Nicholas Bellono, a professor of molecular and cell biology at Harvard University, was worried about his first octopus. “It’s not trivial to have an octopus in the lab,” he says. They’re wily creatures who require specific water conditions and diets, and have a penchant for carrying out elaborate escapes. But those worries were no match for Bellono’s curiosity. “We just thought, ‘This animal is pretty crazy, so we should just study that,’” he says.
The result of that curiosity is a paper published Thursday in Cell in which Bellono’s lab reveals yet another very cool thing about these invertebrates: a unique type of receptor in the tissue of their suction cups that can taste surfaces by touching them. “Octopuses’ arms are like big tongues that are probing around and making contact,” Bellono says. As they brush their arms across surfaces, molecules on those surfaces bind to receptors in the suckers, which send signals to a long axial nerve running the length of the octopus’s limb.
The new paper also shows that the signal doesn’t have to travel all the way to the animal’s brain to be decoded. Instead, it’s processed and acted on by nerves distributed in the arms, independent from the octopus’s central nervous system. The findings help explain more about how the cephalopods sense and explore their environments and about how their limbs act on stimuli independently.
“This is a really exciting finding,” says Charles Derby, a professor of neurobiology and biology at Georgia State University, who was not involved in the research. He says any time scientists find a new type of sensing cell, it’s a big deal. “Animals are cool in that they are really plastic, in an evolutionary sense,” he says. This study helps add to the big picture of how animals have evolved and adapted to their surroundings over time.
Bellono specializes in researching how animals adapt their sensory systems to survive in particular environments. In just two short years, he’s brought around 30 species into his lab, including sharks, squid, jellyfish, photosynthetic sea slugs, and anemones. He likes to step into the animal room and marvel at each creature’s unique adaptations. And when it came to the octopus, Bellono was especially interested in its limbs. The creature would explore surfaces by running its arms over objects, and sometimes, when specific chemicals were present, an octopus would alter the type of touch it was using, quickly tapping the surface. Previous studies had characterized this “taste by touch” behavior, but there was no research about the stimuli, cells, receptors, or neural processing involved in the process. So Bellono set out to find what sensory mechanisms might explain this unique behavior, and what molecules might be interesting to the octopus.
Just defining what the sense of taste is and how it works for aquatic organisms can be counterintuitive for land dwellers. For those of us above the water line, taste happens when soluble molecules—chemicals dissolved in liquids or fats—come into contact with receptors on the tongue. Insoluble molecules, which aren’t dissolved and can be floating through the air, are sensed through the olfactory neurons in the nose. But in water the opposite is true. Soluble molecules float easily through aquatic environments, while insoluble molecules—the stuff that won’t dissolve—stick to surfaces and have to be physically touched in order to be sensed. So for the octopus, Bellono asks, “Is it just based on the molecule that’s detected? Is it based on the organ? Is it based on the distance?”
“In the case of the octopus, it really seems to be contact-dependent,” he concludes. To find these taste receptors, the researchers started by looking at cells in the places where the octopus makes most contact with objects: its suction cups. The Harvard team was able to identify mechanoreceptors, which respond to touch, but the team couldn’t find any chemoreceptors, which react to chemical signals.