Invertebrates
Argonaut
Argonauta argo · also called paper nautilus, paper argonaut
The argonaut is an octopus that behaves like almost nothing else in the ocean. The female builds a shell that is not a shell, uses it as a diving weight, and broods her eggs in it. The male is a dwarf a fingernail long whose mating arm swims away without him.
Overview
The argonauts are a group of about four species of open-ocean octopus in the genus Argonauta, drifting in the warm surface waters of the world’s oceans. Despite the common name paper nautilus, they are true octopuses and are not related to the chambered nautilus.
The shell
Only the female makes it: a thin, white, ridged case secreted from two widened arms, which she lives in and can leave. It is both an egg case, where she broods her young, and, as a 2010 study showed, a buoyancy device — she seals a gulp of surface air inside it and dives to a depth where the trapped air cancels her weight.
The vanishing male
The female’s body reaches about 10 cm; the male is only 1–2 cm and lives just long enough to mate. His sperm-loaded arm detaches and swims into the female on its own — early naturalists mistook these for parasitic worms.
Status
Least Concern and widespread, though rarely seen alive. The empty shells are a common beachcomber’s find worldwide.
Argonaut: common questions
Is the argonaut the same as a nautilus?
No, and the resemblance is only in the name and the fact that both make a shell. The chambered nautilus is a member of an ancient cephalopod lineage with a many-chambered coiled shell it lives in permanently, dozens of small tentacles, and simple eyes. The argonaut is a modern octopus, with eight arms, a single crop-like body, and the complex eyes and behaviour of octopuses, and its shell is not a true shell at all but a brittle secreted egg case that the female makes with two of her arms and can leave and re-enter. "Paper nautilus" was coined for the shell's thinness and its superficial resemblance to a nautilus, but the two animals are not close relatives.
Why does the female argonaut make a shell?
For two purposes that took a long time to establish. The obvious one is reproduction, since she lays her eggs in strings inside the case and broods them there, carrying her developing young with her as she drifts. The second, confirmed by researchers in 2010, is buoyancy control. The female rises to the surface, takes a gulp of air into the shell, clamps it in place with the webs of two arms, and then dives; at a certain depth the compressed air exactly offsets her body weight, so she becomes neutrally buoyant and can hover and move around without constantly swimming. The shell is thus both a nursery and a ballast tank.
What is going on with the tiny male argonaut?
The argonaut has one of the most extreme size differences between the sexes of any animal. The female is a substantial creature; the male is a dwarf a centimetre or two long that matures quickly, does not feed much, and exists essentially to reproduce. When he mates, his hectocotylus, a modified arm packed with sperm, breaks off and enters the female's mantle cavity, where it can remain and function while the male, its job done, dies. A single female may carry several detached arms from different males. When these were first found inside females in the nineteenth century, before the connection to the male was understood, they were described as parasites and given the name Hectocotylus, which survives as the name for the arm.
Where can you see an argonaut or its shell?
Live argonauts are rarely encountered, since they live out in the open ocean and near the surface mostly at night, though divers and snorkellers occasionally find a female drifting, sometimes riding on a jellyfish or a piece of debris. The empty shells, by contrast, are a familiar prize for beachcombers, especially after storms, washing up thin, white, and delicately ridged on beaches in Australia, the Mediterranean, Japan, and elsewhere. Because they float, they can travel far from where the animal lived, and a collector may find one on a beach thousands of kilometres from the nearest breeding population.