Curiosity 1 of 10
1. The octopus has three hearts and blue blood
The octopus has three hearts. Two of them sit beside the gills and only push the blood through them; the third, the largest, sends the blood to the rest of the body. And this blood is blue.
Two hearts to breathe, one for the body
The two smaller hearts, called branchial hearts, sit one beside each gill. Their job is to push the blood through the gills, where it picks up oxygen from the water. After that, the central heart, called the systemic heart, pumps this now-oxygenated blood to the arms, muscles and organs.
The most curious detail is that the systemic heart stops beating for a few moments when the octopus swims. Swimming requires strong jets of water, and pumping against that pressure is costly. That's why swimming tires the octopus far more than crawling, and most of the time it prefers to walk along the bottom using its arms.
Besides the hearts, the octopus has a spread-out nervous system: about two-thirds of its neurons are in the arms, not the head. Each arm can sense and react to what it touches with considerable independence, which helps the animal search the crevices between rocks while looking for food.
The blue color comes from hemocyanin, the protein that carries oxygen in the octopus's blood. It uses copper, not iron like the hemoglobin in our blood, and it turns bluish when bound to oxygen. In the cold waters where many octopuses live, it is as good an oxygen carrier as our hemoglobin.
Did you know? Squids and cuttlefish, close relatives of the octopus, also have three hearts and blue blood. The layout of the circulatory system is similar across the whole group of cephalopods.
About this list: these are 10 animal curiosities, one per page. Each fact comes with an explanation of why it happens and an extra detail in the "Did you know?" box. The sources are at the end of the list.
