The paper nautilus is a true anomaly in the cephalopod world. Despite its name, the paper nautilus—also known as an argonaut—is not a nautilus at all. It’s actually an octopus! Read on to learn more about these weird and wonderful invertebrates.

What is the paper nautilus?  

Argonauts are part of the family Argonautidae, which includes four species: the greater argonaut, the winged argonaut, Böttger’s argonaut and the knobby argonaut. Although they look similar to real nautiluses, their thin “shell” is completely different from the complex, chambered shells of their cephalopod cousins.

Paper nautiluses are found in the open ocean in temperate and tropical waters. All are pelagic, meaning that they live in the water column. This is a different strategy than that of other octopuses who live on the ocean floor so they can dart into holes and crevices for protection. They don’t live very long—typically less than a year—and feed on small mollusks, crustaceans and jellyfish.

Paper nautiluses are sexually dimorphic, meaning the males and females look very different from each other. Females are up to eight times larger and 600 times heavier than males, and they’re the only ones who have shells. Tough luck, fellas.

What’s the deal with their “shell”?

The primary function of the paper nautilus’ thin shell is to protect eggs! Unlike other octopuses, who hide their eggs in protective nooks on the ocean floor, paper nautiluses secrete a thin calcite shell in which they lay their eggs. The mother then then carries her eggs—up to 170,000 of them—around with her until they hatch.

But a nursery isn’t the only function of the paper nautilus’ shell—it can also serve as a flotation device. The animals can trap bubbles of air in the thin shell, which allows them to stay buoyant in the water column. This can help the females save energy as they drift along.

These critters are so cool!

Yes, they are! These gorgeous and unusual critters have captivated people for centuries. Aristotle even speculated that they used two of their webbed arms as a “sail” to capture breezes and drift along the ocean surface. This idea was even included in the novel Twenty Thousand Leagues Under the Sea—Jules Verne wrote about a “shoal of argonauts traveling along on the surface of the ocean” that he compared to an “elegant skiff”. Although hundreds of tiny octopus boats is certainly a cool concept, it is, alas, not true.

Fortunately, the truth about the paper nautilus and its mysterious shell is cool enough!

Can’t get enough octopus content? Celebrate World Octopus Day on October 8 by learning about the mimic octopus, blue ringed octopus and dumbo octopus!