The paper nautilus is an octopus that builds its own portable, floating nursery

Nature · Published 3 October 2026 Swipe more like this →

In short: Unlike its seafloor-dwelling cousins, the argonaut drifts near the ocean surface. To protect its eggs in the open water, the female secretes a delicate, papery shell from its own tentacles. This unique structure serves as both a secure brood chamber and a sophisticated ballast tank for buoyancy control.

The argonaut, or paper nautilus, is a pelagic octopus that defies the typical cephalopod lifestyle. While most octopuses inhabit the seafloor, argonauts spend their lives in the open ocean. They exhibit extreme sexual dimorphism; females are significantly larger than males, growing over 15 inches long, while males remain less than an inch. This size gap is so pronounced that the male's specialized mating arm, the hectocotylus, was once mistaken for a parasitic worm when discovered inside a female.

The female's most striking feature is its shell, a translucent, papery structure made of calcite. Unlike true nautilus shells, which are chambered, the argonaut's shell is an evolutionary innovation used primarily as a brood chamber for eggs. The female secretes this shell from the tips of its dorsal tentacles. Once the shell is formed, the animal uses it to manage buoyancy by trapping air at the surface. By adjusting this internal bubble, the argonaut can bob at specific depths without expending energy, a feat effective in the upper 10 meters of the water column.

Historically, the origin of these shells was a subject of intense debate. Early observers speculated that argonauts were kleptoparasites, stealing shells from other organisms like hermit crabs. This hypothesis was debunked in the early 19th century by marine biologist Jeanne Villepreux-Power, who successfully reared argonauts and observed them manufacturing their own eggcases. Some researchers suggest that the argonaut's ability to create these shells may have evolved as a response to the extinction of ammonites, potentially replacing the discarded shells they once utilized. Regardless of its origins, the argonaut remains a master of the open sea, capable of rapid movement via jet propulsion and even hitching rides on jellyfish or driftwood to traverse the vast, empty blue.

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