It’s a science fiction trope… A few soldiers walking down a dimly-lit spaceship, weapons drawn, as unsettling splatters of blood cover the walls—only some of it red. The blood that circulates in our bodies has a cultural significance that is difficult to overstate, its deep scarlet hue being used to imply everything from rage to romance to power. Perhaps this is why differently colored blood is used to show that the very essence of alien beings is completely foreign to the human experience. What you may not know, however, is that these aliens walk—and swim—among us.
Many invertebrate animals have blue blood, and octopuses are no exception. Our blood is red due to the protein hemoglobin, more specifically the iron contained within hemoglobin, which binds to oxygen and carries it throughout our body to each of our cells. The octopus blue blood, on the other hand, uses copper instead of iron to carry oxygen. This comes with the added benefit of remaining functional in a wide variety of temperatures, which is particularly useful for ectothermic organisms. This is part of why octopuses have been found everywhere from the Caribbean to the Arctic circle. Hemocyanin does not, however, come without its costs. For one, it is highly sensitive to changes in pH. This means that if octopuses do not adapt quickly, they may be negatively impacted by the acidification of our oceans from the mass emission of carbon dioxide. In addition, hemocyanin is not as efficient as hemoglobin.
This, along with their sophisticated brain, astronomical growth rate, and relatively active lifestyle, causes octopuses to have a massive metabolism, and consequently, require a lot of oxygen. In fact, while most other mollusks, as well as fish, require a 3% oxygen saturation in their blood, octopuses demand a hefty 11%. This has also influenced their evolution. To compensate for this gluttonous oxygen demand, their cephalopodan ancestors developed a closed circulatory system, while other mollusks maintained an open system. In other words, the blood of an octopus travels through capillaries to perfuse its tissues while snail blood simply mixes with the extracellular fluid. If this sounds too familiar, don’t worry, for they have not one, nor two, but THREE hearts. One brachial heart is paired with each gill and a third systemic heart pumps the blood throughout the rest of the body. As strange as this may be, we vertebrates had to optimize blood flow to keep pace with our energy demands as well. While we divided our hearts into chambers, keeping the oxygen-rich blood on the right side for maximum concentration, octopuses and their relatives developed entire separate organs for this same purpose.
The blue blood of an octopus, along with its bizarre cardial anatomy, may evoke images of extra-terrestrials, and while this may be technically inaccurate, it is perhaps not far from the truth. After all, you would expect any organism that diverged from our lineage hundreds of millions of years ago to be drastically different from humans. But even stranger is that, despite all their differences, they still seem so eerily similar to ourselves.