The phrase “blue blood” is often used to describe people of aristocratic origin. They didn’t work in the fields like peasants, nor did they tan, so their skin was pale, making their bluish veins more visible. However, the blood running through those veins was perfectly ordinary. So, this expression is simply an idiom.
But there are creatures with blue blood in nature, and they aren’t aristocrats—they are octopuses and squids.
Why do they have blue blood? First, let’s understand why ours is red. It’s fairly simple: the red color of our blood comes from hemoglobin—a protein that contains iron. Hemoglobin transports oxygen throughout the body to oxygenate tissues. It evolved more than 400 million years ago in the common ancestor of humans and sharks.
Cephalopods, however, are farther from us on the evolutionary tree, and they followed a different path, choosing to retain an older, familiar protein—hemocyanin. Hemocyanin is found in the blood of mollusks and the hemolymph of arthropods.
In octopuses, hemocyanin is used to transport oxygen from the gills and skin throughout the body. It works more efficiently in the cold waters of the ocean but makes the blood more viscous, which is why these creatures have relatively high blood pressure—sometimes exceeding 75 mm Hg.
Unlike hemoglobin, hemocyanin contains copper instead of iron. It is this copper that gives octopus blood its blue color.
Despite this difference, their circulatory system is quite sophisticated. Octopuses have three hearts: two smaller ones that pump blood through the gills, one for each gill, and one main heart. The main heart shuts down when the octopus swims and reactivates when it crawls. The main heart tires quickly, so octopuses prefer to move along the ocean floor rather than swim.
Interestingly, hemocyanin doesn’t always make animals’ blood blue. For example, some crabs have a slightly different blood composition from octopuses, giving their tissues a purple hue.
