How many hearts does an octopus have?

Octopuses have three hearts. Two branchial hearts push blood through the gills to pick up oxygen, while a larger systemic heart pumps oxygen-rich blood to the organs and arms. Their blood is blue because it uses hemocyanin, a copper-based protein, instead of the iron-based hemoglobin in human blood; hemocyanin works well in cold, low-oxygen water. Remarkably, the systemic heart stops beating while an octopus swims, which may be why they prefer crawling. An octopus also has around 500 million neurons, with about two-thirds of them located in its eight arms.