Even when their eyes are in the dark, octopuses can “see” light with their arms, according to a recent study by researchers at the Ruppin Academic Center in Israel. After detecting the light, the octopus will pull its arms in, close to its body.
In general, the cephalopod’s sense of where its body is in space is quite poor, so this complex instinctive behavior may act to protect the arms from undetected predators nearby, which may mistake the tips of the octopus’s arms as fish or worms.
That octopus arms react to light has long been known. Its skin is covered in chromatophores, pigment-filled organs that change color when light falls upon them. They are behind the octopus’s color-changing camouflage ability.
Researchers Tal Shomrat and Nir Nesher were studying these chromatophores when they noticed something unusual.
“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,” said Shomrat. “We shifted our experiment to explore this behavior after we found out that nobody had described it before.”
In their new experiment, they placed an octopus in a black tarp-covered tank. While the octopus was in the dark, it was trained to reach an arm through a small opening above to get some pieces of fish. As it was blindly feeling around for the morsels, the scientists randomly shined a bright light on its arm. They found that the octopus would pull its arm away 84 percent of the time, which suggested that through its arm, it was able to sense and react to light, even when it could not see the light with its eyes.
“We often feel the heat from intense light, but for the octopus, this is not the case,” Neshir said. “In our experiments, we checked for changes in temperature and there were not any. The effect is from pure light.”
Further experimentation revealed that the tip of the octopus’s arm was the most sensitive to light. In addition, they found that while the chromatophores in a sleeping octopus reacted to light, the octopus did not pull its arms away. The same was true when muscles at the base of the octopus’s arms were disabled.
The findings of the study, which was published in the Journal of Experimental Biology, suggest that the octopus’s arm senses the light and sends a message to the brain through the nerves in its muscles. Then the brain tells the octopus to move its arm. This sequence was confirmed, when light was shone on a piece of fish, which the octopus avoided initially, before finally deciding to grab the fish anyway, overriding its instincts.
Eduardo Sampaio, an octopus behavior researcher at the University of Lisbon in Portugal, who was not involved in the study, said, “The fact that this behavior is not a reflex, but instead controlled by higher-level cognition in the brain is fascinating.”