The idea of an animal possessing seven hearts captures a fascination with the variety of life on Earth. While the number seven is a specific biological inaccuracy, the curiosity points toward creatures that have evolved complex, multi-heart circulatory systems to meet demanding physiological needs. The most compelling example of such an adaptation is found in a group of marine invertebrates whose anatomy allows for a high-energy lifestyle beneath the waves. This complex system is far removed from the single-pump organ common to mammals.
Setting the Record Straight on the Number of Hearts
The specific notion that any animal has seven hearts is a common biological misconception. No known animal species possesses precisely seven true hearts. The animal most often associated with having a complex, multi-heart system is the cephalopod, a class of marine mollusks that includes the octopus, squid, and cuttlefish. This group of animals, known for their intelligence and active predatory behavior, utilize a total of three hearts to manage their entire circulatory function.
The three-heart arrangement is a defining feature of the cephalopod circulatory system, not a redundancy. This specific number—one systemic and two branchial hearts—is a necessity driven by the animal’s body chemistry and high metabolic demands. Understanding the function of this trio of pumps is the first step toward appreciating the physiological complexity of these invertebrates.
The Unique Anatomy of the Cephalopod Circulatory System
The cephalopod circulatory system is a closed network, similar to that of vertebrates, where blood remains contained within vessels. This design supports a much higher metabolic rate than the open circulatory systems found in many other invertebrates. The three hearts are functionally specialized and divided into two distinct types: the two branchial hearts and the single systemic heart.
The two branchial hearts are situated at the base of each gill. Their function is to receive deoxygenated blood returning from the body and push it through the capillaries of the gills, where gas exchange occurs and the blood is oxygenated. This arrangement is necessary because passing blood through the restrictive gill structure causes a significant drop in blood pressure.
Once the blood is oxygenated, it flows into the single, larger systemic heart, located centrally in the mantle cavity. The systemic heart is responsible for pumping the freshly oxygenated blood at high pressure throughout the rest of the body, including the arms, head, and internal organs. This three-part system ensures that the blood pressure remains high enough to quickly deliver oxygen and nutrients to all the tissues, supporting the animal’s active existence.
Physiological Demands Requiring Multiple Hearts
The evolution of a three-heart system in cephalopods is a direct consequence of their blood chemistry and demanding lifestyle. Unlike mammals, which use iron-based hemoglobin to transport oxygen, cephalopods use a copper-based protein called hemocyanin, which gives their blood a blue color when oxygenated. Hemocyanin is generally less efficient at carrying oxygen than hemoglobin, especially in warmer conditions.
The use of hemocyanin means that the cephalopod’s circulatory system must work harder to deliver the necessary amount of oxygen to its tissues. This lower efficiency is compounded by the anatomical necessity of the two branchial hearts. Pushing blood through the fine capillary network of the gills substantially reduces the pressure, making it impossible for a single pump to then circulate the blood effectively throughout the entire body.
The systemic heart is required to receive the low-pressure blood from the gills and re-pressurize it for distribution to the organs. This two-stage pumping mechanism—gill circulation followed by systemic circulation—is the solution to the pressure drop problem. This setup is a prerequisite for the cephalopod’s high-energy requirements, such as rapid jet propulsion.
Jet propulsion, which involves forcefully expelling water from the mantle cavity, requires a massive and immediate supply of oxygen to the muscles. The high-pressure, three-heart system ensures that oxygen can be delivered quickly enough to support such bursts of activity. This constant demand for oxygen is further fueled by the cephalopod’s large brain and complex nervous system, which also require a continuous supply of resources.
The systemic heart must briefly stop beating while the cephalopod is actively swimming with jet propulsion. The muscle contractions required for jetting alter the pressure within the body cavity, which interferes with the action of the systemic heart. This temporary cessation of the main pump highlights the metabolic cost of their locomotion and the complex physiological trade-offs required to maintain their active, predatory existence.