Octopuses Have Three Hearts, Blue Blood, and Nine Brains

Octopuses are among the most alien creatures on Earth. An octopus has three hearts: one main heart pumping blood throughout the body and two branchial hearts pumping blood through the gills. Two of the three hearts stop beating when the octopus swims — which is why they prefer crawling.

Their blood is blue because it contains hemocyanin (a copper-based molecule) instead of the iron-based hemoglobin found in red human blood. Most remarkably, they have nine brains: one central brain and eight mini-brains (one per arm) that can act semi-independently. Octopus arms can continue reacting and moving even when severed from the body. They have short lifespans — most species live just 1–2 years.