The fact that octopuses have three hearts is partly due to their blue blood. Blood is pumped into the gills by their two peripheral hearts, where it picks up oxygen. The oxygenated blood is subsequently circulated throughout the body by a central heart to supply energy to the organs and muscles.
Cephalopods, which literally translates to “head foot,” describe the truncated anatomy of octopuses. Squids, cuttlefish, and nautiluses, the other three members of the group, also have blue blood, which transports oxygen using a copper-rich protein. This clarifies why they require three hearts.
The iron-based molecule known as haemoglobin, which is transported by red blood cells, gives our red blood its colour. Haemocyanin, a copper-based protein that is significantly bigger and circulates in the plasma, is used by cephalopods. Compared to haemoglobin, hemocyanin is less effective at binding oxygen. Octopuses make up for this by having three hearts: one “systemic” heart that takes the oxygen-rich blood and boosts its pressure before circulating it throughout the remainder of the body; two “branchial” hearts that take deoxygenated blood from the body and pump it through the gills.
The other cephalopods provide one hint that the three-heart system is required to support an octopus’s active lifestyle. The Nautilus, which is more sedentary and energy-efficient than the others, is the only member of the group who does not share this anatomical defect. Furthermore, octopuses’ vast neurological system may make them particularly dependent on a healthy circulation of oxygenated blood. Octopuses contain nine brains, including a large one in the middle between the eyes and smaller ones in each arm. This brain tissue uses a lot of energy.
Of course, oxygen is also necessary for octopuses’ muscles to function. They prefer to crawl around the seafloor for propulsion. They can swim quickly as well because of the water jets they discharge out of a tube known as a syphon. Yet, because their systemic heart does not beat when they are swimming, they quickly become exhausted.
There are almost 300 different types of octopus, with sizes ranging from the enormous Pacific octopus, which can weigh up to 50 kilogrammes, to the teeny Octopus wolfi, which weighs less than a gramme.
The majority of octopuses live alone. They inhabit a variety of habitats, from deep water to intertidal zones, and it may be advantageous to be blue-blooded in these environments. In habitats with a wide range of temperatures and oxygen concentrations, hemocyanin appears to aid octopuses in efficiently transporting oxygen. For animals like the Antarctic octopus, it is especially effective in the frigid seas. Yet, as acidity rises, hemocyanin loses its capacity to bind to oxygen. As oceans become warmer and more acidic due to climate change, that is not good news for octopuses.