Why is Octopus Blood Blue?

Why is Octopus Blood Blue?

The octopus is a marvel of the natural world with a laundry list of fascinating attributes and abilities. They are invertebrates, meaning they have no spine. In fact, they have no bones at all, unless you count their parrot-like mouth, a beak, which they use to break through the shells of other creatures like crabs. They can move by either shooting jets of water through their heads or using their eight sucker-covered arms to pull themselves along the seafloor. They can also use those arms to grab prey, open containers, and even carry items, like rocks, around with them. Each tentacle is lined with suction cups which they use for grip, touch, and taste.


The octopus is a member of the cephalopod class along with the squid, cuttlefish, and nautilus. The name cephalopod comes from the Greek word Cephalopoda which means “head foot”. This name suits these animals fairly well as, upon inspection, their body seamlessly transitions from head to feet with nothing in between. That said, their “feet ”, the tentacles, are more akin to an arm or fingers. In English, the name for the octopus is perhaps more accurate as the prefix “octo-” implies the number eight, addressing its eight arms. In conjunction with these eight arms octopuses also have nine brains, each of which has a specific purpose. Eight of the brains each control one of the octopus’ tentacles, with the ninth brain in control of the rest of its body’s functions. The strange numbering continues with the octopus’s three hearts. Two of the hearts move oxygenated blood to and from the gills whereas the third heart works to pump that blood throughout the rest of the body.


The Intelligence of the Octopus

In Aristotle’s History of Animals part 37, he states “The octopus is a stupid creature, for it will approach a man’s hand if it is lowered in the water”. Given the ‘observations’ that he then performs on the octopus, we can conclude that it was indeed a stupid decision (though I think we should hardly blame the octopus for that). In reality, modern observations have found the many existing species of octopus to be extraordinarily intelligent! Octopuses have repeatedly displayed higher-order thinking skills and behaviors such as using tools, solving mazes, and opening jars. They have also been seen in the wild using discarded shells, rocks, and coconut halves to shelter themselves from predators and the environment. In lab settings octopuses have been taught to discern between and attack certain objects in order to gain a reward. This kind of memory and processing is a learned behavior that not all organisms can achieve.


The remarkable intelligence of the octopus has led some scientists to suggest using the octopus as a model organism. A model organism is a standard replacement for humans in certain biological tests and procedures. Examples include the fruit fly for studying genetics, the mouse for studying the brain, and the pig for its internal organs and layout. One problem with this plan, however, is how difficult octopuses are to care for. Model animals are preferred to be easy to keep alive, handle, and propagate. Octopuses are none of these! They require specific living conditions and large tanks, are known to hold grudges against people they don’t like, and only lay eggs once, after which they quickly die. Depending on the species a brood of eggs can number from one hundred up to the tens of thousands. The mother octopus then stops eating and guards the eggs until they hatch which can take anywhere from 2-53 months!

This level of cognition and complexity has left many people uncomfortable with the idea of eating octopuses and others to speculate that they may have the closest mind of living creatures to a human level of intelligence.

A Colorful Creature

One of the octopus’s more well-known traits is its ability to change the color of its skin to aid in camouflage, hunting, and communication. An octopus’s skin uses two unique organs in tandem to achieve its variable display. The first of these organs are called chromatophores which store little sacks of pigment within them. The octopus is able to stretch these sacks out using muscles that encompass the chromatophores, which causes the pigment to be more or less visible on the surface of its skin. The pigment in chromatophores is normally only red, yellow, or brown, but many species of octopus are capable of producing even more colors. This is where the second set of organs, called iridophores, come in. The iridophores lay in thin sheets over the top of the chromatophores in the skin. These thin sheets can alter the wavelength of the light passing through and reflected off of them. In combination with the expanding and contracting chromatophores beneath, octopuses are capable of shifting through a gallery of colors in just fractions of a second!


There are so many fascinating facts about how the octopus looks and works but there is another hidden just below the surface. Octopuses have blue blood.


Human and (Most) Animal Blood

The reason having blue blood feels so strange is because almost every living creature on earth with a circulatory system has red blood. While blood is not interchangeable between creatures (or even between all humans) it serves the same kind of role in every organism. It carries nutrients, waste, information, and oxygen from one part of an organism to another. When we eat food our stomachs and intestines work to break that food down into its constituent molecules and compounds. These then get absorbed into our bloodstream and carried through the vascular system to every corner of our body. They get picked up by the cells lining the walls of these channels and carried into the cells to be used for energy, construction, or other chemical processes. Depending on what gets taken in and what it is used for there are sometimes chemicals and compounds left over. This waste gets put back into the bloodstream so that it can be filtered out by other organs in the body like the liver or the kidneys. But sometimes our body will inject things into our circulatory system not to get rid of them but to communicate with other parts of the body.


A lot of our intrabody communication is done through the nervous system sending electrical impulses from our brain to the muscles at the end of our toes. Another way our body communicates with itself is with hormones like adrenaline. When the brain detects a threat or stressful situation it will send a signal to the suprarenal gland on top of each kidney causing them to secret adrenaline into the blood that is passing through them. As the circulatory system carries the hormone through vesicles and capillaries it causes the smallest of them to tighten. This restricts blood flow in the extremities and increases its concentration near core muscles and organs so they have more oxygen and nutrients available to them during a time of crisis. It also causes airways in the lungs to expand, absorbing more oxygen for the body to use. This oxygen comes in bonded pares (O2) which can attach to molecules in the blood called hemoglobin. Hemoglobin is stored on the surface of red blood cells with over 25,000,000 per cell! Each Hemoglobin can bond to 4 O2 pairs and then release them when they come to other cells in the bloodstream. These cells will use their shipment of oxygen in many ways but it most often leaves the oxygen atoms bonded to a carbon atom in the form of CO2. The cells can then give this back to the hemoglobin molecules to be transferred to the lungs and exhaled. These hemoglobin molecules each contain 4 iron atoms which, when bonded with oxygen, turn bright red; giving red blood cells and blood itself that distinctive red appearance.


