Is an octopus a fish?

Youโ€™re swimming in the ocean and spot a curious octopus gliding by. With its smooth head, fins, and torpedo-shaped body, it sure looks like it could be some kind of fish! But is an octopus actually a fish? The answer may shock you.

Despite their fish-like appearance, octopuses are in no way fish at all. In fact, they belong to a completely different phylum of animals. Read on as we explain the key differences between fish and octopuses, what classifies a fish, and why an octopus could never be mistaken for one.

selective focus photography of octopus

What Makes a Fish a Fish?

Fish are aquatic vertebrate animals that possess gills, fins, and an elongated shape covered by scales or skin. They are cold-blooded, meaning they cannot internally regulate their own body temperature. Fish breathe underwater via gills that extract oxygen from the water.

All fish belong to the phylum Chordata. What defines them as chordates is their distinguishing spinal column and internal skeleton.

Octopuses Are Mollusks, Not Fish

Octopuses have some fish-like traits, including smooth skin, fins, and a torpedo-shaped body. However, they belong to the phylum Mollusca and class Cephalopoda, making them mollusks. Mollusks include clams, snails, slugs, and other soft-bodied creatures.

Unlike the vertebrate fish, octopuses are entirely invertebrate, meaning they lack any kind of internal skeletal structure or backbone. Other key differences include:

  • Octopuses breathe with gills when young but develop lungs as adults. Fish only breathe through gills their entire lives.
  • Octopuses move along the sea floor using their flexible arms to crawl and pull themselves. Fish rely on fins for swimming locomotion.
  • Octopuses have high intelligence and complex brains. They use tools, solve problems and demonstrate learning abilities. Fish do not display these traits.
  • While both types of animals live in water, the ocean contains many marine creatures besides just fish. Thus, an underwater habitat alone does not make octopuses fish.

Is A squid A fish or a Mammal?

Squid are neither fish nor mammals – they belong to a completely different taxonomic classification. Squid are invertebrates, which means they lack a vertebral column or backbone that both fish and mammals possess. Instead, squid, like octopuses, belong to the phylum Mollusca and class Cephalopoda, making them mollusks, not vertebrate animals.

Fish are cold-blooded with gills, while mammals are warm-blooded, breathe air, and give live birth. Squid are cold-blooded like fish and breathe through gills, but they do not give live birth like mammals. Squid move through the water by propulsion using their siphon, while fish use fins and mammals use their limbs/flippers for locomotion, making the squid’s method unique.

Though adult squid have lungs to supplement their gills, this does not make them mammals, as many non-mammals like amphibians and reptiles also have lungs. Additionally, squid lack fur or hair which are defining characteristics of mammals. They have smooth skin covered in chromatophores that enable their color-changing abilities.

Finally, squid hatch from eggs laid by the female rather than giving live birth like mammals, and they go through a distinct larval phase very unlike mammal development. In summary, squid are marine invertebrates belonging to their own unique phylum and class, sharing some qualities with fish and superficial similarities with mammals, but taxonomically they are neither fish nor mammals based on evolutionary history and anatomical traits.

The Shocking Conclusion: Octopuses Are Not Fish!

Despite some similarities in appearance, octopuses and fish differ anatomically and taxonomically in every fundamental way. Octopuses are classified as mollusks and belong to the phylum Mollusca, while fish are vertebrates classified under the phylum Chordata. No matter how fish-like they may look, octopuses could never be classified as fish.

FAQ: Common Octopus vs Fish Questions

Still wondering how octopuses could possibly not be fish? Here we address some other frequently asked questions:

  • Are octopuses cold-blooded like fish? No, as invertebrates octopuses can actually regulate their own body heat.
  • Do octopuses swim underwater like fish? While they can propel through the water, octopuses primarily crawl slowly along the ocean floor. Their anatomy is not built for powerful swimming.
  • Why arenโ€™t octopuses fish if they live in the ocean? The ocean contains many types of marine animals besides just fish, including molluscan invertebrates like the octopus. Habitat alone does not dictate taxonomy.
  • Could an octopus evolve into a fish? No, the anatomical differences are too vast, and the phyla are separated by hundreds of millions of years of evolution. Octopuses lack the defining vertebral column to ever be classified as fish.

The next time you spot an octopus, remember – no matter how much they resemble fish, they are marine invertebrates belonging to an entirely different phylum! Their unique anatomy sets them apart from all fish species. Understanding animal taxonomy can reveal shocking truths like this that defy surface-level assumptions.

Adam Docherty

Hi I'm Adam. At Pet Know How we aim to help you learn everything you need to about your pets.

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