Octopuses indeed bleed, and their blood is distinctly blue. The unusual color of their blood, along with their specialized circulatory system, plays a significant role in their survival and active lifestyle beneath the waves.
The Blue Pigment
The striking blue color of octopus blood comes from a copper-containing protein called hemocyanin. Unlike the red blood of vertebrates, which uses iron-rich hemoglobin to transport oxygen within red blood cells, octopuses and many other invertebrates, such as snails, spiders, and horseshoe crabs, circulate hemocyanin directly suspended in their hemolymph. When deoxygenated, hemocyanin is typically colorless or very pale. However, upon binding with oxygen, the copper atoms within the protein undergo a change, causing the hemolymph to appear a vibrant blue. This contrasts with hemoglobin, which turns bright red when oxygenated due to its iron content.
How Oxygen Travels
Hemocyanin’s function is to bind and transport oxygen throughout the octopus’s body. Each hemocyanin molecule contains two copper atoms that reversibly bind a single oxygen molecule. This oxygen-binding mechanism delivers oxygen to tissues and organs, especially given the octopus’s active metabolism and complex nervous system. While generally less efficient at oxygen transport than hemoglobin in terrestrial conditions, hemocyanin offers distinct advantages in the cold, low-oxygen, and high-pressure conditions found in many marine habitats. This makes it a suitable respiratory pigment for octopuses, enabling them to maintain a steady oxygen supply where other creatures might struggle.
A Unique Circulatory System
Circulatory System Structure
To support their high oxygen demands and compensate for hemocyanin’s characteristics, octopuses possess a closed circulatory system featuring three hearts. Two of these are branchial hearts, positioned near the gills, which pump deoxygenated blood through the gills to facilitate oxygen uptake and carbon dioxide release. Once oxygenated, this blood flows to a larger, central systemic heart. The systemic heart then circulates the freshly oxygenated blood to the rest of the body, powering their muscles and organs.
Circulation Efficiency
This multi-heart system enables efficient blood circulation at higher pressure, which is beneficial given that hemocyanin is less efficient than hemoglobin at transporting oxygen per amount of blood. When an octopus moves by swimming, its systemic heart may temporarily stop beating, which is why octopuses often prefer crawling to conserve energy.