Unique, free-swimming octopuses called argonauts, use their stunning white shells to remain neutrally buoyant beneath the sea surface, say Australian researchers.
Dr Julian Finn and Dr Mark Norman from Museum Victoria in Melbourne have for the first time observed the animals, Argonauta argo, in the wild, in the Sea of Japan.
They report their findings today in the Proceedings of the Royal Society B.
The females of these rarely-seen octopuses sport the fragile white shells, and since antiquity the animals have been spotted floating on the surface of the sea, sometimes in long chains of 20 animals or more.
The ancient Greek philosopher and scientist Aristotle postulated that argonauts used these shells to 'sail' across the sea surface, hence their name.
This was proved to be wrong, and scientists then thought that the shells acted predominantly as brood chambers for eggs, and that air caught in the shells could cause mass strandings.
Now, Finn and Norman say instead females actively fill their shells with air, and then jet down into the water column, where the air compresses as water pressure increases with depth.
This allows argonauts to remain neutrally buoyant at depths of up to 10 metres, with the volume of air in their shells exactly compensating for their weight, they say.
Finn took three female argonauts captured by Japanese fishermen scuba diving in Okidomari Harbour on the western coast of Honshu, and released them at depths of 2-7 metres. Prior to release, the shells were depleted of air.
All three argonauts jetted to the surface and rocked their shells forward to 'gulp' air, which they then sealed in their shells with specially-adapted tentacles. The argonauts then dived until buoyancy from the trapped, compressed air cancelled their weight.
"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.
"Female argonauts released with no air in their shells flailed from side-to-side when swimming, struggling to maintain vertical orientation. Argonauts released with ample air in their shells at the water surface displayed no difficulty in diving to depth."
A "whacky" octopus
The study demonstrates a convergent evolution between argonauts and true nautiluses, unrelated cephalopods which achieve buoyancy through controlling fluid levels in their chambered shells, to which they are attached.
"Our study has demonstrated how the dexterity and morphological plasticity of the female argonaut has allowed neutral buoyancy to be attained with far less morphological architecture and complexity than that of the [true] nautiluses," says Finn.
Finn says that the animals, whose females can be 600 times heavier than the shell-less, minute males, were "pretty whacky".
While females cruise the surface, the smaller males actively swim over a greater depth range. When they encounter a female they transfer their sperm to an arm which they detach and leave with her.
"Males grow to a size comparable to other octopus [larvae] that migrate with the plankton," says Finn. Males don't appear to counteract their weight because they are so small, while the heavier females need to counteract their weight to avoid expending energy, he says.
"They have made a lot of strange adaptations [in order to] live in the open ocean."