Scientists solve millennia-old mystery about the argonaut octopus

By | May 20, 2010

The argonauts are a group of octopuses unlike any other. The females secrete a thin, white, brittle shell called the paper nautilus. Nestled with their arms tucked inside this beautiful, translucent home, they drift through the open ocean while other octopus species crawl along the sea floor. The shell is often described as an egg-case, but octopus specialists Julian Finn and Mark Norman have discovered that it has another function – it’s an organic ballast tank.

An argonaut uses its shell to trap air from the surface and dives to a depth where the encased gas perfectly counteracts its own weight, allowing it to bob effortlessly without rising or sinking. Finn and Norman filmed and photographed live animals in the act of trapping their air bubbles, solving a mystery that has been debated for millennia.

Scientists have long wondered about the purpose of the argonaut’s paper nautilus. No less a thinker than Aristotle put forward a hypothesis. In 300 BC, he suggested that the female octopus uses its shell as a boat, floating on the ocean surface and using her tentacles as oars and sails. Despite a total lack of evidence for this ‘sailing hypothesis’, it was later championed thousands of years later by Jules Verne, who wrote about sailing argonauts in Twenty Thousand Leagues Under the Sea.

Since 1923 and the work of Adolf Naef, the shell has been viewed as a container for the argonaut’s eggs. After mating with a male (who is around 8 times smaller and 600 times lighter), the female secretes the papery shell using the tips of two large tentacles. She lays her eggs within the structure before snuggling inside herself. Besides her eggs, her only housemate is one of the male’s arms – the hectocotylus. The arm doubled as a penis, snapped off during sex and stays inside the female’s body.

Naef was clearly wrong. The air isn’t life-threatening or even unintended – the argonaut deliberately introduces it and has total control over it. Once the animals dived again, Finn and Norman grabbed them and rotated them through 360 degrees – not a single bubble emerged. “To my delight the argonauts immediately put to rest decades of conflicting opinions, demonstrating their expert ability at obtaining and managing surface-acquired air,” says Finn.

Argonaut_dive

This neutral buoyancy is a big boon for animals that live in the open ocean, because they don’t have to expend energy on keeping their place in the water column. Other cephalopods use a combination of fins, jets of water and, in the case of the actual nautilus, chambered shells. The argonauts are the only species known to use bubbles, but it’s clearly an efficient tactic. Finn and Norman observed that once they had trapped their air pockets and reached the right depth, they could swim fast enough to outpace a human diver.

By rocking at the surface, the argonaut can also trap a sizeable volume of air, which, in turn, allows it to reach a greater depth before becoming neutrally buoyant. Finn and Norman think that this may allow these unusual octopuses to avoid the surface layers of the ocean, where they would be vulnerable to birds and other top-level hunters.via Scientists solve millennia-old mystery about the argonaut octopus | Not Exactly Rocket Science | Discover Magazine.

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