All About the Paper Nautilus
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We first encountered this incredible little creature while blackwater diving in 2017. At the time, we had absolutely no idea what we were looking at.
The first Argonaut we encountered was apparently a female, as males are 10 times smaller, rarely passing the size of 2 cm, and don’t have a shell. Since then, we’ve been lucky enough to spot Paper Nautiluses many more times — including during blackwater dives in the Similan Islands — and finally managed to capture a few decent photos.
After some research, we discovered this strange and beautiful animal is called a Paper Nautilus, also known as an Argonaut.
Despite the name, the Paper Nautilus is not actually related to the true nautilus. It belongs to the octopus family and is one of the ocean’s rarest pelagic cephalopods.
What Is a Paper Nautilus?
The Paper Nautilus (Argonauta argo) is a species of open-ocean octopus found in tropical and subtropical waters around the world, including the Andaman Sea, Thailand, Myanmar, and the Similan Islands.
The delicate white “shell” carried by females is not a true shell like a nautilus shell. Instead, it is a thin egg case secreted by the female using two specially adapted arms.
The female uses this paper-thin structure to:
- Protect her eggs
- Control buoyancy
- Shelter herself in open water
Male Argonauts are tiny — often less than 2 cm long — and do not produce a shell at all.
One of the Weirdest Mating Systems in the Ocean
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Argonaut reproduction is famously bizarre.
One of the male’s specialized arms acts as a reproductive organ called a hectocotylus. During mating, this arm detaches from the male’s body and remains inside the female to fertilize her eggs.
The female then secretes the papery shell with the tips of two tentacles. She lays her eggs inside the paper shell and watches them until they are ready to hatch, and her job is done.
For many years, scientists thought these detached arms were actually parasitic worms because nobody had ever seen a complete male Argonaut. Nature is weird sometimes.
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The Paper Nautilus and Its Incredible “BCD”
One of the most fascinating things about the female Paper Nautilus is its ability to control buoyancy.
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Argonauts are the only known octopuses capable of trapping air from the ocean surface inside their shell-like egg case.
The female swims to the surface, captures a small air bubble, seals the opening using her arms, then descends back into deeper water.
This trapped air acts almost like a diver’s BCD (buoyancy control device), helping the Argonaut achieve neutral buoyancy and drift effortlessly through the open ocean.
It’s one of the most unique buoyancy adaptations in the marine world.
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5 Amazing Facts About the Paper Nautilus
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🐙 1. The “shell” isn’t a shell at all!
Female Paper Nautiluses don’t actually have shells; they build a delicate paper-thin case just to store their eggs. It’s basically the ocean’s most elegant Tupperware.
🐙 2. The male is so tiny he looks like he forgot to evolve.
Females can grow up to 20 times larger than males. A male Paper Nautilus is hard to tell from other octopus you see on blackwater dives.
🐙 3. They can “glide” underwater using jet propulsion.
Like other octopuses, Paper Nautiluses swim using jet propulsion, expelling water through a siphon to move surprisingly fast through open water. Females may also use their delicate egg case to stabilise themselves while drifting in ocean currents.
🐙 4. Paper Nautiluses are true pelagic hitchhikers.
They grab floating objects — seaweed, jellyfish, even trash — to ride ocean currents and save energy. They’re the backpackers of the sea, but with much better fashion sense.
🐙 5. They can transform into tiny air-sailors.
By trapping air inside their paper-thin case, females can float at the surface like miniature balloons, rising and falling like they’re on a tiny ocean elevator.
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Where Can You See a Paper Nautilus?
Paper Nautiluses are rarely seen during normal scuba dives because they live in the open ocean.
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The best chance of encountering one is during:
- Blackwater diving (pelagic night dives)
- Night dives at remote offshore locations close to deep water
- Diving in tropical waters
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Why Divers Love the Paper Nautilus
For underwater photographers and blackwater divers, finding an Argonaut is a bucket-list encounter.
They are incredibly delicate, rarely seen, and completely alien-looking.
Watching one drift through the water while carrying its paper-thin egg case feels more like encountering a creature from another planet than a marine animal.
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