Taste by touch is all in the suckers

An octopus can “taste by touch” thanks to the suction-cup-like suckers along each of its eight tentacles.

But how does that work? There have been a number of studies into the biomechanics of the process, but now a team at Harvard University in the US has taken a look at the molecular level.

In a paper published in the journal Cell, they describe how a novel family of sensors in the first layer of cells inside the suction cups has adapted to react and detect molecules that don’t dissolve well in water.

The researchers suggest that the sensors, called chemotactile receptors, use these molecules to help the animal figure out what it’s touching and whether that object is prey.

“We think because the molecules do not solubilise well, they could, for instance, be found on the surface of octopuses’ prey and [whatever the animals touch],” says senior author 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’.”

Bellono and colleagues first showed that the California two-spot octopus (Octopus bimaculoides) responds differently when its suckers touch a prey item rather than another object. The finding confirmed that their suckers have a taste-touch ability.

They then looked more closely at the suckers to identify the sensory cells involved. They discovered that the sucker did indeed include discrete populations of sensory cells.

But how do chemical signals received via those suckers work together with other physical stimuli to decide whether an octopus grabs what it touches?

While there’s much more to learn, the researchers say, their study shows that distinct chemotactile receptors form discrete ion channel complexes that detect specific signals and send them on to the nervous system.

Bellono suggests that this could serve as a signal filtering system suited to the octopus’ uniquely distributed nervous system.

“We also showed that separate and distinct chemo- and mechanosensory cells express specific receptors and exhibit discrete electrical activities to encode chemical and touch information, respectively,” he says.

“Our results demonstrate that the peripherally distributed octopus nervous system exhibits exceptional signal filtering properties that are mediated by highly specialised sensory receptors.”

And there is a bigger picture. “Not much is known about marine chemotactile behaviour 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 lead author Lena van Giesen. “These insights into protein evolution and signal coding go far beyond just cephalopods.”