An octopus has three hearts, blue blood, and can edit its own genes

The octopus is built so differently from familiar animals that it can seem almost designed to break the rules of biology. It pumps blue blood through not one heart but three, carries oxygen with copper rather than iron, and can rewrite its own genetic instructions on the fly to cope with a changing environment. These are not exaggerations but well-documented features of an animal whose evolutionary path split from that of vertebrates more than half a billion years ago.

Together these traits describe a body adapted to the particular demands of ocean life, especially the cold, oxygen-poor waters many octopuses inhabit. Each oddity turns out to solve a real problem, and understanding how reveals why the cephalopods are so often described as the closest thing on Earth to intelligent aliens.

Three hearts and a divided circulation

An octopus has three hearts that share the work of moving blood. Two of them, called branchial hearts, sit at the base of the gills and pump blood through them to pick up oxygen. The third, the systemic heart, then drives that oxygenated blood out to the rest of the body. The overview of the octopus describes this arrangement as central to how the animal keeps its tissues supplied in demanding conditions.

There is a curious consequence to this design. The systemic heart tends to stop beating when the octopus swims by jetting water, which is one reason many octopuses prefer to crawl along the seafloor. Sustained swimming leaves them exhausted, so they conserve energy by moving on their arms instead.

Blue blood built on copper

The octopus’s blood is blue because it carries oxygen using a copper-based protein called hemocyanin, rather than the iron-based hemoglobin that makes human blood red. Hemocyanin is dissolved directly in the fluid, called hemolymph, instead of being packed inside cells, and it shifts from colorless to blue when it binds oxygen. As Smithsonian magazine notes, this copper-based system is comparatively good at capturing oxygen in cold water where the gas is scarce.

That advantage comes with a vulnerability. Because the whole system depends on delicate blood chemistry, octopuses are highly sensitive to acidity, and if the surrounding water becomes too acidic, they can struggle to circulate enough oxygen to survive.

Rewriting genetic instructions through RNA editing

Perhaps the strangest ability of octopuses and their relatives is extensive RNA editing. Genes made of DNA are copied into RNA, which the cell then reads to build proteins. Most animals follow that copy faithfully, but octopuses can chemically edit their RNA messages before they are used, changing the proteins that get made without altering the underlying DNA. This gives them a way to fine-tune their biology on shorter timescales than ordinary evolution allows.

Research has shown that octopuses use this editing especially in their nervous systems, and that they can adjust it in response to temperature, tweaking nerve proteins to keep them working properly in colder water. It is a form of biological flexibility that comes at a cost, since heavy reliance on editing may slow the pace of conventional genetic evolution.

A distributed nervous system across eight arms

The octopus’s intelligence is spread out in a way no vertebrate brain is. It has a central brain, but the majority of its neurons, hundreds of millions in total, are distributed through its eight arms. Each arm contains a large share of nerve cells and can carry out complex movements, sense its surroundings, and even react somewhat independently of the central brain.

This decentralized design means an octopus arm can explore, grasp, and taste on its own while the animal attends to something else. It is a fundamentally different model of a nervous system, one in which thinking is not confined to a single head but woven through the whole body.

Why the octopus seems so alien

What ties these features together is deep evolutionary distance. The lineage leading to octopuses diverged from the lineage leading to vertebrates well over 500 million years ago, so traits such as large brains, sophisticated behavior, and problem-solving evolved entirely independently. The result is a highly capable animal whose solutions to the basic challenges of life, breathing, circulating blood, adapting to cold, look nothing like the ones familiar from mammals. The three hearts, the copper-blue blood, and the self-editing genes are not tricks but the coherent toolkit of one of evolution’s most independent experiments in complex life.

Camouflage without the ability to see color

Another of the octopus’s paradoxes lies in its skin. The animal can transform its color and even its texture in a fraction of a second, blending into rock, coral, or sand with uncanny precision, using specialized pigment cells and muscles to reshape its appearance. Yet the eyes of most octopuses are thought to be color-blind, detecting brightness but not hue in the way human eyes do. How a creature that may not perceive color can match the colors of its surroundings so convincingly remains an open scientific puzzle. Some researchers have proposed that the skin itself contains light-sensitive proteins, allowing the body to detect aspects of the surrounding light independently of the eyes. Whatever the mechanism, the octopus’s disappearing act is one more feature that sets it apart as a genuinely alien intelligence in the sea.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

More from Morning Overview