The heart is a muscular organ that functions as a pump, propelling blood through vessels to circulate it throughout the body. While humans and most vertebrates possess a single, four-chambered heart, some animals have evolved to possess more than one, an adaptation to their unique physiological demands.
Invertebrates with Multiple Hearts
Several invertebrate species have multiple hearts. Cephalopods, such as octopuses, squids, and cuttlefish, are among the most well-known. These marine animals typically possess three hearts: two branchial hearts and one systemic heart. The two branchial hearts pump blood through the gills for oxygenation, and the systemic heart then circulates the oxygenated blood to the rest of the body.
Annelids, such as earthworms, also have multiple heart-like structures. Earthworms have a closed circulatory system, where blood remains entirely within vessels. They possess five pairs of muscular vessels, called “aortic arches” or “pseudohearts,” which function as pumping organs. These ten pulsatile structures are located in the anterior segments, circulating blood through dorsal and ventral vessels throughout their elongated body.
The Biological Necessity of Extra Hearts
Multiple hearts in these animals are linked to their circulatory systems and metabolic requirements. Cephalopods, as active marine predators, need efficient oxygen delivery to support their movements. Their two branchial hearts increase blood pressure for gill oxygen uptake. After oxygenation, the systemic heart then pumps this blood at a higher pressure to the rest of the body, including their muscular arms and mantle.
Cephalopod blood uses hemocyanin, a copper-based protein less efficient than vertebrate hemoglobin, requiring greater pressure for oxygen circulation. Additionally, an octopus’s systemic heart can become inactive during swimming, leading to quick fatigue and explaining their tendency to crawl rather than swim for extended periods.
Earthworms’ elongated, segmented bodies benefit from the distributed pumping of their five pairs of aortic arches. These “hearts” propel blood throughout their entire length, overcoming vessel resistance to efficiently deliver nutrients and oxygen and remove waste.
Understanding What Counts as a Heart
Defining a “heart” is important when discussing multiple pumping organs. A heart is a muscular organ that actively propels blood or hemolymph through a circulatory system. This is distinct from multiple chambers within a single heart, like those in humans, which are part of one organ and facilitate efficient blood separation and pumping.
Some animals have structures that assist circulation but are not true hearts. Insects, for example, have a single tubular dorsal vessel as their main heart, pumping hemolymph in their open circulatory system. They also possess “accessory pulsatile organs” in appendages like antennae and legs, which help circulate hemolymph but are considered secondary pumps, not independent hearts.