Three Hearts and Most of Its Neurons in Its Arms: The Octopus as "Another Intelligence"

The video begins with the octopus's circulatory system. An octopus has two branchial hearts, each of which pumps blood to a gill. Once the blood picks up oxygen in the gills, a large central heart sends it out to the rest of the body.

The blood is blue because of a difference in the substance that carries oxygen. Humans transport oxygen using iron-based hemoglobin, but in octopuses and many other cephalopods, that role is played by copper-containing hemocyanin. The narrator notes that the mystery of an intelligence built on such a strange body has itself become a subject of scientific research today.

As background, the octopus is a mollusk, belonging to the same group as shellfish such as clams and snails, as well as squid and nautiluses. Cephalopods have very ancient origins, and octopuses are said to be a group that appeared relatively late in that lineage.

The video identifies abandoning the shell as a major turning point in their evolution. Modern squid and octopuses have either lost their shells or, where one remains, reduced it to something very small. This allowed them to squeeze into tight spaces and move more freely, and the narrator explains that it laid the foundation for the later evolution of their behavior and senses.

Indeed, apart from its beak, almost the entire body of an octopus is soft, with no bones or joints. Even so, it catches prey, slips into crevices, and carries out complex behaviors. The video draws a contrast: in humans, the skeleton determines the body's shape in advance, whereas in the octopus, neural control holds the soft body together moment by moment.

The first remarkable ability introduced is the sucker. An octopus's suckers are not merely attachment devices. They are equipped with numerous sensory receptor cells and can detect chemicals that dissolve poorly in water, sensing them as if tasting by touch. The octopus touches the world with its arms and identifies the things around it through that contact. For this reason, the narrator says, the arms are not just tools that receive commands from the brain but semi-autonomous information-processing organs that interact directly with the outside world.

Another sense that is hard for humans to imagine is the ability to sense light through the skin. This doesn't mean seeing a scene, but the skin may detect light and dark or localized light stimuli and may be involved in controlling camouflage. The video does not state this definitively, presenting it as a "possibility."

Octopuses can change not only their color but also their brightness, patterns, and even the texture of their skin. Their bodies function as an integrated whole of vision, skin sensitivity, muscle control, and local responses, and the narrator sees this as another clear sign that the octopus's intelligence is inseparable from its body.

Most octopuses reproduce in earnest only once in their lives. After laying eggs, the female guards them while eating almost nothing, and dies shortly after the young hatch. Males are also generally short-lived. The video sums up the octopus as an animal that uses up its life for reproduction.

The narrator foreshadows that this short lifespan connects to a mystery addressed later: "Why did such advanced intelligence evolve in an animal that isn't highly social?"

So how intelligent is the octopus? Cephalopod evolution is thought to have proceeded independently of the brain evolution of mammals and birds. Nevertheless, the video explains, similar endpoints emerge in areas such as learning, memory, problem-solving, and behavioral flexibility.

According to the narrator, octopus intelligence is an intelligence that evolved in a short-lived, largely solitary animal with a soft body and eight arms. Cephalopods have large brains yet are neither long-lived nor strongly social. For this reason, they are often discussed as an example that departs from the conventional picture of how intelligence evolves.

The octopus nervous system can be broadly divided into three parts: the central brain, the optic lobes, and the nervous system of the arms.

The central brain is arranged so that it encircles the esophagus, and it brings together many regions with different roles, making it a highly complex structure overall. The optic lobes support vision and are especially well developed in octopuses. Octopuses rely heavily on information coming in through their eyes, and vision is said to help them not only find prey but also explore their surroundings, escape predators, grasp space, and even camouflage themselves.

And what most sets octopus intelligence apart from human-like intelligence, the narrator says, is the nervous system of the arms. An octopus is said to have about 500 million neurons in total, and 60–70% of them are thought to be in the arms. The octopus is an animal with a large neural network not only in its brain but also in its arms.

As a region particularly important for understanding octopus intelligence, the video highlights the vertical lobe. The vertical lobe has long been thought to be deeply involved in learning and long-term memory. Changes in neural circuits are important for retaining what has been learned, and the vertical lobe is believed to play a major part in those changes. It is involved in short-term learning as well, but its connection to long-term memory is especially strong.

