#171 Why do octopuses have blue blood?

I learned this today. Octopuses have blue blood because they have adapted to live in cold, deep water.

Octopuses live in every ocean on Earth, and they live in a range of different conditions. A lot of them live at great depths and this is the main reason why they have evolved to have blue blood. An octopus has even been photographed at a depth of 6,957m, which is incredibly deep.

At the depths that some octopuses live, the oxygen content of the water is lower than it is at the surface. More importantly, the temperatures are extremely low. Below about 200m depth, the ocean has an average temperature of 4℃. At about 3,000m down, the temperature drops to about 2℃. Octopuses have evolved to live in this kind of environment.

Our blood is red because of the iron that it contains. The iron is used to make hemoglobin, which is a protein that carries oxygen through the blood to wherever it is needed. One molecule of hemoglobin has an iron atom, and it can bind to four oxygen molecules and transport them. The oxygen “oxidizes” the iron in our blood and makes it look red. Iron is a very common mineral and it makes sense that it would be in our blood.

Octopuses use copper in their blood instead of iron. The copper is inside a protein called hemocyanin, which the octopuses use to carry oxygen around their bodies. The hemocyanin protein has 96 pairs of copper atoms that bind to a single oxygen molecule. When iron oxidizes it turns red. When copper oxidizes it turns blue. And this is why octopuses have blue blood.

So, why do they need to use the copper-based hemocyanin instead of the iron-based hemoglobin? There are two reasons.

The first, and most important reason, is that hemocyanin is a more efficient carrier of oxygen in cold conditions. In can transport far more oxygen at temperatures close to freezing than hemoglobin can. However, at cold temperatures, oxygen molecules stick more tightly to proteins, and even hemocyanin loses its ability to carry oxygen perfectly. Octopuses that live in very cold water have evolved to have 40% more hemocyanin in their blood than octopuses that live in warmer waters.

The second reason is that deeper water has more copper in it than iron. On the surface, we are surrounded by iron. Iron is the fourth most common element by mass in the Earth’s crust and it is in almost everything that we eat. As you go deeper into the sea, the amount of iron starts to decrease and the amount of copper increases. It is easier for octopuses to replenish their copper stores than it would be if their blood was based on iron.

Having to evolve a different type of blood to cope with cold water environments seems more difficult than moving to warmer water, but the octopus has never really had this opportunity because they don’t live very long, and they can’t swim very far.

The lifespan of an octopus varies from six months to about five years, with most of them being under two years. Because of this short life expectancy, long distance migration doesn’t make any sense and they had to evolve to cope with the habitat they were in.

Also, they cannot swim very far. Octopuses have three hearts. Two of them pump blood to their gills and the third, larger heart, circulates the blood around their bodies. Because their blood contains high amounts of hemocyanin, it is very viscous and requires a lot of pressure to pump it around their circulation. When the octopus swims, the larger heart stops pumping, which means that the octopus can’t swim very far. They prefer to crawl along the seabed because it means they can breathe while they do it. This inability to travel very far is another reason why they have never moved to warmer water.

Having three hearts sounds impressive, but octopuses also have nine brains. They have a central brain that controls all of the higher-level functions and they have a mini brain at the top of each arm. Two thirds of their neurons are in the nerve cords of their arms. This allows the arms to act almost independently and reduces a lot of the load from the main brain.

So, octopuses have blue blood because their blood uses hemocyanin, which is a protein that is based on copper and carries blood through their bodies. Copper is blue when exposed to oxygen, making their blood blue. And this is what I learned today.

Photo by Pia from Pexels

Sources:

https://www.nhm.ac.uk/discover/octopuses-keep-surprising-us-here-are-eight-examples-how.html

https://animals.howstuffworks.com/animal-facts/5-animals-whose-blood-isnt-red.htm

https://biogeoplanet.com/why-do-octopuses-have-9-brains-8-arms-3-hearts-and-blue-blood-surprising-facts