Octopurse

There's a little bit in the news today about the argonaut. It's often called 'paper nautilus', but it's an octopus; a little one that floats around near the surface of the ocean. Females are bigger then the 1-2cm males, but still only reach around 10cm. What's interesting about them, though, is that before they lay eggs, the females secrete calcite from the tips of a couple of specialised webbed tentacles and form a spiral shell. She will live in the shell, sticking the head and tentacles out the front, but the main purpose seems to be to protect the eggs. Most octopuses lay theirs in holes in rocky substrates. In their lairs. I wish I had a lair.

Argonauts are known to bob around just below the water surface and draw in a pocket of air to store in the shell, and this leads to another function of the shell. This air bubble can be used to control the buoyancy of the animal, greatly reducing the effort needed to move up and down through the water column. Effort that can then be expended by the argonaut in more profitable ways - going to the pub, making rock buns, etc. Obvious advantage.

A squashy, tentacley thing living in a spiral shell in the ocean? Hmm. Sound like an ammonite? The closest living relative of the ammonite is the octopus, not the spiral-shelled, tentacle-flapping nautilus, BUT this isn't a cephalopod family heirloom; the argonaut has developed this afresh, the clever little pulpbag. What's going on? Covergent evolution? Could be. Octopuses are brilliant.