Octopi come in also sorts of shapes and sizes, but the argonaut octopus must be one of the strangest. The females secrete a thin, white shell, which is called a paper nautilus, and it does make them look like a nautilus. They tuck themselves and their eggs into it and drift through the sea. Sometimes these paper nautiluses contain trapped air, and its function has been a mystery for many years. Some people have argued that the air is trapped accidentally and is a major problem for the octopus.
Julian Finn and Mark Norman have shown that they air is deliberately introduced by the females, and is used to acquire neutral buoyancy, meaning they can keep their position in the water column without having to expend any energy.
You can read more about it here.