Squid, marine invertebrates, are known for their intelligence, ability to change color, and jet-propelled movement. These agile creatures possess a unique biological feature: they have multiple hearts. Unlike humans and most mammals with a single heart, a squid’s circulatory system is supported by three distinct hearts. This specialized arrangement allows them to thrive in their active, predatory lifestyle within the ocean’s depths.
The Direct Answer: Three Hearts
Squid have three hearts. These three hearts are not identical but serve specific roles within their circulatory system. Two of these are called branchial hearts, or gill hearts. These smaller, less muscular hearts are positioned at the base of each of the squid’s two gills. Their primary function is to pump deoxygenated blood through the capillaries of the gills, where oxygen is absorbed from the surrounding seawater.
The third heart is the systemic heart, larger and more robust than the branchial hearts. After the blood has been oxygenated in the gills, it flows to this systemic heart. The systemic heart then powerfully pumps this oxygen-rich blood throughout the rest of the squid’s body, delivering oxygen and nutrients to its muscles, organs, and other tissues. This division of labor among the three hearts ensures efficient blood circulation.
A Specialized Circulatory System
The squid’s three hearts work as part of a closed circulatory system, where blood remains within vessels. This is an adaptation, as most other mollusks have an open circulatory system. A closed system allows for more efficient transport of oxygen and nutrients, important for the squid’s high metabolic rate and active predatory nature.
Blood flow in a squid is coordinated. Deoxygenated blood from the body returns to the two branchial hearts. Each branchial heart propels this blood into its gill, facilitating the exchange of carbon dioxide for oxygen. Once oxygenated, blood flows from the gills to the systemic heart. This powerful systemic heart distributes oxygen-rich blood at high pressure throughout the body, reaching all tissues.
This multi-heart system provides several advantages. Their blood contains hemocyanin, a copper-based protein for oxygen transport, less efficient at carrying oxygen than the iron-based hemoglobin in human blood. To compensate for this, the extra pumping power provided by the three hearts ensures sufficient oxygen delivery. The branchial hearts overcome the resistance within the gill capillaries, allowing the systemic heart to focus on circulating oxygenated blood to the body, important for muscles used in rapid jet propulsion. This efficient oxygen delivery system supports their bursts of speed and sustained activity.