Although it is well known for its many arms, the octopus does not seem to know where these eight appendages are most of the time.
“In the octopus, you have no bones or joints, and every point on its arm can go in any direction you can think of,” said Nir Nesher, senior lecturer in marine science at the Ruppin Academic Center in Israel. “So even an arm is something like infinite degrees of freedom.”
So how does the octopus avoid all those wavy, sucker-covered limbs? According to a study published this month in The Journal of Experimental Biology by Dr Nesher and colleagues, the octopus’s arms can sense and respond to light – even when the octopus cannot see it with its eyes on its head. . This light-sensing ability can help cephalopods keep their arms hidden from other animals who might mistake the tip of an arm for a sea worm or other type of meal.
Itamar Katz, one of the study’s authors, first noticed the detecting powers of light while studying a different phenomenon: how light causes the octopus’ skin color to change. Along with Dr. Nesher and Tal Shomrat, another author, Mr. Katz saw that the bright light on one arm caused the octopus to pull it out, even when the creature was sleeping.
Other experiments showed that the arms would avoid light in situations where the octopus could not see it with its eyes. Even when octopuses reached an arm through a small opening in an opaque, covered aquarium for food, the arm quickly retracted when light shone on it 84% of the time.
It was a surprise, as if the octopus “could see the light through the arm, it could feel the light through the arm,” said Dr Nesher. “They don’t need the eye for that.”
The light response behavior of the arm has also adapted to the changing conditions. When the octopuses were kept in the dark for a week or a month, the reflex to withdraw from the light became faster but also required brighter light to activate.
This ability to sense and adapt to light most likely uses a lot of energy, suggesting that behavior is important for the octopus’ survival.
“We think this reflex is to protect the arms and keep them bent so that no crab or fish bite it when they think it’s some sort of worm,” Dr Shomrat said.
However, how the arms of an octopus can sense light, let alone respond to it, remains a mystery. There is some evidence that there are light-sensitive receptors in the skin, but no direct evidence yet that they are responsible for the behavior, Katz said.
Surgical experiments, conducted under anesthesia and with care to minimize suffering, have provided some clues. The arm stopped retracting from the lumen when it was separated from the body or when the octopus was anesthetized. Although the behavioral response did not require sight, it still required the brain to be intact. Stranger still, the reflex also seemed to depend on something in the muscle in the arm – when only the skin was cut, the reflex persisted, but not when the muscle below was cut as well.
Finding out how exactly an octopus arm avoids light is the next step in understanding this unexpected behavior.
“But it’s always like that in octopus,” Dr. Nesher said. “You see some very strange or very interesting phenomena and then you say, ‘Oh, no one has ever looked at this before.'”