Octopus arms have minds of their own.
Each of these eight supple yet powerful limbs can explore the seafloor in search of prey, snatching crabs from hiding spots without direction from the octopus’ brain. But how each arm can tell what it’s grasping has remained a mystery.
Now, researchers have identified specialized cells not seen in other animals that allow octopuses to “taste” with their arms. Embedded in the suckers, these cells enable the arms to do double duty of touch and taste by detecting chemicals produced by many aquatic creatures. This may help an arm quickly distinguish food from rocks or poisonous prey, Harvard University molecular biologist Nicholas Bellono and his colleagues report online October 29 in Cell.
The findings provide another clue about the unique evolutionary path octopuses have taken toward intelligence. Instead of being concentrated in the brain, two-thirds of the nerve cells in an octopus are distributed among the arms, allowing the flexible appendages to operate semi-independently (SN: 4/16/15).
“There was a huge gap in knowledge of how octopus [arms] actually collect information about their environment,” says Tamar Gutnick, a neurobiologist who studies octopuses at Hebrew University of Jerusalem who was not involved in the study. “We’ve known that [octopuses] taste by touch, but knowing it and understanding how it’s actually working is a very different thing.”
Working out the specifics of how arms sense and process information is crucial for understanding octopus intelligence, she says. “It’s really exciting to see someone taking a comprehensive 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 California two-spot octopus (Octopus bimaculoides). Detailed imaging identified what appeared to be sensory cells, some with fine branched endings, at the surface of suckers. 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 fish extract contained receptors, proteins that detect specific stimuli, unlike any seen in other animals.
To study how these “chemotactile” receptors work, the researchers inserted them into human and frog cells in the lab using genetic tools and then exposed them to a variety of chemical compounds an octopus might normally encounter. Only one class of molecules, insoluble terpenoids, elicited a response from the cells. Terpenoids, natural compounds found in the bodies of many marine creatures, are thought to be used in defense by some animals.
Initially the finding struck Bellono as somewhat odd, since these compounds don’t dissolve well. “For aquatic sensation, we usually think of molecules that diffuse well through water,” he says, similar to how humans smell compounds that diffuse through air. But then Bellono realized that this might make sense given how octopuses move through the world “by touching everything.”
Specialized terpenoid detectors might cue an octopus to quickly grasp something it touches lest it swim away, or withdraw and keep searching.
This played out in the lab, where octopuses in tanks explored normal surfaces without terpenoids with broad, sweeping arm movements. But once an arm touched a surface infused with different terpenoids it stopped, either quickly tapping the spot and moving on, or immediately withdrawing and avoiding that part of the tank.
While it’s not clear just what these behaviors mean, they confirm that octopuses do use these receptors to sense chemicals by touch. “We equate it to taste by touch just so that we can sort of understand what it might mean to the octopus, but it’s very different than our taste,” Bellono says.
His lab is already working on identifying other compounds detected by these sensors, as well as investigating how the receptors might be tuned to respond to different sorts of stimuli depending on the context, such as how hungry the octopus is.