Three Hearts, Nine Brains, Blue Blood: The Extraordinary Biology of the Octopus

Three Hearts and Why the Arrangement Makes Sense

An octopus does not have one heart — it has three, each performing a distinct function. Two branchial hearts, located near each of the octopus's two gills, pump deoxygenated blood through the gills where it picks up oxygen. A single systemic heart then receives the newly oxygenated blood and pumps it to the rest of the body's tissues and organs.

The reason for this arrangement relates to the efficiency challenges of the octopus's circulatory system. Octopus blood uses hemocyanin rather than hemoglobin to carry oxygen. Where hemoglobin is an iron-containing compound that gives human blood its red color, hemocyanin is copper-based — and it is blue when oxygenated. Hemocyanin is less efficient at carrying oxygen than hemoglobin, particularly in warm conditions, which is one reason octopuses typically prefer cold ocean waters. The three-heart system compensates for hemocyanin's lower oxygen transport capacity by providing dedicated pumping power for the respiratory exchange.

The systemic heart stops beating when an octopus swims — which is why octopuses prefer to crawl rather than swim. Active swimming is genuinely exhausting for them because their primary oxygen-delivery system shuts down during it.

Nine Brains and the Distributed Intelligence Model

The description of nine brains is slightly metaphorical but fundamentally accurate. Octopuses have one central brain located in their head that processes information and makes higher-level decisions. But each of the eight arms contains a ganglion — a dense cluster of neurons — that controls that arm's movement semi-independently of the central brain.

This distributed nervous system means that an octopus arm can make its own movement decisions without waiting for instructions from the central brain. If an arm is exploring a crevice or manipulating an object, the arm's own neural cluster processes tactile information locally and generates responses. The central brain provides general direction; the arm neurons handle local execution. Approximately two-thirds of an octopus's roughly 500 million neurons are located in the arms rather than the central brain.

This architecture allows an octopus to do multiple complex things simultaneously — solve a problem with one arm while using others to manipulate prey, anchor itself, and change the texture of its skin — in a way that a centralized neural system would struggle to coordinate.

Blue Blood and Copper Chemistry

The blue color of octopus blood is not a metaphor or a misidentification — it is genuinely, visibly blue when oxygenated. Hemocyanin, the copper-containing protein that performs the same oxygen transport role as hemoglobin in vertebrate blood, produces this color through the same principle as hemoglobin's red: the protein changes its light-absorption characteristics when it binds oxygen.

Hemocyanin evolved independently from hemoglobin in a different branch of the tree of life. It is found not only in cephalopods (octopuses, squid, cuttlefish) but also in some crustaceans, spiders, and horseshoe crabs — a diverse group united by their evolutionary history rather than their current appearance. The convergent evolution of copper-based oxygen transport in such different lineages is a fascinating example of how similar chemical solutions can arise from different evolutionary starting points.

What Evolution Tells Us Through the Octopus

The octopus's extraordinary body plan is the product of an evolutionary lineage that separated from our own hundreds of millions of years ago, before the Cambrian explosion. Yet octopuses demonstrate intelligence, problem-solving ability, and behavioral flexibility that rivals many vertebrates. They can open jars, navigate mazes, recognize individual human faces, and demonstrate what appears to be something like play behavior.

The existence of intelligence in an animal with such a radically different body plan — no bones, distributed nervous system, completely alien physiology — suggests that intelligence is not narrowly dependent on a specific biological architecture. It can emerge through multiple different arrangements, given sufficient selection pressure and sufficient neural complexity.