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

The octopus is, by almost any measure, one of the most biologically improbable animals on Earth. It shares a common ancestor with humans, but that ancestor lived more than 750 million years ago — before complex nervous systems, before bilateral body plans, before the emergence of most of the biological features we associate with higher intelligence. In the vast expanse of time since that common ancestor, the octopus and human evolutionary lines have been developing independently, and what the octopus line produced is almost entirely unlike what produced us.

Consider the cardiovascular system. Octopuses have three hearts. Two branchial hearts — one positioned beside each gill — pump deoxygenated blood through the gills, where it picks up oxygen from the surrounding water. The third, the systemic heart, receives this oxygenated blood from both gill hearts and pumps it through the rest of the body. When an octopus moves by jet propulsion — expelling water forcefully through its siphon — the systemic heart actually stops beating temporarily under the muscular demands of propulsion, which is one reason octopuses typically prefer crawling to jetting: sustained jet propulsion is physiologically exhausting.

Blue Blood and Copper Chemistry

Octopus blood is blue. This is not a quirk or a trick of light — it reflects a fundamental difference in how octopus blood carries oxygen. Human blood uses hemoglobin, an iron-based protein that binds oxygen molecules and gives blood its characteristic red color. Octopus blood uses hemocyanin, a copper-based protein that serves the same oxygen-transport function but contains copper atoms where hemoglobin contains iron. When hemocyanin binds oxygen, the copper-oxygen complex produces a blue color rather than red.

Hemocyanin is less efficient than hemoglobin at transporting oxygen under normal conditions, but it performs substantially better at low temperatures and low oxygen partial pressures — the conditions found in cold, deep ocean water. Octopuses are well-adapted to cold environments, and their copper-based blood chemistry is one of the reasons. The tradeoff is that they cannot sustain high-intensity activity for prolonged periods; their oxygen delivery system, efficient in cold water but less suited to aerobic exertion, limits their endurance.

The Nine Brains

The octopus's neural organization is as remarkable as its cardiovascular system. It has approximately 500 million neurons — comparable to some mammalian species — but only about a third of these are located in the central brain. The remaining two-thirds are distributed throughout the eight arms, with each arm containing a semi-autonomous nerve cluster capable of generating basic movements and responses without instructions from the central brain.

This distributed intelligence means that an octopus arm, severed from the body, will continue to respond to stimuli for up to an hour after detachment — it retains the neural machinery to react and move independently. In an intact octopus, the central brain sets high-level goals and intentions while the arm-brains handle the detailed execution: the central brain "decides" to reach for prey, but each arm calculates its own path and movement independently. The coordination is seamless to an observer but reflects genuinely distributed computation rather than centralized control.

Intelligence Without a Vertebrate Template

What makes octopus cognition particularly interesting to neuroscientists is that it evolved completely independently from vertebrate intelligence. The problem-solving abilities octopuses demonstrate in laboratory settings — opening jars, navigating mazes, recognizing individual human faces, using tools — emerged from an entirely different neural architecture than the one that produced comparable abilities in mammals and birds.

This independent evolution of intelligence suggests that the development of sophisticated cognition is not dependent on any specific biological substrate — that the vertebrate brain is not the only solution to the problem of building a mind capable of flexible, adaptive behavior. Studying how octopus cognition works, and how it differs from vertebrate cognition despite producing some similar outcomes, is one of the more productive approaches neuroscience has for understanding what intelligence fundamentally is, as opposed to how one particular lineage happened to build it.