Photograph by Fred Bavendam, Minden Pictures, Nat Geo Image Collection
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The argonaut, or paper nautilus, is actually an octopus. Females make a parchment-like shell to carry incubating eggs and control where they swim in the water column.

Photograph by Fred Bavendam, Minden Pictures, Nat Geo Image Collection


This species of octopus is as unusual as it is beautiful. The argonauts, also known as paper nautiluses, spend their lives drifting near the surface of tropical and subtropical seas far from their cephalopod cousins on the seafloor. Females do this tucked into fragile, translucent shells that they create themselves.

Females are much bigger than males, about eight times larger and 600 times heavier. During mating, the male’s hectocotylus, an arm that contains sperm, is released and stays inside the female.

Without the nooks and crannies of the ocean floor in which to lay eggs, the female takes matters into its own tentacles. After mating, it begins secreting calcite from the tips of two of its arms, a continuous process. That forms the delicate, papery shell. It lays its eggs inside, then squeezes in.

Shell's game

The shell serves another purpose: a ballast tank. The argonaut rises to the water’s surface, gulps in air, then seals it inside. It then dives down until the trapped bubble counteracts its own weight. It then can bob, without having to expend energy keeping its position in the water column. Paper nautiluses also can swim quite quickly—faster than a human diver—by using jet propulsion.

Female argonauts sometimes hitch rides with jellyfish or attach themselves to driftwood. They’ve also been seen forming a long chain with other females.

Not much else is known about the argonaut, though its numbers are not believed to be threatened.

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