Why octopuses don't get knotted

Why don't octopuses end up tying their arms in knots?  

Guy Levy at the Hebrew University in Jerusalem thinks he has found the answer - a mysterious chemical in their skin. Kat Arney spoke to him to find out why he got involved in this knotty problem.

Guy - It might seem a little weird in the beginning when I'm asking such a question because our arms never interfere with each other and it seems natural for us. But it has to be understood that the unique morphology and structure of the octopus arms being so flexible and with the ability to deform at any point along their lengths, it limits the ability of the octopus to understand and know their exact positions. On one hand, we have all of this, that the octopus is not aware of the exact position of its arms. It cannot see them all the time. On the other hand, it has these hundreds of suckers along each arm that have the reflecting tendency to attach to anything.

Kat - So, you have all these arms going everywhere, the octopus doesn't know exactly where they are. They're covered with suckers that will grab anything. How did you go about investigating how the octopus doesn't tie itself in a knot?

Guy - So, it has to be said that an octopus arm is partially autonomic. That means an amputated octopus arm, a freshly amputated octopus arm will continue to live for more than an hour after amputation. In this time, it will behave. It will grab items.  What we did is we tried to put two amputated parts in the same dish. We saw that they don't grab each other. They would grab almost anything or let's say, anything else but not each other. This was the first step to show us that there is a built-in mechanism inside the arms that prevents this unwanted grabbing of each other.

Kat - I have this weird vision of two flailing arms in a dish. What was the next step to try and find out what's actually going on?

Guy - We suspected this has to do something with the skin of the octopus. So, we peeled the skin off one of the amputated arms in the dish. The first one that was not peeled did grab the flesh of the peeled arm. We took the skin and stretched it over a plastic disk and submitted this disk to the amputated arm and it didn't grab it. So then we knew it's something inside the skin that prevents the suckers from using their reflex for attaching.

Kat - So, what do you think it is that's in the skin that repels the other arms of the octopus?

Guy - We suspected it has to be something chemical, but we were not sure but it could have been also something - the texture of the skin. And then what we did in the next experiments is to extract molecules out of  the octopus skin in order to see if the extraction, we also prevent the suckers from grabbing. We mixed those molecules inside gel and we coated plastic disks with this gel and we showed that amputated octopus arms indeed avoided grabbing this gel that contained molecules from the octopus skin. But when we did the same thing with fish skin, we embedded molecules that were extracted in exactly the same way from fish skin, then the amputated octopus arm indeed grab the gel.

Kat - So, do you know what this mystery molecules are yet?

Guy - I'm sorry to say that no. We don't know which molecules. This is our next step to try and isolate the precise molecules that are involved in this process.

Kat - This is really fascinating because octopuses are just amazing animals. But why would you want to know how it doesn't tie itself in knots?  Is there a wider picture of knowledge that you're trying to build on?

Guy - Other than being very interesting, there are also other implications of this and we are also part of a project of the European Commission. This project is called STIFF-FLOP and it aims at building a soft manipulator that will be in the shape of an octopus for medical purposes. Our part in this project is to provide the biological information of how the real octopus controls its arms and engineers are supposed to take ideas from the biology and implement them inside the controlling system of this soft manipulator.

Kat - So maybe one day, there could be an octopus surgeon inside you that won't tie itself in a knot.

Guy - This project is aiming at building one arm. So, it won't have other arms that it will have to avoid, but maybe this mechanism can be extended and this manipulator can be programmed to avoid manipulating obstacles on the way to the target, in the same way that the octopus is avoiding grabbing its own arms. This manipulator can use similar mechanisms to avoid for example, let's say, it will be used for operations inside the intestines of a human. Then it will have to crawl inside to reach the target. We don't want it to stick to the walls of the intestines on the way.