How Many Hearts Does an Octopus Have? Octopus Anatomy

Why does an octopus have three hearts?

The answer lies in its blood and its gills. Octopuses have a closed circulatory system, in which blood stays inside vessels, unlike the open systems of snails and clams, where blood washes loosely around the organs. Their blood carries oxygen with hemocyanin, a large copper-based protein dissolved in the fluid rather than packed into blood cells. This makes the blood thick and less efficient at carrying oxygen than ours, so it has to be pushed hard to move fast enough.

Pushing blood through the fine vessels of the gills costs a lot of pressure. If a single heart had to drive blood through the gills and then on around the body, the flow would be weak by the time it reached the arms. Fish manage with one heart because their blood carries oxygen efficiently and they need less pressure. Mammals solve the same problem with a double heart: the right side pumps blood to the lungs, and the left side pumps it to the body.

The octopus does something similar with separate organs. The two branchial hearts act like our right side, boosting blood into the gills, and the systemic heart acts like our left side, sending refreshed blood to the body.

What does each octopus heart do?

The branchial hearts (from the Greek word for gills) receive used blood returning from the body through large veins. Each squeezes that blood into its own gill, where it picks up oxygen from the water flowing through the mantle cavity. Attached to each branchial heart is a small appendage that helps filter the blood, part of the octopus’s kidney system.

The systemic heart is larger and has one muscular ventricle and two atria, one receiving blood from each gill. Three main arteries, called aortas, leave it. The biggest, the dorsal aorta, supplies the head, brain and arms, while two smaller ones serve the gut and the reproductive organ.

The three hearts beat in a steady sequence: the two branchial hearts contract together, followed by the systemic heart. The octopus also gets help from its blood vessels. Many of its veins have muscular walls that contract, helping to squeeze blood back toward the hearts.

Why the main heart stops when an octopus swims

To swim fast, an octopus fills its mantle with water and then contracts it hard, forcing a jet out of its funnel. All three hearts sit inside the mantle, and the pressure of this squeeze is so great that the systemic heart stops beating while the octopus jets. Laboratory studies on the common octopus have also recorded brief pauses in the heartbeat after a sudden shock or disturbance.

The octopus heart is not built for endurance. Its rate hardly rises with exercise, and its main way to deliver more blood is to pump a bigger volume with each beat, up to about three times the resting amount. Fast swimming therefore leaves the animal short of oxygen within moments.

That explains a familiar sight to divers: octopuses usually crawl across the seabed with their arms, and save jetting for short escapes from danger. Squid, which swim most of their lives, have a more streamlined body and fins that share the work.

Blue blood and high blood pressure

Octopus blood is almost colorless when it lacks oxygen and turns blue when it carries oxygen, because of the copper in hemocyanin. Being thick and carrying less oxygen than red blood, it needs high pressure, and blood pressure in octopuses can exceed 75 mmHg (10 kPa), very high for an invertebrate. The full story of how this blood works, and why it suits cold, low-oxygen water, is in why is octopus blood blue?

Other octopus anatomy facts

The three hearts are only one of many surprises inside an octopus. Here is the basic body plan of the common octopus, the best-studied species:

Because the esophagus runs straight through the brain, an octopus bites its food into small pieces with its beak before swallowing. The eyes are camera-like, much like ours, but have no blind spot, because the nerve fibers leave from the back of the retina. Balance organs called statocysts keep the slit-shaped pupils horizontal, whichever way the octopus is turned.

There is no skeleton at all, which is why a large octopus can squeeze through a gap little wider than its beak; see do octopuses have bones?

Do other animals have more than one heart?

Squid and cuttlefish, the octopus’s closest living relatives, have three hearts too. Most other animals manage with one, but some have several pumps. Earthworms have five pairs of muscular blood vessels near the front of the body, often called hearts, as explained in how many hearts do earthworms have? Hagfish have a main heart plus several smaller accessory pumps.

A popular piece of trivia says an octopus has three hearts, nine brains and blue blood. Two of those are true. The “nine brains” figure counts the central brain plus a nerve cord in each arm, but those arm cords are not brains in any usual sense, even though they let each arm react and grab on its own.