Why Blue Blood?

If blood is red because of hemoglobin, which is used for transporting oxygen and is necessary for all multi-cellular life, why don’t octopuses also have red blood? Because, octopuses don’t use hemoglobin as their oxygen transporter. Instead, octopuses use a molecule called hemocyanin. Hemocyanin consists of copper rather than iron which, when bonded to O2 turns a bold blue color. Another significant difference between hemoglobin and hemocyanin is that while hemoglobin bonds with four O2 or CO2 molecules, hemocyanin can bond with up to 96. Naturally, this means that hemocyanin is much larger than hemoglobin. Despite being so much larger it’s considerably easier for hemocyanin to get around in the circulatory system because instead of millions being bonded to a single cell for transport, like with hemoglobin, each molecule of hemocyanin floats freely in the bloodstream.

Advantages of Blue Blood

But what advantage does blue blood give the octopus? Well, none really. The blue is just a bonus that comes along with hemocyanins’ true advantages. To best understand these advantages we need to consider where the octopus makes its home. While octopuses can swim in a way, as mentioned earlier, they much prefer to pull themselves around on the floor of the ocean. This comes with a few hurdles as the majority of the ocean floor is miles below the surface. The water at these depths has little chance to absorb oxygen from the surface of the ocean. On top of that, it’s remarkably cold with some species of octopus living in waters that are only fractions of a degree above freezing. There are some red-blooded species that can live in these areas but they each have their own unique adaptations for surviving.

One advantage given by hemocyanin is its capacity for oxygen. With the ability to bond to 96 pairs of oxygen, as well as each hemocyanin molecule being free-floating, the octopuses’ gills can make sure to grab every O2 available. This is especially important as octopuses require an above-average amount of oxygen due to their complex brains. In addition to that, hemocyanin can continue to carry out the reaction to trade oxygen and carbon dioxide at much lower temperatures than a hemoglobin-covered red blood cell can. This means that octopuses don’t need to spend as much energy keeping themselves warm.


With these advantages, it’s easy to see why octopuses in the deep ocean or near-freezing waters have evolved to bleed blue. But aren’t there many species that live in warmer waters? Indeed there are and their blood works just the same. This has led scientists to think that octopuses originally evolved in the deep ocean or in frigid environments and then adapted to warmer waters as they spread. This is likely how we come to find hemoglobin-filled creatures living in those deeper, colder environments. They evolved for warmer oxygen-rich waters and then adapted as they made their homes deeper.


Are There Other Animals with Blue Blood?

Actually, there are other animals that have blue blood! One animal that many know of already is the horseshoe crab. While it is not actually a crab, it is actually more closely related on the family tree to spiders and scorpions, its blue-hued blood is worth quite a lot of money. These creatures’ blood is used widely in the medical field. Medical manufacturers use the blood to test their products for dangerous bacteria that can cause fevers and even fatal sickness in humans. In 1971, scientists discovered that when horseshoe blood was exposed to the E. coli bacteria, the crab’s blue blood clotted. As we know, these bacteria can cause extreme sickness in humans, so learning this information about the blood of the horseshoe crab was, and will continue to be, beneficial to the medical community.


Other sea creatures that have blue blood include lobsters, scallops, shrimp, crayfish, mussels, and squid. Snails, which are mollusks, small worms, and pillbugs (commonly called Roly Polys) also have this unique blood coloring. Last, but not least, most spiders have blue blood.


Is Blue Blood the Only Other Unique Color Type?

Blue blood is not the only other unique color type in the animal kingdom. While blue blood is produced from the protein hemocyanin, some marine worms have the protein hemerythrin as their oxygen binder and this gives their blood a purple color.


There are other marine worms that have chlorocruorin as the protein oxygen binds to. This protein also uses iron to bind to oxygen, like hemoglobin, so it is red when very concentrated. When you dilute blood with chlorocruorin, though, it looks bright green in color! Another organism with green blood is the green-blooded skink, or Prasinohaema, of New Guinea. Rather than using the chlorocruorin protein to gain its unique hue, the green-blooded skink has high levels of biliverdin in its blood. This is a green bile pigment and can also cause jaundice, so scientists believe that this skink has developed a resistance to the toxicity that is produced by this pigment. It definitely gives it a cool color, though!


The icefish, a member of the Channichthyidae family, has white blood. It has neither red blood cells nor hemocyanin in its hemolymph. Oxygen diffuses, moves from an area of high concentration to an area of low concentration without the body using energy, straight from the seawater into the plasma of the fish!


The octopus is one of the most interesting creatures on earth! From its amazing intelligence to its innate ability to camouflage itself. From its unique blue-hued blood that helps it more efficiently carry oxygen throughout its body, to its interesting body shape and function, the octopus is unlike any other animal on land or sea. The octopus is among a small number of organisms on earth that can boast blue blood. Unlike most other organisms that use hemoglobin in their blood which contains iron and gives a red hue, octopuses have evolved the use of hemocyanin. Hemocyanin is a copper-containing protein that carries oxygen throughout an octopus’s body. By using copper, an octopus’s hemolymph (blood cells, platelets, etc.) can more efficiently carry oxygen in colder climates. This unique blood type will forever brand the octopus as one of the most interesting creatures.

Why is Octopus Blood Blue?
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