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February 6, 2012
Since ancient biologists have wondered why, among all octopuses, only one, the female Argonaut, is able to generate around a thin spiral shell. So fragile (to the point of being called "paper nautilus"), which serves to protect themselves from predators. And so light that allows the animal to float weightless on the fence.
For centuries, given that female Argonaut put and keep their eggs inside the shell, it was thought that, offering a sort of nest for their offspring, was precisely the main function of the curious structure.
A study by Australian biologists and published in the Journal of the Royal Society discovered the truth: the main function of the shell of the Argonauts is to allow the animal to ascend and descend in the water at will, thanks to "air capsules" ably used to regulate the depth without making any physical effort.
Which puts an end to long speculations on the role of those curious air bags, which many considered responsible for the octopus floating and drifting uncontrollably to be stranded on the beaches half a world. But biologists say the study shows that the air inside the shells of female Argonaut is not only beneficial for them, but essential for survival.
Indeed, thanks to these bags the Argonauts can choose exactly how deep they want to be by the simple method of "adjusting" the amount of air which absorb on the surface. A mechanism that gives them a great advantage over other octopuses, which to scroll up and down need to use a lot of muscle power.
The Argonaut, however, uses a kind of jet to descend effortlessly. As low (up to 750 m depth), increasing water pressure is reduced the volume of air within the shell, reaching an equilibrium in which the animal's weight and it vanishes just remains floating in a state similar to weightlessness.
Researchers believe that this extraordinary ability had remained hidden until now due to the fact that most previous studies have been conducted in aquariums, too shallow to allow the octopus to demonstrate their skills.
Despite its resemblance to the nautilus, another cephalopod with which he is often confused, the Argonaut is a real octopus with eight tentacles equipped with suckers around its central mouth. There are four known species live in temperate and tropical waters and is an important food source for whales, seals, fish and seabirds.
Females can grow to 50 cm and are able to make their shells, calcium carbonate secreting two bodies, similar to those spiders to spin off its cloth, placed in two of its tentacles. Males are much smaller, barely two inches, and can not make shells.