This makes it tempting to call the vertical lobe "the octopus version of the hippocampus," but the narrator cautions that it is not exactly the same thing. Even if the roles are similar, the structure of the brain itself is not the same. So, the narrator suggests, it is best thought of as an important center for learning and long-term memory that functions somewhat like a hippocampus.

What's more, in recent years even the wiring diagram of its neural circuits has begun to come into view. In 2023, a study traced part of the vertical lobe of a Mediterranean common octopus at the electron-microscope level, examining in detail which cells connect to which. What emerged, the video reports, is that while octopus learning and memory mechanisms share some features with those of other animals, the structure of the neural circuits themselves is quite distinctive.

In the octopus, the brain does not direct every part of the body in fine detail. A thick nerve runs through each arm, and the arm itself can process information to a certain extent. So rather than the brain issuing every command, the system is thought to work more like this: the brain gives general instructions, and the arms decide the fine movements on their own to some degree. The video sums this up by saying the octopus is a creature in which not only the brain but also the arms play a very important role.

Octopuses have long attracted attention for their learning and problem-solving abilities. The example the video gives is an experiment in which a live crab, a favorite food, is placed in a transparent jar with the lid closed, and researchers observe how the octopus gets it out. The octopus applies the results of trial and error to its next attempt, improving its method. The narrator frames this as an important result in that it shows the octopus can carry experience forward into its next actions.

However, the video adds a caveat that octopuses cannot do everything. While they are quite strong at problem-solving and learning, caution is still warranted regarding recognizing themselves in a mirror and higher-order metacognition. Even so, the behavioral flexibility octopuses display is exceptional. The video's view is that octopus cleverness is not confined to the head but shows up as the animal uses its body to act on its environment and find answers on the spot.

Does an animal that behaves so flexibly have something like emotions? The video distinguishes this as a somewhat different question from whether the octopus is smart. The question is whether states and behaviors that look like emotions at first glance are mere reflexes, or whether they are accompanied by subjective sensory experience, even if not to the same degree as in humans.

In research, the term "sentience" is sometimes used in connection with this question. The video explains that, for now, it can be understood roughly as meaning "whether something is a feeling being." According to recent assessments, there is said to be strong evidence that octopuses and squid have such sensory experiences.

So what kinds of emotions can octopuses plausibly be said to have? The narrator says it is relatively easier to make the case for states involving negative emotions such as pain, discomfort, and fear. The evidence presented is a 2021 study. Octopuses injected with acetic acid avoided the place where they had experienced pain and preferred a place where the pain had been relieved. In addition, they were seen rubbing the injected area as if attending to it, and it was confirmed that this behavior disappeared when a local anesthetic was applied. The video explains that this is widely cited as strong evidence suggesting pain with an emotional component, rather than a mere reflex.

Finally, the video asks whether octopuses are truly solitary animals. Octopuses are generally considered highly solitary, but recent research reportedly shows that this solitude is not absolute.

The example given is a high-density habitat of Sydney octopuses known as "Octopolis." Here, multiple individuals live in a cluster of closely spaced dens, encountering one another frequently, clashing, and in some cases even exhibiting behavior like throwing objects at each other. According to the narrator, Octopolis is less a cooperative society like a human city than a group in which interactions between octopuses arise as a result of gathering in a place with good resources and habitat conditions.

In fact, behavior that is the opposite of cooperation is better known. It has been reported that Sydney octopuses use jets of water to launch shells, silt, and algae, which often hit other octopuses. The video describes this as closer to "friction between neighbors" than to living harmoniously in a group.

On the other hand, the relationships are not purely hostile either. In Octopus laqueus, it has been reported that multiple individuals shared the same tank or dens in an experimental setting and showed social tolerance. However, this report reportedly also noted that while they do share, they often choose to be alone when given the option. The video's summary is that octopuses do not have strong sociality, but neither are they absolutely incapable of living anything but a solitary life.

The narrator says the uniqueness of octopus intelligence lies in the fact that, despite not being strongly social, octopuses display such complex interactions depending on the situation. The video concludes that simply calling the octopus "a smart animal" is not enough: it is another intelligence, one that arose from a design entirely different from humanity's.

As for the question foreshadowed at the beginning—"Why did advanced intelligence evolve in an animal that is short-lived and not highly social?"—the video does not offer a clear answer. What it presents goes only as far as positioning the octopus as an example that does not fit within the conventional explanatory framework of long lifespans and strong sociality.