Why Is Octopus Blood Blue? Hemocyanin Explained

How hemocyanin makes blood blue

Hemocyanin means “blue blood protein”. Each oxygen-binding site in the molecule holds a pair of copper atoms, which together grip one molecule of oxygen. When oxygen attaches, the copper atoms change their electrical state, and the protein starts absorbing orange and red light while letting blue light through. That is why blood leaving an octopus’s gills has a clear blue tint.

After the blood delivers its oxygen to the muscles and organs, the copper returns to its original state and the color fades. Blood in the veins of an octopus is almost colorless or faintly gray. So an octopus’s blood is blue only when it is doing its job.

The protein was described and named in 1878 by the Belgian physiologist Léon Fredericq, who studied the blood of the common octopus.

Hemocyanin vs hemoglobin

Both proteins do the same job, picking up oxygen where it is plentiful and releasing it where it is needed, but they are built very differently.

The biggest practical difference is packaging. Red blood cells let us cram huge amounts of hemoglobin into our blood without making it thick. Octopus hemocyanin floats loose, so adding more would make the blood too thick and sticky to pump. As a result, octopus blood carries far less oxygen than ours.

Why do octopuses use copper instead of iron?

Part of the answer is history. Hemocyanin is old and widespread among mollusks, and octopuses inherited it from shelled ancestors. Interestingly, arthropods such as crabs and spiders also use hemocyanin, but theirs evolved separately and has a different structure, an example of two groups arriving at a similar solution.

The other part is that hemocyanin suits the conditions where many octopuses live. It works well in cold water and can still take up oxygen when the surrounding water holds little of it. Octopuses also have unusually efficient gills that can extract a high share of the oxygen from the water, and they absorb some oxygen through their thin skin.

The extreme case is the Antarctic. A 2015 study of an Antarctic octopus, Pareledone charcoti, which lives in water close to −1.8 °C (29 °F), found unusually high levels of hemocyanin in its blood, a likely adaptation for keeping its tissues supplied with oxygen in near-freezing seas.

How blue blood connects to three hearts

Thick blood that carries little oxygen needs a strong pump. Octopuses have three hearts: two branchial hearts drive blood through the gills, and the systemic heart pushes it around the body at pressures that can exceed 75 mmHg (10 kPa), very high for an invertebrate. Even so, the system has little spare capacity. Fast swimming quickly leaves an octopus short of oxygen, which is why most octopuses crawl and jet only to escape. The details are in how many hearts does an octopus have?

Weak spots: warm and acidic water

Hemocyanin in octopuses and squid is very sensitive to acidity. When carbon dioxide builds up in active tissues, the blood becomes slightly more acidic and hemocyanin lets go of its oxygen more easily, which helps feed hard-working muscles. But it also means that the blood picks up oxygen less well when the surrounding water is warmer or more acidic.

Warm water holds less dissolved oxygen, and many octopuses prefer cool, well-oxygenated water for this reason. Scientists worry that warming seas and ocean acidification could make breathing harder for octopuses and squid in the future, and some octopus populations may shift into deeper, cooler water.

Other animals with blue blood

Octopuses are far from alone. Hemocyanin is the second most common oxygen carrier in the animal kingdom after hemoglobin, and it is found in two big groups.

• Mollusks: squid, cuttlefish such as the common cuttlefish, nautiluses and many snails.
• Arthropods: crabs, lobsters, crayfish and shrimp, horseshoe crabs, many spiders including tarantulas, scorpions and some centipedes.

The American lobster, for example, has hemolymph (the arthropod equivalent of blood) that looks grayish inside the body and turns pale blue when it meets oxygen. The most famous blue blood of all belongs to the horseshoe crab, explained in why is horseshoe crab blood blue?

Other blood colors in nature

Red blood is not limited to vertebrates. Earthworms have red blood too, colored by a giant hemoglobin dissolved in the fluid. Some marine worms have green blood containing a protein called chlorocruorin, and peanut worms and lamp shells use hemerythrin, which turns violet-pink with oxygen. A few New Guinea skinks have bright green blood because of a green bile pigment, and Antarctic icefish have nearly clear blood with no hemoglobin at all.

One common myth deserves a correction: human blood is never blue. Our veins look bluish because of the way light penetrates and reflects from skin, but the blood inside them is dark red.