Nicholas Bellono, en professor of molecular and cell biology at Harvard University, was concerned about his first octopus. “It is not trivial to have an octopus in the laboratory,” he says. They are clever creatures that require specific water conditions and diets, and have a penchant for performing detailed escapes. But those concerns did not match Bellano’s curiosity. “We just thought, ‘This animal is pretty crazy, so we just have to study it,'” he says.
The result of this curiosity is a paper published Thursday in Cell where Bellono’s laboratory reveals 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. “The squid̵
The new paper also shows that the signal does not have to travel all the way to the animal’s brain to be decoded. Instead, it is treated and traded by nerves distributed in the arms independent of the octopus’ central nervous system. The results help explain more about how octopuses sense and explore their surroundings, and about how their limbs work independently of stimuli.
“These are really exciting findings,” said Charles Derby, a professor of neurobiology and biology at Georgia State University who was not involved in the research. He says that when researchers find a new type of sensory cell, it’s a big deal. “Animals are cool because they’re really plastic in the evolutionary sense,” he says. This study helps to add to the big picture of how animals have evolved and adapted to their environment over time.
Bellono specializes in studying how animals adapt their sensory systems to survive in specific environments. In just two short years, he has brought about 30 species into his laboratory, including sharks, squid, jellyfish, photosynthetic sea snails and anemones. He likes to step into the animal room and marvel at the unique adaptations of each creature. And when it came to the squid, Bellono was particularly 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 change the type of touch it used and quickly knock on the surface. Previous studies had characterized this “taste by touch” behavior, but there were no studies on stimuli, cells, receptors, or neural processes involved in the process. So Bellono sat down to find out what sensory mechanisms could explain this unique behavior, and what molecules might be of interest to the squid.
Just defining what the sense of taste is and how it works for aquatic organisms can be counterintuitive for farmers. For those of us above the waterline, taste happens when soluble molecules – chemicals dissolved in liquids or fats – come in contact with receptors on the tongue. Insoluble molecules that are not dissolved and can flow through the air are felt through the olfactory neurons in the nose. But in water, the opposite is true. Soluble molecules flow easily through aquatic environments, while insoluble molecules – the things that do not dissolve – adhere to surfaces and must be physically touched to be detected. So for the squid, Bellono asks, “Is it just based on the molecule that has been discovered? Is it based on the organ? Is it based on the distance? ”
“In the case of the squid, it really does seem to be contact-dependent,” he concludes. To find these taste receptors, the researchers started by looking at cells in the places where the squid has the most contact with objects: its suction cups. The Harvard team was able to identify mechanoreceptors that respond to touch, but the team could not find any chemoreceptors that respond to chemical signals.