Most complex organisms, from humans to insects, share a similar circulatory design: a single, centralized heart. A few animal groups, however, have developed unique solutions involving multiple hearts to meet high metabolic demands. This rare adaptation is often a response to challenges within their respiratory or circulatory mechanics.
Cephalopods: The Three-Hearted Exception
The animals that most definitively answer the question of having multiple hearts are the cephalopods, a class of marine mollusks that includes the octopus, squid, and cuttlefish. Contrary to the common query about animals with two hearts, these highly active invertebrates possess three distinct hearts. This trio of pumps supports a closed circulatory system, which is unusual among most mollusks that rely on a less efficient open system.
Cephalopods are known for their intelligence and rapid movements, such as jet propulsion, which demands a constant supply of oxygen. Their blood uses a copper-rich protein called hemocyanin for oxygen transport, which is less efficient than the iron-based hemoglobin found in vertebrates. To compensate, the three-heart system is necessary to generate the required pressure and flow rate throughout the body.
The Function of Branchial and Systemic Hearts
The three hearts in a cephalopod are not identical; they are functionally divided into two branchial hearts and one systemic heart. The two branchial hearts are positioned at the base of the gills, one for each gill, and are responsible for moving deoxygenated blood through the respiratory structures. These branchial hearts effectively boost the blood pressure before it enters the capillaries of the gills, maximizing gas exchange with the surrounding seawater.
After the blood passes through the gills and becomes oxygenated, its pressure drops significantly due to resistance within the delicate gill structure. The third pump, the systemic heart, then takes over. It receives the freshly oxygenated blood from the branchial hearts and forcefully pumps it out to the rest of the body, including the mantle, organs, and arms.
The systemic heart’s function is so demanding that it ceases to beat entirely when a cephalopod swims vigorously using jet propulsion. This temporary inactivity highlights the metabolic trade-off of the three-heart system, forcing the animal to rely on crawling or short bursts of movement. The synchronized contraction of the two branchial hearts is immediately followed by the contraction of the systemic heart, creating a highly coordinated, phased circulatory cycle.
Single Heart Efficiency Versus Multiple Hearts
The single heart found in most vertebrates, such as the four-chambered heart in mammals, is a design optimized for efficiency and integration. This single organ effectively acts as two pumps joined together, completely separating the oxygenated and deoxygenated blood streams. This separation allows the blood to be repressurized after passing through the lungs, ensuring high-pressure, oxygen-rich blood is delivered consistently to all body tissues.
The cephalopod’s multiple-heart system, while successful, is a less integrated and more energetically expensive solution. Some animals are occasionally mistaken for having multiple hearts, such as the earthworm. An earthworm possesses five pairs of aortic arches, which rhythmically contract to move blood, but these are muscular sections of a major blood vessel, not distinct organs like the cephalopod’s hearts.
Another example is the hagfish, which has one main heart and three accessory pumps. However, this animal is sluggish and maintains a low-pressure circulatory system. Ultimately, the three-heart system of cephalopods remains the most prominent example of this specialized adaptation.