Octopuses can 'see' light with their arms

Octopuses can "see" light with their arms, even when their eyes are in the dark, researchers have found. When the arms of the octopus detect light, the eight-armed creature pulls them close to their body.

Because octopuses generally have a poor sense of where their body is in space, this complex instinctive behavior might help protect their arms from the pincers of predators nearby that they might otherwise not sense. 

Scientists have long known that octopus arms react to light. Their skin is covered in pigment-filled organs called chromatophores that reflexively change color when exposed to light. These chromatophores are responsible for the octopus's color-changing camouflage superpowers. In fact, it was while studying these light-induced chromatophore responses that Tal Shomrat and Nir Nesher of the Ruppin Academic Center in Israel noticed something odd. 

Related: 8 crazy facts about octopuses

At the time, an undergraduate student in their lab was shining bright lights on octopus arms to elicit a chromatophore response. But the octopus was not cooperating. 

"We were using a very strong flashlight and when we illuminated the tip of the arm, it would always pull away. It was very surprising," Shomrat told LiveScience. "We shifted our experiment to explore this behavior after we found out that nobody had described it before."

Their new experiment involved placing an octopus in a tank covered in an opaque black tarp. The octopus, kept in the dark, was trained to reach an arm through a small hole in the top of the tank to find pieces of fish. While the octopus was blindly feeling around for a piece of food, the researchers would shine a bright light on the octopus's arm at a random time; about 84% of the time when they shone the light, the octopus would rapidly pull its arm away,  suggesting that the octopus is able to sense and react to light with its arms, even when they can't see the light with their eyes. 

"We often feel the heat from intense light, but for the octopus, this isn't the case," Neshir said. "In our experiments, we checked for changes in temperature and there weren't any. The effect is from pure light." 

Having established that octopus arms can sense and react to light, their next step was to explore what controls this reaction. Is it a simple reflex controlled completely by neurons — or special nerve cells —in the arm, or is it controlled by the brain?

To answer this question, they performed a few additional experiments. First, they illuminated different parts of the octopus's arm to determine the region most sensitive to light. They found that the tip of the arm was the most sensitive to it.

Next, they illuminated the arms of several anesthetized octopuses. If light avoidance were entirely based on a local reflex, it might occur in an unconscious octopus. However, while the chromatophores in the sleeping octopus reflexively reacted to the light, the arms didn't pull away. 

When the scientists cut the muscles at the base of the arms, that also eliminated the arm retraction. Put together, the studies suggest that the arm is sensing the light, sending a message to the brain through nerves in the muscle, and the brain is telling the octopus to move the arm.

One of Shomrat and Neshir's experiments confirmed this, too. When they would shine the light on a piece of fish, the octopus would initially avoid the food before seemingly deciding to override its instincts and grab the fish anyway.

"The fact that this behavior is not a reflex, but instead controlled by higher-level cognition in the brain is fascinating," said Eduardo Sampaio, an octopus behavior researcher at the University of Lisbon in Portugal, who was not involved in the study. 

Shomrat and Neshir think this reaction may have evolved as a way for the octopus to protect its arms from predators. They say that predators that hunt by sight might mistake a misplaced octopus arm for a worm or small fish. Being able to feel light with the tip of the arm might help the octopus keep its arms out of harm's way. 

"The behavior is so robust and obvious, it is interesting that nobody had described it before," Shomrat said. "Now, our next steps are to investigate the evolution and purpose of the behavior."

Shomrat, Nesher and Itamar Katz, also of the Ruppin Academic Center, published their findings online Feb. 3 in the Journal of Experimental Biology.

Originally published on Live Science.