Octopus arms have their own minds.
Each of these eight flexible but powerful limbs can explore the bottom of the sea in search of prey, snatching crab from hiding without direction from the octopus brain. But how each arm can tell what it is grabbing remains a mystery.
Now, researchers have identified specialized cells that have not been seen in other animals that allow octopuses to “test” with their arms. Built into the suction cups, these cells allow the arms to do the dual duty of touch and taste when detecting chemicals produced by many aquatic creatures. This can help an arm quickly distinguish food from rocks or poisonous prey, Harvard University molecular biologist Nicholas Bellono and colleagues reported online in Cell.
The findings provide another clue as to the unique evolutionary path octopuses have taken toward intelligence. Instead of concentrating in the brain, two-thirds of an octopus’s nerve cells are distributed between the arms, allowing the flexible appendages to operate semi-independently (SN: 4/16/15).
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“There was a huge gap in the knowledge of how the octopus (the arms) actually collects information about their environment,” says Tamar Gutnick, a neurobiologist studying octopus at the Hebrew University of Jerusalem who did not participate in the study. "We know that (octopuses) know by touch, but knowing it and understanding how it really works is a very different thing."
She states that it is crucial to determine how the arms perceive and process information to understand the octopus ’intelligence. “It’s really exciting to see someone taking a full look at the cell types involved” and how they work.
Bellono and his colleagues weren’t sure what they would find when they took a close look at the arms of a two-spotted California octopus (Octopus bimaculoides). Detailed images identified what appeared to be sensory cells, some with thin branched ends, on the surface of the suction cups. The researchers isolated the cells and tested their response to a variety of stimuli, such as fish extract and pressure. One class of cells turned out to be similar to those that detect touch in a variety of animals. But the cells that responded to the fish extract contained receptors, proteins that detect specific stimuli, unlike any other animal.
To study how these “chemotactile” receptors work, researchers inserted them into human and frog cells in the laboratory using genetic tools and then exposed them to a variety of chemical compounds that an octopus could normally find. Only one class of molecules, the insoluble terpenoids, elicited a response from the cells. Terpenoids, natural compounds found in the bodies of many sea creatures, are believed to be used in defense by some animals.
Initially, the finding seemed to Bellono somewhat strange, as these compounds do not dissolve well. “For the aquatic sensation, we usually think of molecules that diffuse well through water,” he says, similar to how humans smell compounds that diffuse through the air. But then Bellono realized that this could make sense given how octopuses move around the world "touching everything."
Specialized terpenoid detectors can make an octopus to quickly catch something it touches so it doesn’t swim or retreat and keep looking.
This was developed in the laboratory, where tank octopuses explored normal terpenoid-free surfaces with wide, wide arm movements. But once an arm touched a surface infused with different terpenoids it stopped, quickly touching the point and moving forward, or withdrawing immediately and avoiding that part of the tank.
While it’s not clear just what these behaviors mean, they confirm that octopuses use these receptors to detect chemicals by touch. “We match it to taste to the touch just so we can understand what it can mean to octopus, but it’s very different to our taste,” Bellono says.
His lab is already working on identifying other compounds detected by these sensors, as well as researching how receptors can be tuned to respond to different types of stimuli depending on the context, such as octopus hunger.