The Argonaut Octopus

The female of the argonaut species of octopus produces a thin shell called a paper nautilus. You may remember these animals from the book Twenty Thousand Leagues Under the Sea. Scientists have pondered the purpose of these shells for thousands of years. The octopus lays her eggs inside the shell, but that couldn't be the only reason they developed this ability that is so different from other octopuses. Octopus experts Julian Finn and Mark Norman had a chance to observe argonauts in the wild and found they deliberately filled their paper nautiluses with an exact amount of air in order to keep themselves floating at a particular ocean depth.
This neutral buoyancy is a big boon for animals that live in the open ocean, because they don’t have to expend energy on keeping their place in the water column. Other cephalopods use a combination of fins, jets of water and, in the case of the actual nautilus, chambered shells. The argonauts are the only species known to use bubbles, but it’s clearly an efficient tactic. Finn and Norman observed that once they had trapped their air pockets and reached the right depth, they could swim fast enough to outpace a human diver.

See how the argonaut octopus does it in a video at Discover magazine. Link

(Image credit: Julian Finn)

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I thought Captain Nemo named his ship after the Chambered Nautilus - another cephalodod which actually lives and travels in its spiral shell. They are not closely related to the Paper Nautilus, and the latter's shell-shaped egg carrier evolved independently.
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