The circulatory system, a network responsible for transporting essential substances throughout an organism, typically features a single, central pump: the heart. This muscular organ drives blood or hemolymph to deliver oxygen and nutrients while removing waste products. While a single heart is common across many animal species, nature presents fascinating exceptions where multiple hearts perform these functions, showcasing unique biological adaptations.
The Unique Case of Cephalopods
Among the most prominent examples of animals with multiple hearts are cephalopods, a class of marine invertebrates that includes octopuses, squids, and cuttlefish. These intelligent creatures typically possess three hearts: two branchial hearts and one systemic heart.
The branchial hearts are smaller, muscular pumps located at the base of each of the two gills. Their primary role involves pushing deoxygenated blood through the capillaries of the gills, where gas exchange occurs. Following this oxygenation, the blood then flows to the single, larger systemic heart. This systemic heart is responsible for circulating the oxygenated blood to the rest of the body’s tissues and organs.
Cephalopods also possess a closed circulatory system, meaning their blood remains contained within vessels, unlike the open systems found in many other mollusks. Their blood contains hemocyanin, a copper-based protein, instead of the iron-based hemoglobin found in vertebrates. This copper content gives their blood a distinct blue color when oxygenated.
How These Hearts Function
The three hearts of a cephalopod work in a coordinated sequence to ensure efficient blood circulation. Deoxygenated blood, returning from the body, first enters the two branchial hearts. Each branchial heart then propels this blood through its respective gill. As blood passes through the fine capillaries of the gills, it picks up oxygen from the surrounding seawater and releases carbon dioxide.
Once oxygenated, the blood flows from the gills to the single systemic heart. This powerful systemic heart then takes over, pumping the now oxygen-rich blood at higher pressure throughout the rest of the cephalopod’s body. This includes delivering oxygen and nutrients to the muscles, organs, and particularly the highly active arms and tentacles.
The systemic heart, however, can become inactive when the cephalopod is swimming. When this occurs, the animal may lose energy quickly and often resorts to crawling, highlighting the importance of the systemic heart for sustained activity.
The Evolutionary Advantage
The presence of multiple hearts in cephalopods provides a significant evolutionary advantage, particularly given their active lifestyles and unique blood chemistry. Blood passing through the gills experiences a drop in pressure. The two branchial hearts compensate for this pressure loss, boosting the blood pressure before it reaches the systemic heart. This ensures that the systemic heart receives blood at a sufficient pressure to effectively circulate it throughout the entire body, including to their complex nervous systems and highly mobile appendages.
The copper-based hemocyanin in cephalopod blood is less efficient at oxygen transport compared to iron-based hemoglobin. To overcome this, cephalopods require a more rapid and higher-pressure circulation to deliver enough oxygen to their tissues. The three hearts facilitate this increased circulatory efficiency, enabling the high metabolic rates necessary for their predatory behaviors, rapid movements, and complex cognitive functions.