Octopuses have three hearts, blue blood, and a freakishly strange nervous system like no other creature. They possess around 500 million neurons, over two-thirds of which are located within their arms and body. As such, they are sometimes said to have nine brains – a donut-shaped one in their head and eight other “mini-brains” located in each tentacle – leading some scientists to wonder whether the arms effectively have a “mind of their own” that can act independently from the central brain.
However, recent research suggests this isn't strictly true (though rest assured, the new insights show that their nervous system is still deeply unusual). The new study, reported in the journal Current Biology, has looked at the central nervous system of octopuses and revealed that their arms don't actually act independently from the centralized brain, but are, in fact, more connected than previously thought.
“This study makes it clear that octopus's arms don't behave totally independently from the centralized brain – there's information flow between the peripheral and central nervous system. Rather than talking about an octopus with nine brains, we're actually talking about an octopus with one brain and eight very clever arms,” lead author Dr Tamar Gutnick, an octopus researcher formerly at the Okinawa Institute of Science and Technology Graduate University in Japan, said in a statement.
To reveal this, the researchers looked at common Mediterranean octopuses to see if single arms were able to provide the brain with two different types of sensory information through a number of maze experiments.
The maze consisted of a Y-shaped pipe, in which the octopus can either put its tentacle down the right or left path to find a food reward. They appeared to explore the maze using fast movements, by pushing or unraveling their tentacle straight through the tube into the end box. If they got the correct tube then they would find the food, but if they entered the wrong tube, the food was blocked by a net and the scientists removed the maze.
The study showed that five out of the six octopuses eventually learned the correct direction to push or unroll their arm through the maze in order to get the food. Most crucially, they could successfully navigate the maze using arms that hadn’t been used before. This process of learning, the researchers say, shows that the tentacles require the central brain and they are not autonomously acting like an independent mind of their own.
“We conclude that although octopus arms have a great capacity to act independently, they are also subject to central control, allowing well-organized, purposeful behavior of the organism as a whole,” the study reads.
Nevertheless, there are still many mysteries that surround the octopus brain. For starters, it’s unclear how much information is communicated between all these different neutral structures and why their nervous system is structured in this way. More fundamentally, it's unclear why octopuses and other cephalopods are so intelligent. While scientists have some grasp of how a high level of intelligence has evolved among certain species, such as chimps or dolphins, most of their theories fall flat when applied to the curious world of cephalopods.
“The brain of octopuses is so different – it’s still a black box to us really,” concluded Dr Gutnick. “There’s so much more to learn.”