I suppose many of us would like to have a pair of extra hands, you know, just to hold that cup of coffee in one hand, a book in another and maybe an iPhone in the third. Although, then we might also require an extra pair of eyes too… But it’s not all that easy, apart from the fact that clearly under the specific circumstances of human evolution two hands proved to be an optimal option, too many hands can pose a problem: when it comes to managing them all at the same time they might end up just tangling all over the place!
But there are animals that do have quite a few hands and not just any hands but hands full of suckers that will stick and hold on to anything in their path. Yes, I am talking about cephalopods, which include octopuses, squids, cuttlefishes and others. So how and do these animals manage to distinguish between their own hands and other objects and prevent the suckers from catching the limbs of their own?
Well, this is the question that a recent paper called “Self-Recognition Mechanism between Skin and Suckers Prevents Octopus Arms from Interfering with Each Other” has attempted to answer.
To begin with the first observation made was that an amputated arm of an octopus, which is still active for around an hour after amputation, can grasp and stick to all kinds of objects, if and only if the object is not covered with octopus’s skin. For example, if you place a skinned piece of octopus on the amputated arm it will grab and stick to it tightly but if the piece still has the skin on it the amputated arm will not react to it. This applies to inanimate objects as well; when a petri dish, which was half-covered in an octopus skin, was placed on an amputated arm it only grabbed the uncovered part. See video 1 and 2. When the force used to stick to different objects was measured scientists confirmed ‘with skin-no stick’ observations, and also found that the octopus’s arm exhibited the strongest grasping force when holding a food object such as fish.
Amputated octopus’s arm stimulated by peeled and non-peeled piece of arm. http://goo.gl/ZcRvym
Amputated Octopus’s arm stimulated by skin covered petri plate. http://goo.gl/ZcRvym
So it is now quite clear that the skin has something that prevents the octopus from grasping it and the question is- what is it? It’s a simple question with a no easy answer because skin is a very complex system containing many chemicals, receptors and other elements, all of which could be involved in attachment inhibition. In order to answer the question the study basically took some octopus skin and used different solvents to extract general groups of substances from the skin. They then took these skin extracts, coated petri dishes with them and again measured the grabbing forces applied to these dishes. The idea is that if some chemicals are involved in preventing the grabbing then the grabbing force should be reduced for the plate which is coated with this substance. Indeed, the grabbing force was 10 times reduced for the dishes that were coated with skin extracts obtained using hexane as a solvent (as compared to controls of either just hexane coated petri plates of fish skin hexane extracts).
So this is great, we now know that skin contains some chemical that prevents octopus suckers from sticking to it. However, an amputated arm is a highly isolated system and will not necessarily reflect the real-life behavior of an octopus, which is quite a sophisticated organism. Therefore, the scientists next used Octopus vulgaris to test if life octopuses show the same behaviour. When an animal was provided with an amputated arm from a different octopus the animal grabbed it in 94% of the trials but when the arm came from the same animal in less that 40% of the cases the arm was grabbed. So, although the results are not as clear-cut as the ones observed with an amputated arm, same conclusions can still be made. In addition, this suggests that there is a higher order mechanism, probably one that involved octopus’s nervous system, which can override the chemical signals from the skin.
In addition, scientists have noted that octopuses tended to hold the amputated arms in a peculiar way, which they called “spaghetti holding” (see video 3 and figure 2). The octopus would bring an amputated arm to its mouth (holding to it by the amputation site where no skin is present) and only touch it with its beak (rather than arms or interbrachial web, as the food is usually held) and the amputated arm would just hang like this neither being eaten nor released. The spaghetti holding mechanism has probably evolved as a part the self-avoidance behaviour and also supports the idea that the different systems in the octopus have coevolved in an integrated fashion to facilitate the control of a complex animal morphology, rather then relying on a top-down control in which one system manipulates all others.
Spaghetti holding. http://goo.gl/ZcRvym
In conclusion, the study shows that octopuses have a mechanism to prevent their suckers from sticking to their own arms. The essence of this mechanism lies in the skin of an animal which contains some kind of chemical signal that inhibits the grabbing behaviour. This mechanism is also integrated with the nervous system, which can override the grabbing inhibition as suggested by the live animal experiments.
I am sure the research will not stop here as there are still many questions left; which specific chemicals are involved, how they differ between animals and how neuronal signals integrate the system? The complex workings of the motor system in octopuses have been an inspiration for biology-based design in robotics and other engineering fields and I am sure that someone will find an application for this system as well. As the authors nicely put it:“Peripheral self-avoidance is a striking addition to the list of surprises in the motor system of this uniquely embodied animal.”
Nesher, N., Levy, G., Grasso, F., & Hochner, B. (2014). Self-Recognition Mechanism between Skin and Suckers Prevents Octopus Arms from Interfering with Each Other Current Biology DOI: 10.1016/j.cub.2014.04.024