With a flexible mind, changeable skin and soft bodies fed by three hearts, octopuses can play all sorts of pranks. Their mastery of disguise can allow them to remain hidden while they discreetly explore their surroundings with limbs, each with its own mini-mind.
We can now have some idea of how this ability of touch works.
As their tentacles stretch across the seabed, exploring thousands of independently moving, finger-like suckers, it turns out that octopuses use their sense of taste as well as unique sensory cells to map their surroundings.
Molecular biologist Lena van Giesen and her colleagues at Harvard University have identified these chemosensory cells – cells that detect molecules similar to our smell and taste cells – in the skin of the suckers of a California two-point octopus (Octopus bimaculoides).
Chemotactyl cells with thin, branched ends can signal continuously (tonic arousal), but they depend on being close enough, like our tongue. Chemosensory cells can react to several odors, including chemicals in cephalopod ink and 'warning' chemicals released by potentially toxic prey.
An attacking California two-point octopus. (Peter Kilian)
In the skin of the suckers, the team also found the expected and more familiar mechanosensory cells with short, branched ends. These cells are triggered only during the start of the contact before the signal ends (phase triggering).
This type of signaling allows octopuses to determine whether they are touching inanimate objects (where the signal stops when contact is stationary) or wriggling about the victim, where the signal will be triggered again in response to the loss and restoration of contact.
“We find that octopuses explore their environment using stereotypical touch movements that change markedly upon contact with different [molecules that trigger chemotactyl receptors],” the researchers explain in their paper.
They identified these skills by observing octopuses, performing tests, and studying which proteins are expressed by genes in specific suction cells. This technique is called transcriptomics, and it allows researchers to see what a cell is doing by analyzing which proteins are being actively used within it.
The team found that some of the chemotactyl cells were highly activated in response to fish and crab extracts. But they suggest that, in addition to detecting prey, this ability to taste by touch can also cause a rapid retreat from repulsive odors that indicate danger. They also observed how octopus ink blocks the ability of the limbs to taste.
“Our results were unexpected because water-based chemosensitivity has long been associated with remote signaling through water through water-soluble chemicals,” Bellono said. “Our research shows that octopuses, and possibly other aquatic animals, can also detect poorly soluble molecules in a contact-dependent manner.”
The octopus is tasting the cup. (Lena van Giesen).
Chemotactyl receptor genes were found in three different octopus species the team studied, but University of California biologist Rebecca Tarvin, who was not involved in the study, explains that other cephalopods, such as squid, don't seem to use their suckers for taste.
“We're really interested in how this unique sensorimotor system evolved in other cephalopods,” Bellono said, explaining that there are many questions about its evolution, physiology, and use.
Although they have carefully examined only a few genes associated with specialized taste cells, there are hints of a large number of cells in the rest of the genome, with nearly 100 genes associated with sensation yet to be characterized.
Bellono said that the mini-brain in the octopus's tentacles must have the exceptional ability to filter information from a multitude of highly specialized receptors. This may help explain why two-thirds of an octopus's neurons reside in its tentacles.
So, octopuses, in fact, have eight smart and dexterous languages that allow them to taste food, regardless of their main body, in the dark depths of the ocean. How much weirder can life be?
This study was published in Cell.
