An octopus might be one of the most intelligent invertebrates, but it doesn't always know what, exactly, its arms are doing. How these animals manage to avoid tangling themselves up is a major feat. But another—of no small concern—is keeping free of the strong grasp of its own suckers.
New research, published May 15 in Current Biology, reveals an elegantly simple solution.
Octopus suckers are powerful and can grasp onto just about any type of surface—smooth or bumpy, rigid or supple. In fact, note the researchers in their paper, "the hundreds of suckers along each arm have a tendency to stick to almost any object they contact." Which could make for a very sticky situation. Or, as the researchers tactfully put it: "This reflex could pose significant problems with unplanned interactions between the arms."
Like the arms, the suckers seem to be under relatively local control. So rather than bothering the central brain for instruction, as our bodies are set up to do, the octopus relies on closer-at-hand nerve bundles for information processing and control. But that raises the question: how do the suckers "know" a feisty fish from the octopus itself?
To figure out how these animals are managing this, researchers at Hebrew University of Jerusalem studied the behavior of octopus arms that could no longer talk to the main brain at all. (Warning: this is where things get a little gross.)
Octopuses often discard their own regrowable arms, whether for mating or to distract a predator. And a severed octopus arm can respond to stimuli and even activate its suckers for more than an hour. The research team noticed that amputated octopus arms suctioned onto various objects—but never onto itself or other octopus skin.
The amputated arms wouldn't even attach their suckers to Petri dishes that had been covered in octopus skin. If only half of the dish was covered with octo-skin, the arm would adhere only to the exposed plastic section. The suckers seem to be responding individually to some substance in the skin, the researchers, hypothesize, because in the latter instance, a sucker on the skin would avoid sticking while a neighboring sucker on the plastic would adhere right away.
To further support this idea, bodiless arms did adhere to an octopus arm that had been skinned—but with less force of a normal grasp. And when researchers coated Petri dishes in an extract made from octopus skin, the suckers attached, but very gently.
"We were entirely surprised by the brilliant and simple solution of the octopus to this potentially very complicated problem," Nir Nesher, a Hebrew University researcher and co-author of the new study, said in a prepared statement.
"The results so far show, and for the first time that the skin of the octopus prevents octopus arms from attaching to each other or to themselves," the researchers wrote in their paper, suggesting that the mechanism is likely a chemical compound.
The suckers are not completely in their own charge, however. We already know that octopuses often eat one another. But how could they, if suckers could not hold onto octopus skin at all? The researchers found that a live octopus could, indeed, grasp onto an amputated arm. This suggests that "the peripheral mechanism appears to be overridden by central control," the researchers noted in their paper. However, octopuses treated arms differently if they came from a different octopus or from their own bodies. "Surprisingly, octopuses seem to identify their own amputated arms, as they treat arms of other octopuses like food more often than their own," the researchers noted.
The new discovery might also come in handy for future robots—especially those that draw inspiration from the octopus. "We hope and believe that this mechanism will find expression in new classes of robots and their control systems," study co-author and lab leader Binyamin Hochner said in a prepared statement. We'll call it octopus sucker sense.
Read more about the amazing capabilities of octopus suckers in Octopus! The Most Mysterious Creature In the Sea.
Illustration courtesy of Ivan Phillipsen