Scientists discover how octopuses can taste with their tentacles

Researchers at Harvard University are sharing intriguing new details about how octopuses can “taste” by using their tentacles to touch objects along the seafloor. The scientists investigated the nervous system of octopuses on a molecular level to gain a better understanding of how sensors located within their suction-cup covered tentacles actually work. 

The experts have identified a previously unknown family of sensory cells inside the suction cups that have evolved to detect molecules that do not dissolve well in water. The sensors, called chemotactile receptors, rely on the poorly soluble molecules to determine what type of object the suckers are touching and whether or not it is prey.

“We think because the molecules do not solubilize well, they could, for instance, be found on the surface of octopuses’ prey and whatever the animals touch,” said study senior author Professor Nicholas Bellono. “So, when the octopus touches a rock versus a crab, now its arm knows, ‘OK, I’m touching a crab [because] I know there’s not only touch but there’s also this sort of taste.'”

The researchers also found variation in the types of objects that triggered a response from the receptors, as well as in the signals that were transmitted from the receptors to the nervous system.

“We think that this is important because it could facilitate complexity in what the octopus senses and also how it can process a range of signals using its semi-autonomous arm nervous system to produce complex behaviors,” explained Professor Bellono.

Study co-author Peter B. Kilian said that the strategies used by octopuses to solve problems in their environment are unique to them, which inspires a great deal of interest from both scientists and non-scientists. “People are drawn to octopuses and other cephalopods because they are wildly different from most other animals.”

The team set out to determine how the receptors are able to sense chemicals and detect signals in what they touch, such as a tentacle around a snail. 

Octopus arms contain about two-thirds of the body’s neurons, yet they can partially operate independently from the brain. This is why, if an octopus arm is severed, it can still reach for, identify, and grasp items.

When the experts conducted experiments to pinpoint which cells in the suckers are responsible for detecting objects, they were surprised to find that only the poorly soluble molecules activated the receptors.

In the lab, the researchers confirmed that the only odorants that activated the chemotactile receptors were a non-dissolving class of naturally occurring chemicals known as terpenoid molecules.

“The octopus was highly responsive to only the part of the floor that had the molecule infused,” said Professor Bellono. This led the researchers to believe that the receptors they identified pick up on these types of molecules and help the octopus distinguish what it’s touching. “With the semi-autonomous nervous system, it can quickly make this decision: ‘Do I contract and grab this crab or keep searching?'”

According to the study authors, further research is needed to explore a great number of natural compounds that could also stimulate these receptors to control complex behaviors. “We’re now trying to look at other natural molecules that these animals might detect,” said Professor Bellono.

The researchers believe the findings can help uncover similar receptor systems in other cephalopods. They hope to determine how these systems work on a molecular level and how they evolved.

“Not much is known about marine chemotactile behavior and with this receptor family as a model system, we can now study which signals are important for the animal and how they can be encoded,” said study lead author Lena van Giesen. “These insights into protein evolution and signal coding go far beyond just cephalopods.”

The study is published in the journal Cell.

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By Chrissy Sexton, Earth.com Staff Writer