What are Mammals?

Mammals are warm-blooded animals. They belong to a group of higher vertebrates whose bodies are covered with hair or fur. They give birth to live young and feed them with milk from mammary glands.

What are Fish?

Fish are mostly cold-blooded animals that live in water. They may or may not have scales. They breathe through their gills and are equipped with two sets of paired fins plus several unpaired ones.

Is a Shark a Mammal or Fish?

Breathing Mechanism of Marine Mammals

Beluga whale surfacing to breathe through its blowhole
Beluga whales (true marine mammals) must surface regularly to breathe air through a blowhole on the top of their head.

Marine mammals like whales and dolphins breathe air into their lungs through a blowhole on the top of their head. The blowhole works a bit like a nostril. To breathe, they have to swim up to the surface at regular intervals. Once they dive back under the water, they close their blowhole and hold their breath.

Breathing Mechanism of Fish

Shark showing gill openings on the side of its head
Sharks breathe through gill openings on the sides of their head, extracting oxygen directly from water, no surfacing needed.

Fish have gills that let them breathe oxygen directly from water. There is no need to surface at all. A fish takes water in through its mouth, pushes it over the gills, and out through the gill slits. The gills filter the oxygen, which then enters the blood vessels. Sharks do not have lungs. Instead, they have between five and seven gill openings.

Shark or Mammal? Take the Quiz!

5 quick questions. Can you tell a fish from a mammal in the deep ocean?

Marine Mammal Skin

A smiling bottlenose dolphin showing its smooth, rubbery skin
Dolphins have smooth skin, a sign of their warm-blooded nature. Unlike fish, they don't need scales for temperature regulation.

Skin is another big difference between warm-blooded mammals and cold-blooded fish. Marine mammals are warm-blooded. That means they can keep their body temperature steady even in cold water. Because of this, they have smooth, rubbery skin instead of scales.

Fish Skin

A lemon shark close-up showing its rough, sandpaper-like denticle-covered skin
The lemon shark's skin up close: covered in tiny dermal denticles that look and feel like rough sandpaper, and work like it too, protecting against parasites.

Fish have skin covered in scales, smooth or rough, depending on the species. Fish are cold-blooded, which means their body temperature changes with the water around them. They cannot keep themselves warm like mammals do.

Sharks have a special type of rough, tooth-like scale called dermal denticles. Thousands of these tiny teeth cover their skin. The denticles protect sharks from parasites and help them move really fast in water.

Frequently Asked Questions

Are sharks mammals or fish?

Sharks are fish, specifically cartilaginous fish in the class Chondrichthyes. They breathe through gills, are cold-blooded, and have scales.

Why do some people think sharks are mammals?

About 70% of shark species give birth to live young (pups) rather than laying eggs, which is a trait often associated with mammals. But live birth also occurs in many fish species.

How do sharks breathe?

Sharks breathe through gill openings. They have between five and seven gill slits that extract oxygen directly from water, they never need to surface to breathe.

Are dolphins and whales fish or mammals?

Dolphins and whales are marine mammals. They breathe air through a blowhole, are warm-blooded, and nurse their young with milk.

Dorsal Fin of a Marine Mammal

A beluga whale playing at the surface, note the lack of a dorsal fin
Beluga whales have no dorsal fin at all, one of the features that sets marine mammals apart from most fish.

Marine mammals are much simpler when it comes to fins. They either have just one dorsal fin, or none at all. Beluga whales, for example, have no dorsal fin whatsoever.

Dorsal Fin of a Fish

A shark cutting through the water, its prominent dorsal fin slicing the surface
That iconic triangular fin cutting through the water? Sharks have multiple fins (dorsal, pectoral, pelvic, anal, and caudal) for stabilizing, steering, and thrusting forward.

Fish have more than one dorsal fin, plus pelvic and anal fins too. Depending on the species, sharks can have a first dorsal fin, a second dorsal fin, pectoral fins, pelvic fins, an anal fin, and a caudal fin. They use all of these for stabilizing, steering, lifting, and thrusting forward.

One interesting thing about shark fins: they cannot swim backwards. If a shark swims toward something, it can only drift away from it, it cannot move sideways or reverse.

Side-by-side comparison: a dolphin breathing through a blowhole above water vs. a shark breathing through gill slits underwater
The key difference at a glance: marine mammals (left) surface to breathe air through a blowhole; sharks (right) breathe entirely underwater through gill slits.

Once you know the basic differences between mammals and fish, it becomes clear: the shark is a species of fish, not a mammal, even though many people believe it to be one.

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