Argonaut octopuses use shells as flotation devices
Melbourne: Australian researchers have found that unique, free-swimming octopuses called argonauts, use their stunning white shells to remain neutrally buoyant beneath the sea surface.
For the first time, Dr Julian Finn and Dr Mark Norman from Museum Victoria in Melbourne have observed the animals, Argonauta argo, in the wild, in the Sea of Japan.
The research says that females of these rarely-seen octopuses 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 researchers 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," ABC Science quoted Finn as saying.
"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," Finn added.
The findings have been reported in the Proceedings of the Royal Society B.
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