Octopuses are fascinating marine invertebrates, recognized for their intelligence and complex behavioral patterns. Many aspects of their biology differ significantly from those found in mammals and other vertebrates. One striking difference lies within their internal anatomy, where a distinct mechanism handles the transport of oxygen throughout their bodies.
The Direct Answer
The blood of an octopus is a deep blue color when saturated with oxygen. This hue results from the respiratory protein they use to carry oxygen, which is called hemocyanin. Unlike the red blood found in humans and most other vertebrates, which relies on the iron-containing protein hemoglobin, octopus blood utilizes a copper-based protein. Hemocyanin is a common adaptation among various mollusks and arthropods.
The Chemistry Behind the Color
The shift from red to blue blood is determined by the metal atom at the core of the oxygen-carrying molecule. Human hemoglobin contains iron atoms that bind to oxygen, and the resulting oxidation gives blood its bright red appearance. In contrast, hemocyanin employs two copper atoms at its active site to bind a single oxygen molecule. When the copper binds with oxygen, it changes its oxidation state from cuprous (Cu(I)) to cupric (Cu(II)), which makes the solution appear a vibrant blue.
When the blood releases its oxygen to the tissues, the copper reverts to its deoxygenated state, causing the hemocyanin to become colorless or clear. This copper-based protein is dissolved directly into the hemolymph, or blood plasma, rather than being contained within blood cells. Hemocyanin is efficient at binding and transporting oxygen in the cold, low-oxygen conditions common in the deep ocean. This provides an evolutionary advantage for survival in challenging marine habitats.
The Role of the Circulatory System
The unique chemistry of hemocyanin requires a unique mechanical system to function effectively. Hemocyanin is less efficient at carrying a high volume of oxygen compared to hemoglobin and needs high pressure to circulate. To overcome this limitation, the octopus has evolved a closed circulatory system powered by three separate hearts.
Two of the hearts are known as branchial hearts; their function is to pump blood through the gills to pick up oxygen. Once oxygenated, the blood travels to the third heart, the systemic heart, which distributes the blood to the rest of the body’s tissues. The systemic heart must generate considerable pressure to push the copper-based blood throughout the octopus’s active musculature.
This specialized system comes with a physiological trade-off, however. When the octopus engages in strenuous swimming, the systemic heart temporarily stops beating. This cessation in circulation means the octopus must rely on its two branchial hearts, which reduces the efficiency of oxygen transport to the body. Consequently, octopuses tire quickly during prolonged swimming and generally prefer to crawl along the seafloor or use jet propulsion in short bursts.