The Argonaut Octopus differs from most other types of octopus because it lives near the surface, rather than on the sea floor, and the females have a brittle white shell full of air which has earnt them the name of "paper nautilus".
The purpose of this shell has puzzled marine biologists since the time of Aristotle in ancient Greece, who believed it to be a primitive sail. Many have suggested theories for its purpose, and the most popular was Adolf Naef's theory in 1923 that it's a casing for eggs -- inhabited by the female, who lays her eggs inside before clambering in herself, alongside one of the male's arms, used as a penis, which snaps off during sex.
But Museum Victoria scientists Dr Julian Finn and Dr Mark Norman have discovered that it's primarily used as a buoyancy device that lets the argonaut octopus rise to the surface of the sea and descend to the bottom, when required.
When the octopus is at the surface, the researchers observed it rocking the shell around to capture a quantity of air, which it then seals off using its arms. That then allows the creature to maintain neutral buoyancy in the ocean, in the same way that a hot air balloon stays in the sky. Previously, air trapped inside the shell was thought to be harmful to the creature.
The scientists' findings have been published in the Proceedings of the Royal Society B: Biological Sciences, and describe how three of the 20cm-long creatures were caught in commercial nets in the Sea of Japan, and transported to a tank containing sea water and fed. One at a time, they were then released into Okidomari Harbour, where they were observed and photographed underwater by scuba divers. Before, release, all air was drained out of their shells.
In every case, the animal immediately swam towards the surface, before "gulping" in as much air as it could and sealing it off using its arms. It then jetted back down below the surface again, halting at a point where the trapped air cancelled out the animal's weight -- allowing it to attain neutral buoyancy. It was then able to swim parallel to the water's surface at great speeds -- faster than a diver.
The scientists discovered that the rocking motion of the shell at the surface was more efficient than a passive shell would be, allowing it to capture a larger volume of air and therefore maximise the depth at which neutral buoyancy can be achieved. That allows the creature some protection from wave motion and predators like birds striking from above.
The males of the species, in comparison, are tiny and shell-less -- rarely surpassing 2cm in length. Each only mates once in its short life, whereas the females mate many times. As a result of the male's size and lack of a distinctive shell, they were only discovered in the late 19th century, whereas the females had been known since ancient times. Originally, it was thought that the arm left behind in the female's shell had been a parasitic worm.
The impacts of the research could allow for more sophisticated buoyancy systems for both divers and submarines, as well as allowing for better knowledge of marine ecosystems and how these rare creatures live. The scientists are still investigating the Argonauts, and have set up a survey to try and find out a little more about the creatures. Members of the public who spot the creature should head on over to the survey website and get in touch with ArgoSearch.
By Duncan Geere
(Submitted by Chad Arment)
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