Most organisms, from humans to common household pets, rely on a single heart to propel blood throughout their bodies. This central pump efficiently circulates oxygen and nutrients, sustaining life functions. However, the animal kingdom holds exceptions to this rule, where some creatures possess more than one heart. These additional circulatory organs serve distinct, specialized biological purposes.
Animals with Multiple Hearts
Several animal groups possess multiple hearts. Cephalopods, marine mollusks like octopuses, squids, and cuttlefish, are prominent examples, each having three hearts. These animals possess one larger systemic heart that circulates blood to the body and two smaller branchial hearts dedicated to pumping blood through their gills.
Earthworms, common inhabitants of soil, also have multiple heart-like structures. These segmented invertebrates have five pairs of pseudohearts, also known as aortic arches, running along their bodies. These muscular, pulsating vessels encircle the esophagus and contract to push blood through the worm’s closed circulatory system.
Hagfish, an ancient, jawless deep-sea dwelling fish, are another example. They have four distinct hearts. Beyond their main systemic heart, they possess three accessory hearts: a portal heart in the liver, a cardinal heart in the head, and a caudal heart in the tail.
How Additional Hearts Function
The operation of multiple hearts varies depending on the animal’s circulatory system and physiological demands. In creatures like cephalopods, the branchial hearts increase blood pressure before blood enters the gill capillaries. This higher pressure ensures efficient gas exchange, allowing oxygen to diffuse into the blood. The systemic heart then distributes this oxygenated blood throughout the body.
The pseudohearts of earthworms act as a series of pumps along the length of their elongated bodies. These five pairs of aortic arches contract sequentially, propelling blood both anteriorly and posteriorly. This distributed pumping mechanism is effective in maintaining blood flow throughout their segmented vascular network.
Hagfish use their multiple hearts to overcome circulatory challenges. The main systemic heart drives general circulation, while the accessory hearts provide localized boosts to blood flow. For instance, the caudal heart in the tail assists in returning blood from the tail to the main circulation, which is important given their low-pressure circulatory system.
The Biological Imperative for Extra Hearts
The evolution of multiple hearts in these animals is linked to physiological and environmental pressures. For active predators like cephalopods, their high metabolic rate demands an efficient oxygen delivery system to their muscles and brain. The additional branchial hearts overcome the resistance of gill capillaries, ensuring oxygenated blood is pushed into the systemic circulation.
Earthworms, with their elongated, hydrostatic bodies, face the challenge of circulating blood over a long distance without a single central pump. Their multiple aortic arches address this by providing localized pressure points along the body, ensuring that blood reaches all segments. This distributed pumping mechanism is well-suited for their burrowing lifestyle and low metabolic needs.
Hagfish have adapted to deep-sea environments, which often have low oxygen and require specific circulatory solutions. Their multiple accessory hearts maintain blood flow in a low-pressure system and support specialized functions like osmoregulation. Independent pumps help them manage energetic demands and survive in challenging habitats.