The Evolutionary Secrets of the Octopus: How Three Hearts Support Its Complex Life

The octopus is one of the world’s most amazing creatures, and nothing puts that into perspective more than the physiological differences between these fantastic sea-dwelling mollusks and the creatures we’re more familiar with. There are over 300 octopus species in the order Octopoda, and they’ve managed to carve out a niche in practically every major body of saltwater around the globe. The octopus has nine brains, which it uses to control and manipulate its eight distinct tentacles. The octopus’s hearts, of which there are three, reveal secrets about its evolutionary history while informing our understanding of how it manipulates its environment. Continue reading to discover the secrets contained in the octopus’s three hearts.

How the Heart Works

Not all animals have a heart, or hearts. Sea anemones and starfish get by without them, while jellyfish are the largest animals without a heart, which speaks to the necessity of this blood-pumping organ. The notion of a circulatory system driven by a centralized pump that delivers blood throughout the body is approximately 600 million years old.

Most animals have a single heart, but even those can vary significantly in terms of design. Bumblebees have an open circulatory system with a heart that runs the length of the body and simply spills blood over the organs. The exceedingly long neck of the giraffe requires an incredibly powerful heart with thick walls that is capable of pumping blood efficiently all the way up that long neck to its brain. The cheetah, the world’s fastest land mammal, also has an incredibly fast heart. Research indicates that a cheetah’s resting heart rate is approximately 120-170 beats per minute, and it can rise to 200-250 beats per minute during a chase.

Octopus Hearts

Octopuses and other cephalopods have three hearts. All three of these hearts are located within the animal’s head, with the systemic heart serving as the primary circulatory pump and two branchial hearts that assist. Functionally, this is not very different from how the heart works in other creatures, although in octopuses, the duties are divided among multiple hearts. The two branchial hearts pump blood through the octopus’s gills, where it becomes rich in oxygen, so that oxygen and nutrients can be distributed throughout the body. The blood completes its cycle when it returns to the systemic heart, where it is pressurized and sent back through the body.

The size and volume of an octopus’s extremities illustrate why these cephalopods require such specialized circulatory systems. The amount of energy required to keep nine separate brains oxygenated is another consideration. The nautilus is the only cephalopod not to have three hearts, and it’s both physiologically less complex and significantly more sedentary than the squid and the octopus.

Octopuses have two primary modes of navigation: crawling along the surface of the seafloor and using their tentacles and the current to propel themselves forward. Octopuses rely primarily on crawling because their systemic heart stops beating when they swim, making sustained swimming energetically costly. This inability to exert themselves for extended periods may have contributed to octopuses developing ambush hunting methods and environmental camouflage as a primary defense mechanism.

Why Do Octopuses Have Blue Blood?

The mystery behind an octopus’s blue blood is critical to understanding its need for three hearts. Human blood appears red when it leaves the body because of the presence of an iron-based protein called hemoglobin. Octopuses instead use a protein derived from copper known as hemocyanin. Hemocyanin is heavier than hemoglobin, and its unique chemical structure causes the blood to appear blue when exposed to oxygen. Octopuses are very efficient at using all the oxygen their hearts pump through their bodies, but this efficiency may be more a result of necessity than of an especially effective circulatory system. Hemocyanin is only about a quarter as effective as hemoglobin at transporting oxygen, thus necessitating three separate hearts with two specialized for circulation and one for processing.