Argonaut buoyancy video

Transcript

 

Dr Julian Finn: Argonauts, or paper nautiluses, are really strange because they’re closely related to the octopuses that live on the sea floor that you’d commonly see near the beach, but these octopuses live up in the water column. The other wacky thing is that the female grows to a large size whereas the male only ever gets to about a centimetre in length. And she, unlike any other octopuses, makes herself a shell. The female argonaut secretes the shell, which is made of calcium carbonate, she secretes it from these expanded webs on their first pair of arms.

In the early days people didn’t really understand what was going on. They would find an octopus within this shell with these long webs and this mythical story began around the time of Aristotle that the argonaut female actually lived in the shell and she raised those webs as sails and she sailed across the ocean. And that’s why when Linnaeus came to renaming this group, he called them the argonauts.

In the 1800s there was even more confusion because people started to believe that the octopus that they found within the shell was actually a parasite, something that had eaten the actual animal [that] made the shell. So people believed that the octopus and shell didn’t go together. And it was only in the mid 1800s when they actually realised that an octopus makes this shell.

Argonauts often get confused with chambered nautiluses. They’re two distantly related cephalopods but they get confused because of the similarity in their shell shape. When you actually look at the shells they’re actually very different. The argonaut shell is single chambered, it’s thin, it’s papery, and very lightweight, whereas the chambered nautilus shell consists of multiple internal chambers and the animal is permanently bound to the shell but when you remove it from the shell it looks nothing like an octopus.

Over the last couple of hundred years there’s been debate in the literature about whether or not air getting caught in the top of the female argonaut shell was actually beneficial or detrimental to the female survival. Argonaut shells are highly prized by beachcombers. They tend to wash up on beaches after storms and in some cases they can wash up in their tens of thousands. The theory was that air got caught in the top of the shells when the argonaut went close to the surface and because it couldn’t get away from the surface they would wash up on beaches in these massive numbers.

I was very interested in this idea and when I got the opportunity to actually encounter live argonauts, the first thing I did is I took them diving. The animal that was the basis for this study was Argonauta argo. It’s known as the greater Argonaut, it’s found all around the world but I studied at Shimane Peninsula in the Sea of Japan.

All the argonauts that were released did exactly the same thing. First they would jet straight up to the surface of the water and then using their funnel they would rotate the shell, bobbing counter-clockwise, getting as much air to enter the shell as possible. They would lock off this large volume of air using their arms, using their webbed arms, and then they would turn their funnel down, and rotate the shell clockwise and dive away from the sea surface.

A problem that all animals have that live in open ocean, that live away from the sea floor up in the water column, is that they need to maintain their position. For the female argonaut the way that we found that she is attaining mutual buoyancy is that she’s going up and gathering air. And the air is extremely buoyant, it’s a large volume. But as you push the air down, the pressure of the water shrinks that air and it becomes more compressed and the buoyancy changes. And she pushes the air down to the point where the flotation nature of the air and her weight cancel out so that she becomes mutually buoyant, she becomes perfectly balanced between the upward pull and the downward pull and then she’s able to swim effortlessly.

I’ve studied argonauts for many years and I’ve looked at thousands of shells in museums and I’ve gone through old texts and read up on the old writings, but it wasn’t until I actually got an argonaut in the water that I really saw the true marvel of these animals.

This female argonaut knows exactly what she was doing. We as scientists thought, oh the poor argonaut is getting the air caught in its shell, or it doesn’t know how to get rid of it. Underwater she was completely in control. She went straight to the surface, go the air she wanted, and swam out of sight. It was a long way from Port Phillip Bay but it seems that the two species are probably doing the same thing. The footage we have that’s shot of local animals shows them also going up to the surface before jetting away.

About this Video

Dr Julian Finn explains explains what argonauts are, how they differ from true nautiluses, and some findings of his PhD project on these unusual octopuses.
Length: 05:14