Argonaut

In open water, without rock or seagrass to catch the eye, a white shell appears in the blue. As you approach, it reveals two eyes, a siphon and eight arms: it is not a nautilus, but an octopus. In the argonaut, only the female carries this calcified shell, which she produces herself to shelter her eggs; her first two arms, widened into large membranes, participate in its formation and repair.

In the nineteenth century, in Messina, Jeanne Villepreux-Power conducted experiments on Argonauta argo and showed that the female produces and repairs her own shell, ending the idea that she borrowed it from another animal. But the most astonishing occurs at the surface.

Experiments have shown that the female can capture air in her shell at the surface, retain it with her arms and then dive again. Under pressure, this gas compresses until it allows her a buoyancy close to neutrality.

The male, for his part, possesses a detachable reproductive arm, the hectocotylus, whose nature was so bewildering that the first zoologists mistook it for a parasitic worm. In December 2020, in the Strait of Messina, a stranded shell seemed empty.

Approximately seven hours after its collection out of the water, researchers discovered a hectocotylus still alive within it, which became very active after reimmersion. The female had disappeared; the arm of a male was still moving at the bottom of her shell.