Are sharks fish? Here's where the top of the food chain ranks in the animal kingdom.

Clare Mulroy, USA TODAY
Updated ·3 min read

Did you know there are more than 500 species of sharks out there in the world? These fascinating underwater creatures have been swimming in our seas for about 455 million years, and they can travel thousands of miles in a single year.

There are more than 30,000 species of fish in the world, but are sharks fish? Here’s how the animal kingdom is divided up, and where one the ocean’s apex predators falls in the lineup.

A man watches fish and sharks swim at the Miami Seaquarium on November 27, 2019, in Miami, Florida.
A man watches fish and sharks swim at the Miami Seaquarium on November 27, 2019, in Miami, Florida.

Are sharks fish?

Yes, sharks are fish, and they belong to the subclass “elasmobranch.”

There are six types of animals:

  • Mammals: Warm-blooded vertebrates with hair

  • Birds: Warm-blooded vertebrates with feathers

  • Reptiles: Cold-blooded vertebrates with scaly or bony dry skin

  • Amphibians: Cold-blooded vertebrates without scales who live both on land and in water

  • Invertebrates: Animals without a backbone or bony skeleton

  • Fish: Vertebrates that live in water and breathe using gills

Fish are defined as vertebrates (meaning they have a backbone) that live in water and use gills to breathe. Elasmobranch fish, which includes sharks, rays, sawfish and skates have five to seven gill openings on each side. Elasmobranch fish are distinct from other fish because they are made of cartilage rather than bones.

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Are sharks mammals?

No, sharks are not mammals. Mammals are defined as warm-blooded vertebrates with hair who produce milk to feed their young. Mammals also have more developed brains than other types of animals. Humans are mammals.

This question often gets confusing because some sharks are "viviparous," meaning they give birth to live young. This is a hallmark characteristic of mammals. These types of sharks even have a placental connection similar to the umbilical cord of a mammal. It’s important to remember that while many fish lay eggs, not all do and it’s not a requirement to be considered a fish.

There are three main ways shark fetuses develop:

  • Viviparity: The egg develops inside the womb and leads to a live birth

  • Oviparity: The shark lays eggs in a deposit in the water – the eggs are encased in a leathery pouch often known as a “mermaid’s purse” before hatching

  • Ovoviviparity: The egg hatches inside of the womb and the shark has a live birth

Sharks can also reproduce asexually, and there are two documented species that were separated from male sharks and gave birth to pups only containing the female’s DNA.

Are fish mammals?

No, fish are not mammals. Fish and mammals are two different types of animals, but are both vertebrates. Vertebrates, according to Britannica, have backbones to support their bodies, a distinct head, a differentiated brain and nasal, optic and otic (ear) sense organs.

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This article originally appeared on USA TODAY: Is a sharks a fish or mammal? Where they belong in the animal kingdom.