The amazing arms of octopuses can even sense light Free

Our eyes are the way we see the world around us. But imagine if you could see with other parts of your body too. For the octopus, this feat isn't imaginary. Although their arms are more famous for their number and being covered in suction cups, octopuses can ‘see’ with their arms as well. However, they don't ‘see’ as we usually think of the term. Their arms aren't covered in tiny eyes, but they do sense light. For example, when an octopus's arm senses light, they reflexively pull it back, away from the light and into the safety of the shadows. This discovery led Weipang Chang and Melina Hale of the University of Chicago, USA, to ask what part of the arms are sensing the light and what the octopus does with this information once they've sensed it.

After getting some young California two-spot octopuses (Octopus bimaculoides), Chang and Hale first needed to see whether shining a white light on the arm caused the miniscule axial nerve cord in the arm to respond. Sure enough, the nerve cord started firing shortly after the researchers turned on the light. However, white light is made up of different wavelengths from red (longest) to blue (shortest). So, is the octopus responding to all light or just light of a certain color? To find out, the team next shone a red, green or blue light on the octopus's arm and waited to see whether its nerve cord started firing. Interestingly, light increased how frequently the nerve cells fired regardless of its color, but they fired even more frequently and with a higher amplitude when bathed in a blue light. ‘Blue light penetrates to deeper water depths. Octopus arms might be tuned to detect blue light for an ecological reason – perhaps helping them sense their environment or coordinate movements in dim underwater conditions’, explains Chang.

With the knowledge that the arms sense blue light better, Chang and Hale turned their attention to where the light sensors were located on the arm. Making sure that the light couldn't reach the top of the arms, the scientists shone a red, green or blue light on the arm. While the nerve cord still fired a little in blue light, blocking the skin on top of the arms stopped the nerve from firing if a red or green light was shone on them. When the researchers blocked light from reaching the suction cups and the skin around them, the nerve cord still fired regardless of the light's color, though not as frequently as when the light was allowed to reach the skin on the top. This suggests that the skin on the top of the arms is more important for sensing light than the underside, but also that these sensors are spaced all around the arms. But does the skin send a message all the way to the brain when it senses the light?

To test this, Chang and Hale stopped the light from shining on anything but the very tip of the octopus's arm and checked to see whether the electrical signals the nerves sent could be detected near its body. The team found that the electrical impulses were headed towards the octopus's body, suggesting that the light sensors are sending messages all the way to the brain. Because the signals are traveling to the brain, this suggests that the arm's ability to sense light could help the octopus to move or camouflage itself. While the team is still unsure of why an octopus's appendages can sense light, they are certainly sensory multitools. The ability of their arms to sense light, chemicals and touch is just another reason to appreciate these wonderful creatures.