For centuries, biologists have puzzled over the evolutionary function of the female's delicate off-white casing.
The mystery of the paper nautilus, a translucent shell that belongs to the argonaut, has been solved.
The shell's primary role is to allow the animal to ascend and descend in the ocean.
Argonauts are an important food source for whales, seals, fishes and other animals.
Scientists said Wednesday they had cracked the mystery of the paper nautilus, the brittle and translucent shell belonging to rarely-seen species of open-water octopuses called argonauts.
For centuries, biologists have puzzled over the evolutionary function of the female's delicate off-white casing, which is too thin to offer protection from predators.
The most widely-held view was that it served as a brood chamber to house eggs.
But a new study reveals its primary role is to allow the animal to ascend and descend in the ocean by using trapped air to regulate depth.
"Through underwater observations of wild argonauts, we discovered the five-step process by which females gather air from the water surface and transport it to depth, where neutral buoyancy is attained," said Julian Finn, a researcher at Museum Victoria in Melbourne, co-author of the study.