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Researchers Solve Argonaut Mystery

May 20, 2010

Scientists said Wednesday they had solved the mystery surrounding the paper nautilus, a brittle and translucent shell that belongs to the rarely-seen species of octopi known as argonauts.

For centuries, biologists have pondered about what evolutionary function the female’s delicate off-white casing has served, only knowing it is too thin to offer any protection from predators.

The most popular theory throughout time was that it had served as a brood chamber to house eggs.

But a new study has revealed the primary role of the casing is to allow the animal to ascend and descend in the ocean by using trapped air to regulate its depth.

“Through underwater observations of wild argonauts, we discovered the five-step process by which females gather air from the water surface and transport it to depth, where neutral buoyancy is attained,” said Julian Finn, a researcher at Museum Victoria in Melbourne, co-author of the study.

The finding lays rest to a long-believed notion that pockets of air get trapped inside the shell which has led to mass amounts of the creatures being washed up on beaches around the world.

“This study demonstrates that air in the shells of female argonauts is not only beneficial, it is essential,” Finn said in a statement.

Argonauts determine the depth at which they can remain suspended by adjusting the amount of air they take in at the surface. As an octopus swims to depth using a natural form of jet propulsion, the increasing pressure of the water gradually reduces the volume of captured air until it exactly cancels out body weight, which thus reduces buoyancy.

Finn believed that the remarkable feat of controlled equilibrium went unnoticed because in previous experiments, scientists studied the animals in aquariums that were too shallow for the specimens to show off their skills.

Finn’s observations were conducted in the wild with argonauts caught by Japanese fishermen. “I took each argonaut scuba diving,” he explained. “I removed all the air from each argonaut shell, released it and observed.”

Each octopus immediately jetted to the surface of the water, where it took in a precise volume of air and sealed it with its arms before returning to the depths.

Argonauts are found in all temperate and tropical waters. They are an important food source for whales, seals, fish, aquatic birds and other marine predators. There are four known species, varying in size up to 12 inches in diameter.

Unlike the chambered nautilus, with which they are sometimes confused, argonauts are true octopi, with eight arms and suckers surrounding a central mouth.

Finn’s research was published in Proceedings of the Royal Society B on May 19. The paper, ‘The argonaut shell: gas-mediated buoyancy control in a pelagic octopus’ was co-authored by Dr Mark Norman, Head of Sciences.

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