Squids are captivating marine invertebrates recognized for their intelligence and unique biological characteristics. Among their most intriguing features is their circulatory system, which includes three hearts. This unusual arrangement allows these cephalopods to thrive in diverse ocean environments, supporting their active lifestyles.
The Squid’s Unique Circulatory System
Squids possess a specialized circulatory system that features two distinct types of hearts. Two of these are known as branchial hearts, positioned at the base of each of the squid’s two gills.
In addition to the two branchial hearts, squids have a single, larger systemic heart. This systemic heart is centrally located within the squid’s mantle cavity. Unlike the branchial hearts, the systemic heart is more muscular and robust. Together, these three hearts form a closed circulatory system, meaning blood remains within vessels, unlike some other mollusks.
Pumping Life: How the Hearts Work
The three hearts of a squid work in a coordinated sequence to ensure efficient blood circulation and oxygen delivery throughout its body. The two branchial hearts play an initial role by receiving deoxygenated blood from the squid’s body. They then pump this blood to the gills. This action facilitates the exchange of gases, where carbon dioxide is released and oxygen is absorbed from the surrounding seawater.
Once the blood becomes oxygenated in the gills, it flows to the single systemic heart. This larger heart then propels the oxygen-rich blood to the rest of the squid’s body. It distributes this blood at high pressure to the mantle, arms, brain, and other organs. This two-stage pumping mechanism, with separate hearts for gill and systemic circulation, enhances oxygen delivery efficiency.
An Evolutionary Edge
The specialized three-heart system provides squids with an evolutionary advantage, supporting their active existence. This efficient circulatory design is beneficial for animals that rely on rapid jet propulsion for movement, hunting, and escaping predators. The high-pressure blood flow generated by the systemic heart ensures that oxygen and nutrients reach the muscles and organs quickly, enabling bursts of speed and sustained activity.
Squid blood contains hemocyanin, a copper-based protein, instead of the iron-based hemoglobin found in human blood, which gives their blood a blue color when oxygenated. Hemocyanin is less efficient at carrying oxygen than hemoglobin, especially at varying temperatures. The presence of multiple hearts helps compensate for this reduced oxygen-carrying capacity by ensuring faster and higher-pressure circulation. This adaptation allows squids to maintain high metabolic rates, even in environments with lower oxygen levels or colder temperatures, common in deep-sea habitats.