Octopuses are intelligent creatures with incredible dexterity, capable of probing under debris for starfish and crabs, or even throwing rocks at their rivals. What's more, octopuses perform many of these feats in the dark, such as searching blindly for edible animals under rocks and in corals. Without vision, how can they distinguish mussels from stones, crabs from corals? The answer lies in their incredible arms, which are both delicate and powerful. These boneless and flexible appendages are muscular enough to lift rocks and strangle their prey, but incredibly, can also taste things. They have taste buds on their arms and suckers, as if their arms were eight long tongues. But as Roger Hanlon of the Marine Biological Laboratory, USA, along with a team of scientists from across the USA recently reported, taste is not necessary. Some octopuses can choose their next meal just by touch.
To test how California two-spot octopuses (Octopus bimaculoides) sense things with their arms, the scientists created an artificial rock dome with tiny armholes. They placed crabs in the rock dome, and octopuses learned to feel inside the dark dome to capture their prey without being able to see it. Over time, the researchers started adding 3D-printed cubes, to see how octopuses explored a new object with their arms. They recorded the octopuses with cameras, and carefully analyzed their arm movements. The octopuses were coordinated. After reaching inside, they swept their arms to scan the dome. Once they found a cube, they sometimes explored it by gingerly tapping it with the tip of their arms. Other times they used more of their arms, wrapping around the object in a spiral to get a more stable grip and perhaps try to pull the object out of the dome. And often, they would use multiple arms to handle the plastic model – up to seven at once. So far, so good – these experiments showed that the octopuses were getting lots of information with just their sense of touch, and it was enough to manipulate the objects skillfully.
After several days, Hanlon's group started introducing other 3D-printed objects: in addition to cubes, they added rocks and life-like plastic models of prey such as sea stars, crabs and mussels. These were effectively plastic toys, each made of the same resin as the cubes, and had no distinct taste or scent to identify them as food. Could an octopus's sense of touch be enough to differentiate food from rocks? It turns out that the answer depends on age. Young octopuses (6 to 9 months old) could clearly tell the objects apart even without eyesight or smell. They spent much less time handling the cubes they were used to; they spent longer fiddling with the plastic crabs, which mimicked their food. But older octopuses (18–20 months) could not distinguish plastic crabs from plastic rocks at all. Apparently, age dulls their sense of touch, meaning that older octopuses likely rely more on taste or smell to find their food.
It's incredible how much information these octopuses are integrating at once – multiple arms, each able to bend, stretch and twist at countless points. To manage all this sensory input, more than half of an octopus's neurons are in their arms. Complicated as it may be, discovering how octopuses process their sense of touch could help engineers design robotic replicas of their soft, dextrous arms, allowing robots to detect their environment with their limbs. Engineers and scientists are already working on this science fiction breakthrough – building robots for underwater and space exploration – but have yet to perfect something that even juvenile octopuses can do: ‘see’ using touch.