Is a Shark a Fish? Yes, and Why It Isn't a Mammal

Yes, a shark is a fish. It lives in water and breathes through gills, the way a bass or a catfish does. What makes it special is the skeleton. There are no bones at all, only cartilage, the gristly stuff in your ears and the tip of your nose.

Sharks have been around a very long time. Going by fossil scales found in Australia and the United States, NOAA puts the first sharks in the ocean around 455 million years ago.

Are sharks fish or mammals?

Sharks are fish. NOAA Fisheries keeps the answer short: “Sharks are fish. They live in water, and use their gills to filter oxygen from the water.”

The mix-up is easy to make. A shark and a dolphin both live in the sea and both have a fin on the back. But a dolphin is a mammal, like a dog or like you. NOAA lists what makes it one. A dolphin is warm-blooded and breathes air with lungs. It gives birth to live young and feeds them milk. It even has a tiny bit of hair, right around its blowhole.

A shark fails that test. It gets its oxygen from the water through its gills. Most sharks are cold-blooded, so their body is as warm as the water around them. And no shark looks after its young. Once a pup is born or hatches, it’s on its own.

So when a kid asks “is a shark a fish or a mammal?”, the fast answer is the gills. Fish breathe water. Mammals breathe air. Why a fish counts as an animal but never as a mammal is on are fish animals.

What kind of fish is a shark?

Nearly every fish belongs to one of two big groups. Bony fish are one: every bass, trout, catfish and carp you’ll ever reel in. Cartilaginous fish are the other: sharks, rays, skates and ghost sharks (chimaeras). There are about 1,200 species of them, and about 520 are sharks, the Florida Museum counts.

Sharks, rays, sawfish and skates go by one more name, elasmobranchs. Rays are close kin of sharks, with their gills on the underside.

Cartilage sounds weak, but it isn’t floppy. It’s “strong enough to give support but softer than true bone,” the Florida Museum says. As most sharks get older, they lay calcium into it to stiffen it. A dried shark jaw feels heavy and solid, a lot like bone. Shark ancestors did have bones. Those turned into cartilage, which made sharks lighter in the water.

Cartilage is softer than bone, so a whole fossil shark is very rare. The teeth are a different story. They have enamel, and a shark sheds them its whole life, thousands of them. Its teeth sit in layers, so when one breaks off, a new one takes its place. That’s why shark teeth wash up on beaches. You’ll find teeth of every kind on do fish have teeth. The bony side of the family is on do fish have bones.

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How is a shark different from other fish?

A shark is a fish, but it’s built differently from the bluegill in your bucket. Five things give it away.

Where to look: the shark’s gill slits, spiracle, skin and liver against the bass’s gill cover, scales and swim bladder. Drawing: fishing.info, from the Florida Museum, NOAA and Australian Museum facts on this page.

The gill slits. Look behind the head. A shark has five to seven gill slits on each side, open to the water. A bass or a trout has a single flap over its gills, the gill cover. That’s the quickest check there is.

The skin. Shark skin is covered in tiny tooth-like scales called dermal denticles. They point toward the tail. Rub a shark from head to tail and it feels smooth. Rub it the other way and it feels like sandpaper. A shark doesn’t grow bigger scales as it grows. It grows more of them.

No swim bladder. Many bony fish float with a gas-filled bag called a swim bladder. Sharks don’t have one. A big liver full of light oil keeps them up instead. In many deep-sea sharks the liver is as much as 30 percent of the whole body. The sand tiger can even gulp air into its stomach for extra lift.

Breathing on the move. Many sharks have to keep swimming to push water over their gills. Not all of them do. Nurse sharks and angel sharks rest on the bottom and pump water over their gills through spiracles, small openings just behind the eyes. How gills work in every fish is on how do fish breathe.

Babies. In every shark, the eggs are fertilized inside the mother. Some sharks then lay tough egg cases, called mermaid’s purses. Others give birth to live pups. Either way the baby comes out as a smaller version of the adult. Bony fish have their own ways, and how do fish reproduce has the whole story.

Is a whale shark a fish?

Yes. The whale shark is not a whale. It’s a shark, so it’s a fish, and it’s the biggest fish in the ocean. NOAA puts a big whale shark at 12.2 meters (about 40 ft) and, by some estimates, 40 tons. The longest FishBase lists is 66 ft (20 m). They eat mostly tiny plankton. The basking shark is the second biggest fish, as long as 32 feet. A whale is a mammal, the same as a dolphin. How it stacks up against the biggest fish ever landed on a rod is on biggest fish ever caught.

Is it a fish? Twelve sea animals checked

Lots of animals in the water have “fish” or “shark” in the name, and lots of fish don’t look like fish. Here’s the short answer for the ones you’ll hear about, with the one sign that settles it for you.

