Tasting by touch, the unique way octopuses explore the world

These suckers on an octopus’s tentacle are key to its exploration of its environment. Image credit: ArtTower via Pixabay

Imagine you could taste the things your hands touch. You reach out to grab a slice of apple and can already feel its sweetness before it even touches your mouth. Science fiction? Not for the peculiar creature that is the octopus. This invertebrate is able to touch as well as taste its environment through its suckers. A team of researchers from Harvard University has studied the molecular mechanisms behind this unusual “taste-touch” sensory system, and how it affects the way octopuses explore their environment.

The Harvard team, led by assistant professor Nicholas Bellono, studied the ‘suckers of the California two-spot octopus, and identified sensory cells inside the suction cups. Some were responsible for mediating touch signals and others for taste. In particular, the team uncovered a family of sensors called chemotactile receptors that are activated by hydrophobic molecules. As these molecules don’t mix well with water, they can’t travel long distances in the ocean to be detected from afar.

Among other molecules, their suckers allowed them to taste terpenoids, a group of molecules produced by many marine invertebrates that are insoluble in water. These creatures are prey of choice for the octopus. As terpenoids are often used as a defense mechanism by invertebrates, researchers speculated that detecting these molecules could help the octopus sort between good prey and toxic ones. To investigate this theory, researchers studied changes in octopus behaviour following exposure to these chemicals. After terpenoids were sprayed on the tank floor, the octopuses displayed avoidance behaviours of these areas, suggesting that they do help octopuses identify toxic prey.

As the octopus explores its environment by probing with its arms, a “taste-touch” system is more effective to find prey.

To understand how this information is transmitted, we need to take a look at the octopus’s peculiar nervous system. The octopus belongs to a class of species called Cephalopoda. Cephalopods’ nervous systems have evolved separately from other large-brained species, so show some striking differences. Perhaps most curiously, even though octopuses display a central brain, about two-third of their neurons are located in their arms.

The Octopus’s nervous system is distributed along its body, particularly within its eight arms
Image credit:  OpenStax via Wikimedia, CC BY 4.0

Each arm is considered to be a semi-independent nervous system of its own, capable of performing tasks such as touching and grasping without input from the central brain. The information detected by the sensory cells in the suckers is transmitted directly to the neural cells in the arm of the animal, generating a response there without having to transit through the central brain. This enables the arm to react to itsenvironment more quickly. Because of this independence of action, an octopus arm is still able to grasp objects for a short time after being severed. 

These sensors give the octopus critical information about their environment. Hydrophilic molecules can travel far in water, and allow a predator to detect a prey from a distance. However, that is not the case for poorly soluble molecules like terpenoids: they require direct contact to be detected. As the octopus explores its environment by probing with its arms, a “taste-touch” system is more effective to find prey. “We think because the molecules do not solubilize well, they could, for instance, be found on the surface of octopuses’ prey” explains Bellono.

Sensory cells in the sucker also respond to many other stimuli. For example, you might have wondered how the octopus’s long, flexible arms never get tangled with one another. It is because octopuses are able to ‘taste’ their own skin as well as other octopuses’. Detecting this ‘taste’ sends a signal to the sucker to not grab the object, thus keeping the arms from getting tangled.

This exciting discovery is a firm grounding for more research, as many questions arise from these findings. For example, the team aims to find out if other cephalopods, such as the squid, display similar sensory mechanisms.

It also highlights the variety that exists in the way species explore and apprehend their environment.  For us, a species that tends to value sight above all other senses, the octopus’s world, discovered and explored through touch and taste, would truly feel alien.

Written by Marie Poirot and edited by Ailie McWhinnie.

Marie’s thoughts… I have always been fascinated by the marine world and the species you can find in the ocean. When you think about it, they evolved in an environment that is truly alien to us. As they are not subjected to gravity the same way we are, they get around in a three-dimensional world that’s often cold and dark with a high pressure. This has led to some extreme adaptation and the appearance of the most bizarre species, and there is still so much to discover. It is all the more true to say that we know more about the surface of the moon than the depth of our oceans.

Marie is a former Science Communication and Public Engagement student with a background in cell and molecular biology. Find her on LinkedIn @Marie Poirot.

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