Trapped air in the shells of rare octopuses is the key to their survival in the deep sea, say scientists.

Females of the argonaut family (Argonautidae) release trapped air from their shells to control very precisely their movement through the water.

This ability has puzzled naturalists for over 2,000 years, dating back to observations made by Aristotle in 300 BC.

Research published in the Royal Society journal, Proceedings B, finally explains why it may have evolved.

The Australian researchers describe how the mechanism enables the creatures to conserve energy, avoid predators and protect eggs during the brooding stage.

The study, led by Dr Julian Finn of Museum Victoria in Melbourne, is the first to observe directly how this unique species of octopus captures air at the sea surface and uses it to its advantage.

"It wasn't until I actually got an argonaut in the water that I really saw the true marvel of these animals," said Dr Finn.

Unlike any other species of octopus, the female argonaut, which can be up to 50cm (20 inches) in length, makes itself a paper-thin shell. It secretes this shell, made of calcium carbonate, from two web structures on the sides of its body.

The males are much smaller, typically only a centimetre in length, and do not produce shells.

Mythical Creatures

Air pockets have been observed before within the shells of both wild and captive argonauts, also known as "paper nautiluses", but their origin and purpose has until now been a mystery.

"This mythical story began around the time of Aristotle that the argonaut female actually lived in the shell and raised those webs as sails as she sailed across the ocean," explained Dr Finn.

The new findings show that the female argonaut takes in air at the sea surface through a funnel as it rotates its shell anti-clockwise. It then seals off an air pocket in the top, or apex, of the shell using a second webbed pair of tentacles.

As it dives to depths of up to 750m (almost half a mile) below the surface, it adjusts the amount of air in its shell to match its own density with that of the seawater, keeping it "neutrally buoyant" and enabling it to swim effortlessly.

This contrasts with most other cephalopods - the class of animals that includes octopuses, squid and cuttlefish - which expend vast amounts of energy to maintain their position.

Underwater Control

The female argonaut can also counteract the considerable weight of its eggs, which it releases into its shell during the reproductive period, to carefully avoid bumping them on the sea floor.

By keeping a safe position in mid-water, argonauts can also steer clear of disturbance by surface waves and predators from above, such as birds.

Once believed to hinder the females, it is now thought that argonauts evolved this remarkable mechanism from ancestors that lived on the seafloor, allowing the species to expand its range into mid-depths.

"The female argonaut knows exactly what she was doing. Underwater she was completely in control," added Dr Finn.