Octopuses Have Three Hearts and Blue Blood

Octopuses possess one of the most unusual circulatory systems in the animal kingdom. They have three hearts: two smaller hearts (called branchial hearts) pump blood to their gills, while the larger systemic heart pumps blood to the rest of their body. Their blood is blue because it contains hemocyanin, a copper-based protein that carries oxygen, instead of the iron-based hemoglobin that makes human blood red. Hemocyanin is less efficient at carrying oxygen than hemoglobin, which is why octopuses have multiple hearts to maintain adequate circulation. Remarkably, when an octopus swims, its main systemic heart stops beating, which is why they prefer crawling along the seafloor rather than swimming - swimming exhausts them quickly. This unique physiology allows octopuses to thrive in low-oxygen deep-sea environments where other creatures would struggle.