Is a seahorse a fish?

Yes, a seahorse is a fish. It’s a bony fish, one of 47 seahorse species, in the same family as the pipefish. You’d never guess it from the shape. It swims upright, pushing itself along with the fin on its back. In place of scales, it has thin skin stretched over rings of bony plates, like a suit of armor.

The seahorse’s claim to fame is its dad. Seahorses and their cousins the sea dragons are the only animals in which the male gets pregnant, NOAA says. The female puts her eggs into the male’s brood pouch, and he carries them for two to four weeks. Then he gives birth to anything from a few dozen to as many as 1,000 baby seahorses. Only about half of one percent make it to adulthood, so the big litters matter.

Seahorses live in shallow water near the coast, clinging to stalks of seagrass with their curled tails. Their numbers are falling, NOAA says: many, if not all, of the 47 species are in decline. If one ever comes up in your net in the bay, let it go where you found it.

Are eels fish?

Yes, eels are fish. True eels are bony fish, about 1,000 species of them. Their body is long and their fins are small. They have no fins underneath, and many have none on their sides either.

An American eel. Photo: Sam Stukel, U.S. Fish and Wildlife Service (public domain).

The one you can catch in fresh water is the American eel. It’s the only freshwater eel in North America, the U.S. Fish and Wildlife Service says. Females grow up to 4 feet long, males only to a foot and a half, and an eel can weigh up to nine pounds. They live in rivers and in estuaries, where a river meets the sea. They hunt at night and swim out to the Sargasso Sea, in the ocean, to spawn.

Two “eels” aren’t eels at all. The electric eel of South America is a fish. But it’s closer kin to catfish and carp than to true eels, Smithsonian’s National Zoo says. It even breathes air, coming up to the surface every so often. The lamprey looks like an eel but isn’t related to one. It’s a jawless fish with a skeleton of cartilage and a round sucker mouth full of teeth. If one comes up stuck to your trout, lamprey tells you what to do.

What do you do when you hook a shark?

Keep its gills in the water while you unhook it. A shark gets its oxygen through its gills, the same as any fish you let go. Florida writes that into its rules for the sharks you may not keep.

Florida’s shark rules make a good checklist for any beach or pier. They are the law in Florida only. Other states set their own, so check yours first.

1. Get the permit before you fish from shore. In Florida, take the Shark-Smart Fishing course and get the Shore-based Shark Fishing permit. “Both are required when fishing for shark from shore and must be renewed annually.” The permit is free and is for anglers 16 and older. A kid under 16 skips the permit but takes the course, unless fishing with someone who holds one.
2. Use a circle hook with bait. Florida requires “non-offset, non-stainless-steel circle hooks” when you target or keep sharks with live or dead natural bait. How that hook works, and where else the law asks for one, is on circle hooks.
3. Bring cutters. From shore, you must carry something that can cut the hook or the leader, like bolt or cable cutters. The leader is the heavy length of line or wire in front of the hook.
4. Keep the gills in the water. Prohibited sharks are the kinds you may never keep. They “must remain in the water with the gills submerged.” The advice for every other shark you land is the same. “Keep sharks, especially the gills, in the water.”
5. Not sure what it is? Let it go. “Treat unknown catches as a prohibited species and release them.”
6. Skip the long photo. Release it fast, and “do not delay release just to take pictures.”

Florida sorts the sharks you may keep into groups, each with its own size limit. Here are two of them.

Fork length is measured from the snout to the fork of the tail. The bag limit, what you may keep in a day, is one shark per person, and no more than two per boat.

Want to eat it? NOAA says shark meat “is tasty and nutritious if properly prepared.” It spoils quickly, so ice it. Some kinds taste strong, and NOAA’s fix is 24 hours on ice, then two hours soaking in brine (salt water). Never eat the liver. Its high levels of vitamins can make people sick. Before you feed shark to kids, check your state’s fish advisory.

Can sharks live in freshwater?

Only a couple of shark species can live in fresh water for any length of time: the bull shark and the speartooth shark. Bull sharks have been reported more than 1,800 miles up the Mississippi River, as far as Illinois, the Florida Museum says. They always have a way back to the sea, though. There are no populations in lakes cut off from the ocean, so you won’t meet one in a landlocked lake.

So what about the “shark” in a pet store tank? The bala shark is not a true shark. It gets the name from its torpedo-shaped body and large fins. Some catfish get called sharks the same way. The iridescent shark, another tank fish, is a catfish from Southeast Asia named for its shark-like look. Its meat is sold in stores as swai.

The bull shark also turns up on the scariest fish, as one of the three sharks with the most bites on record. And for more of the questions kids ask on the way to the water, the fish facts page has them all in one place.

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