The ancient Greek thinker Aristotle had a theory about why female argonaut octopuses have a shell: he suggested that they used it as a boat to float on the ocean surface. New research disproves this theory, but sheds new light on their actual purpose.
Shelling out
At last we know why female argonaut octopuses have a shell, and what use they put them to. Previously, scientists thought the females used them to keep their delicate eggs safe. Now, by capturing argonauts and releasing them at different depths, Julian Finn and Mark Norman of in Melbourne, Australia, have established that they use the shells to capture and compress air for buoyancy control. In this picture the octopus has headed to the surface to scoop in some air with her shell.
Journal reference: , DOI: 10.1098/rspb.2010.0155
(Image: Julian Finn/Museum Victoria)
The roll
Next, she rolls the shell away from the surface, positioning it to imprison the air.
(Image: Julian Finn/Museum Victoria)
Downwards propulsion
Using strong jets of water from her funnel, she propels herself and the buoyant shell downward away from the surface.
(Image: Julian Finn/Museum Victoria)
Neutrally buoyant
At depths of around 5 metres, the air is compressed by the greater water pressure, giving a buoyancy force exactly balancing her weight. In this state of “neutral buoyancy”, she can swim around in the open ocean effortlessly, because she neither sinks nor floats upwards.
(Image: Julian Finn/Museum Victoria)
