Blue blood animals on Earth may seem strange, but the idea is less alien than you may think. It’s well known that the blood of most animals on earth is red, but it may surprise you to find out that some species on earth can differ from red, green, violet, and yes, there are even blue-blooded animals.

Spiders, crabs, some mollusks, octopi, and squid are all known to have blue blood, thanks to the presence of hemocyanin.

Below we are going to discuss why hemocyanin makes animals’ blood blue, which ocean animals have blue blood, and what makes these animals so special.

Chemicals Determine The Color of An Animal’s Blood

As mentioned above, blood comes in a variety of colors, which are determined by the chemicals that are used to aid in the transport of oxygen around the body.

Most animals on earth, us included, have red blood because we make use of a chemical protein called hemoglobin. 

Hemoglobin is structured with an iron atom at its center
Hemoglobin is structured with an iron atom at its center

As you can see in the diagram above, there is an iron atom (Fe) in the center of the compound. It is this iron atom that is used for binding oxygen to the red blood cell, which allows for transport around the body. It is the iron atom in this compound that gives our blood its dark red color.

That said, when the center atom changes, so does the color of the blood.

Hemocyanin has a copper atom at it's center
Hemocyanin has a copper atom at its center

Hemocyanin, unlike hemoglobin, has a copper (Cu) atom at the center of the protein. It’s this atom that is used to bond to oxygen and transport it around the animal’s body.

Because of the copper, the blood of animals that make use of hemocyanin has clear blood when it is deoxygenated, but when the blood is exposed to oxygen, it turns a blue color.

Hemocyanin Instead of Hemoglobin: The “Cooler” Choice

Animals that make use of hemocyanin tend to be found in colder climates or where the presence of oxygen is less abundant.

Hemocyanin, under low oxygen or cold environments, becomes more susceptible to binding with oxygen molecules thanks to a phenomenon known as the Bohr effect.

In other words, the copper atoms in hemocyanin bond easier with oxygen than the iron in hemoglobin, so in environments where there is not a lot of oxygen, hemocyanin becomes the more efficient option.

That said, in high-oxygen environments, hemoglobin is far more efficient at moving oxygen around a body but loses this effect as temperatures drop.

Does Having Blue Blood Benefit Animals?

Although blue-blooded animals may seem strange at first, their unique blood color and use of hemocyanin are special adaptations that bring these creatures some key advantages.

  1. Camouflage: In an underwater world where much is blue and green, blue blood can help animals blend into their environment.
  2. High Oxygen Affinity: Hemocyanin increases its ability to bond to oxygen at lower temperatures which makes it more efficient in cold or low-oxygen environments like the deep sea.
  3. Increased Carry Capacity: Hemocyanin is less efficient at transporting blood than hemoglobin; however, because hemocyanin is free-floating, this can allow for a greater density, which could increase oxygen carry capacity with the loss of energy due to increased blood viscosity.
  4. Immune Support: Some research has found that hemocyanin contains antibacterial and antifungal properties that can help protect the host from pathogens which ultimately supports their immune system.

Blue Blood Animals and Their Relation to Humans

If our hemoglobin were replaced with hemocyanin, we would need an entire chemical and physical makeover in order to function properly, but this hasn’t stopped scientists from finding a use for the blue blood.

One such use comes specifically from the Horseshoe crab. These giant blue-blooded crabs contain a chemical in their blood known as Limulus Amebocyte Lysate (LAL).

The LAL test is created from the blue blood of a horseshoe crab
The LAL test is created from the blue blood of a horseshoe crab

It is this chemical that is harvested and used in the medical industry to test for bacterial endotoxins in vaccines and medical products via a test known as the LAL test.

Although this has become a cornerstone in the medical industry, the means of extracting the Limulus Amebocyte Lysate is cruel due to the bleeding process. Due to its importance, there are worries about over-harvesting.

That said, a synthetic version of LAL named Recombinant Factor C (rFC) has been created and is under current debate on whether it is as efficient as Horseshoe crab blood.

Which Ocean Animals Have Blue Blood?

By now, you probably know that most of the animals in the ocean have red blood. However, a few species have blue, green, or violet blood.

Horseshoe crabs have blue blood thanks to the presence of hemocyanin
Horseshoe crabs have blue blood thanks to the presence of hemocyanin

Some animals in the ocean that have blue blood include:

  1. Lobsters: As lobsters are bottom dwellers, the use of hemocyanin aids in better oxygen transportation.
  2. Horseshoe crabs: Due to high levels of hemocyanin, Horseshoe crabs have a high resistance to bacteria and disease, which helps prevent infection.
  3. Sea snails: Sea snails move slowly and require low amounts of energy. This supports increased levels of hemocyanin in their blood, which makes oxygen readily available.
  4. Squid and cuttlefish: Squid and cuttlefish use hemocyanin to ween out competing bacteria on their skin that would otherwise take over their defensive glowing bacteria.


Are there animals with different colors of blood than blue or red?

Blood comes in many colors, including blue, red, violet, yellowish, and green. Some worms have green blood due to the presence of chlorocruorin, some insects have yellow or green blood due to the presence of a yellow or green pigment called hemolymph, while some marine worms have violet blood thanks to the presence of hemerythrin.

How do blue blood animals regulate their copper levels?

As high levels of copper can become toxic, it’s important for animals that make use of hemocyanin to depose excess Cu. To do this they make use of specialized proteins that bond to the unused copper and are then excreted from the body.

Do all animals with hemocyanin have blue blood?

Blue blood is the most common pigment when hemocyanin is present. However, not all animals with hemocyanin have blue blood. One example is some species of squid, which appear to have greenish blood due to other present pigments.

Can blue blood animals survive in other environments besides low-oxygen environments?

Blue blood animals can survive in a range of environments beyond low-oxygen environments, but they may be less efficient at oxygen transport in these situations. As hemocyanin absorbs more oxygen in colder temperatures, warmer weather with more oxygen requires different adaptations for the species to thrive.

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