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Why did the octopus grow a shell?

The ancient Greek thinker Aristotle had a theory about why female argonaut octopuses have a shell, but we didn't know for sure until now.
Shelling out
Shelling out
(Image: Julian Finn/Museum Victoria)

Gallery: Why did the octopus grow a shell?

At last we know why female argonaut octopuses have a shell, but the males don鈥檛. Previously, researchers thought the females used them to keep their delicate eggs safe. Now, by capturing argonauts and releasing them at different depths, Julian Finn and Mark Norman of in Melbourne, Australia, have established that they use the shells to capture and compress air for buoyancy control.

Gallery: Why did the octopus grow a shell?