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Biology

Suck It!: Why Octopus Arms Don’t Stick To Each Other

Background

Octopi (or octopodes, or just octopuses) have a challenging task set out for them: how to control all eight of their arms simultaneously. There are infinite ways an octopus can move its arms, but octopi have simplified the problem of their movement by sticking to (pun intended) predictable movement patterns, which are regulated by their complex neural systems.

Much like a chicken that can run with its head cut off, an amputated octopus arm is highly active for more than an hour after separation from the body. The suckers on the removed limb actively grasp and hold objects, a trait the researchers in this study were able to take advantage of. They did a number of different trials and experiments with octopus arms and octopus skin to find out how the suckers on their arms reacted to different items.

Methods and Results

During the preliminary trials with amputated octopus arms, the researchers never saw the amputated arm grab onto the arm itself or to anything covered in octopus skin. However, when the researchers presented the amputated arm with a skinned piece of octopus arm, the suckers grabbed and held the flesh as if it were a piece of food. To make sure it was the skin causing this response, the researchers covered part of a petri dish with octopus skin and left the other part bare. Once the arm was introduced, it attached only to the bare part of the petri dish and not to the skin at all. (See movie 1)

Octopus Arm Grabs Petri Dish (Movie 1)

The researchers also measured the amount of force the suckers applied to different objects. To do this, they quantified how much force it took to remove the suckers from a reference material: an arm with skin, skin covering the petri dish, a skinned arm, a fish, and a boiled shrimp.

Figure 1

Figure 1

It makes sense that the force applied to the fish was stronger than anything else, as fish is a regular prey item for octopus and often puts up a fight when caught. The figure above also suggests that the peeled arm (the arm without skin) is perceived as food rather than an octopus arm.

Clearly, the skin is what causes the octopus’ suckers to recognize the material as octopus or non-octopus, but the researchers needed to determine if it was the texture of the skin itself or the chemicals within the skin. To do this, they ground up octopus skin, made a gel with the mixture, and coated a petri dish with it. They also made a gel that didn’t contain octopus skin, as well as a gel that contained ground up fish skin. As expected, the octopus suckers attached to the octopus skin gel with much less force than applied to the fish skin gel, so it’s definitely something within the skin causing the suckers to not stick to the octopus’ own arms.

Figure 2

Figure 2

 

Finally, the researchers also wanted to investigate the behavior of the octopus when they were presented with amputated octopus arms as food. It’s well known that octopi are cannibalistic and, in addition to other types of octopus, will even eat members of their own species. In this study, when an octopus was presented with an amputated arm from another octopus, the reaction was interesting: they would touch it with their suckers, but they would not grab or hold on to it. In some cases, the octopus would grab the amputated arm, but only where it had been removed from the octopus (the area without skin). The octopus would then hold the arm only by its mouth (called the beak), a novel behavior the researchers called “spaghetti holding”.

Figure 3

Spaghetti holding

The researchers even amputated an arm off an octopus and fed it back to the same octopus to see how it would behave. They found out that an octopus is capable of recognizing its own arms, even after amputation. In 94% of the trials, the octopus would touch its suckers to an amputated arm taken from another octopus of the same species, as well as hold the arm in its mouth as if it was food. However, if given their own arm, the octopus only touched their suckers to it less than 40% of the time and did not hold in their mouths at all.

Significance

This study is the first to identify two main things: that something within an octopus’ skin keeps the suckers from sticking to each other, and that an octopus will cannibalize another octopus’ amputated arm but not its own. The researchers suggest that because the octopi are capable of identifying their own amputated arms, skin recognition is more elaborate than just an avoidance of the suckers to the skin; instead, they think that it may involve higher brain activity and maybe even intelligence. The octopus is so flexible and the movements that it can make with its eight arms are so unusual and complex that the evolution of unique abilities, like skin recognition and the neural system necessary to support that, are possible.

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