Although known for its many arms, most of the time the octopus doesn’t seem to know where these eight appendages are.
“In the octopus, you have no bones or joints, and any point on its arm can go in any direction you can think of,” said Nir Nesher, a marine science professor at the Ruppin Academic Center in Israel. “So just one arm, it’s something like endless degrees of freedom.”
How does the squid keep all those wobbly, suction cup-covered limbs out of trouble? According to a study by Dr. Nesher and his colleagues published this month in the Journal of Experimental Biology, the octopus’ arms can sense and respond to light – even when the octopus cannot see it with its eyes upside down. This light-sensing ability can help cephalopods hide their arms 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 powers of light detection when he was studying another phenomenon: how light causes the octopus’s skin to change color. With Dr. Nesher and Tal Shomrat, another author, saw Mr. Katz that the glow of one arm caused the squid to pull it back, even when the creature was sleeping.
Further experiments showed that the arms would avoid the light in situations where the squid could not see it with its eyes. Even if the squid reached an arm from a small opening in an opaque, covered aquarium for food, the arm would quickly retract if light fell on it 84 percent of the time.
This was a surprise, as if the squid “can see the light through the arm, it can feel the light through the arm,” said Dr. Nesher. “You don’t need an eye for that.”
The light-light reaction behavior also adapted to changed conditions. If the squids were kept in the dark for a week or a month, the reflex of withdrawal from the light became faster, but also required brighter light to activate.
This ability to sense and adapt to light is most likely very energy consuming, suggesting that behavior is important for the octopus to survive.
“We speculate that this reflex is to protect the arms and keep them folded so that crabs or fish won’t bite them if they think it’s some type of worm,” said Dr. Shomrat.
However, it is still a mystery how an octopus’s arms can detect, let alone respond to, light. There is 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 carried out under anesthesia and with care to minimize suffering provided some evidence. The arm no longer withdrew from the light when separated from the body or when the squid was anesthetized. Although the behavioral response did not require vision, the brain had to be intact. Stranger still, the reflex appeared to be dependent on something in the arm muscle – if only the skin was cut, the reflex persisted, but not if the muscle underneath was also cut.
The next step in understanding this unexpected behavior is figuring out how exactly an octopus arm avoids the light.
“But it’s always like that in the octopus,” said Dr. Nesher. “You see very strange or very interesting phenomena and then you say, ‘Oh, no one has looked at this.'”