Biology
Why do octopuses have three hearts?
Quick fact
One heart circulates blood to the body (systemic heart), and two branchial hearts pump blood through the gills to oxygenate it.
Why this is interesting
Octopuses have three hearts—two are dedicated purely to pushing blood through the gills, while the third does the heavy lifting for the rest of the body.
Read the full explanation
Understanding Why do octopuses have three hearts?
Think of an octopus's circulatory system like a relay race. The first two hearts, called branchial hearts, are dedicated runners that push blood through the gills. There, the blood picks up oxygen using a copper-based protein called hemocyanin—more efficient than iron-based hemoglobin in cold, low-oxygen waters. Once oxygenated, the blood reaches the third heart, the systemic heart, which acts as the anchor leg, pumping that oxygen-rich blood to the rest of the body. However, when an octopus swims, the systemic heart actually stops beating to avoid working against the intense pressure from swimming contractions. That’s why octopuses prefer crawling: it's less taxing on this unique three-heart system. So the mystery of why they have three hearts is resolved: two are specialized for gill oxygenation, and the third handles body distribution, all adapted to support their active predatory lifestyle in challenging environments.
A deeper explanation
Octopuses evolved three hearts as a solution to a fundamental physiological constraint: copper-based hemocyanin, while efficient in cold, low-oxygen environments, carries far less oxygen per unit volume than iron-based hemoglobin. This creates a need for high-volume blood flow to meet metabolic demands. The underlying mechanism is a division of labor between two specialized "gill hearts" that pump blood through the gills in parallel, maximizing oxygen uptake, and a single "systemic heart" that then distributes oxygenated blood to the body in series. A key feedback dynamic occurs during locomotion: when an octopus swims, the systemic heart actually stops beating. This is because swimming involves jet propulsion, which compresses the mantle cavity and increases internal pressure—interfering with the heart's function. The body enters a temporary anaerobic state, making crawling energetically preferable. This trade-off between mobility and circulation reveals how structural constraints shape behavior. The same principle of redundant, specialized pumps appears in other domains. For example, some annelid worms have multiple "aortic arches" acting as hearts along their body to maintain pressure in a long, flexible form. In engineering, hydraulic systems often use dual primary and backup pumps to handle variable loads and ensure redundancy. Gas exchange in fish gills also uses a countercurrent flow mechanism to maximize oxygen extraction, analogous to the octopus's parallel gill hearts boosting throughput. Further exploration pathways include: hemocyanin's thermodynamic properties vs. hemoglobin, the evolution of closed circulatory systems, hydrostatic skeletons and their impact on cardiac function, and convergent evolution in other cephalopods like squid (which also have three hearts but different swimming strategies).