspire the designs of Olympic swimsuits.