Octopuses lost their shells millions of years ago but one small group returned to a hard life. For centuries, scientists have pondered over tiny octopuses called argonauts that swim through open seas inside delicate shells. Some suggested that argonauts attack and eat other shelled creatures, then float off across the waves, using the empty shell as a boat and hoisting two flattened arms as sails.
Eventually it was pioneering scientist Jeanne Villepreux-Power who uncovered the truth about argonauts. In the 1830s, on the island of Sicily, she invented the aquarium tank and conducted ingenious experiments which revealed that argonauts aren’t pirates but make shells themselves, using the ends of two arms.
Villepreux-Power saw female argonauts using their shells as mobile brood chambers, to carry and rear their young. But she saw no sign of the males.
We now know that male argonauts are minute compared to females, and they don’t have shells. They swim around, hoping to come close enough to a female to donate a detachable arm that’s laden with sperm (this had been previously misidentified as a parasitic worm). She then carries around his appendage, and several from other males, using them as required.
Biologist Julian Finn has discovered another use for the argonauts’ shells. Recently, he released several live female argonauts underwater and watched as each one swam upwards, popped her shell above the waterline and trapped an air bubble inside, before jetting off into the distance. Like their cousins, the chambered nautiluses with gas-filled shells, argonauts use their shells as energy-saving buoyancy devices that stop them from sinking.