What colour is fish blood?

In this article, we will answer the question “What colour is fish blood?” and understand more about fish blood, their vascular system, and also about other blood colours.

What colour is fish blood?

In general, fish blood is red, similar colour to human blood. However, there are few colourless- or clear-blooded fish species. 

The red colour is because of the presence of an oxygen transporter protein-rich iron. The transporter cells are known as Red blood cells, and the protein that binds and carries the oxygen is the haemoglobin.

Blood circulation in fish

Differently from birds and mammals, the fish heart has only two chambers, one atrium and one ventricle. In fish, the blood circulation is single-loop motion. This type of circulation drives the CO2-rich blood through the heart, which will pump it through the branchia. In the branchia or gills, is where oxygenation takes place. Then, the blood keeps circulating to reach the body tissues and organs and distributes the oxygen.

In addition to oxygen, the blood is also responsible for transporting and distributing many other compounds. Blood is composed of many cell types. 

Functions of blood in fish

The blood is an important way to distribute substances and also performs multiple other functions. Below are the blood functions in fish:

–       Oxygen supply to all the body tissues and organs;

–       Waste excretion, such as urea, ammonia, carbon dioxide, etc.;

–       Nutrients collection from digestion, with further distribution to tissues and organs;

–       Hormones, nitrogen, electrolytes, and white cells assist in physiological activities, immunity, reproduction, and growth;

–       Water, protein and other solutes distribution to tissues and organs.

What about sharks?

Sharks also have red blood, which is the result of the presence of red blood cells, rich in haemoglobin with iron. 

Sharks and rays are members of a group of cartilaginous fish, which means they do not have a bony skeleton. On vertebrates, the red blood cells are usually produced by the red bone marrow. However, how do sharks produce their red blood cells without bone tissues and, thus, without red bone marrow?

Sharks and rays make their red blood cells by the spleen, specialized tissue nearby the reproductive organs, in some cases, by a structure called “Leydig’s Organ”. The Leydig’s organ, when present, is located close to the oesophagus. It is usually a quite large organ, weighing up to 1.6 kilograms.

Why does blood have different colours?

The main role of the blood is to transport oxygen and distribute it into the different body tissues and organs. However, to complete this function, some animals have specific proteins and specialized cells. These proteins are rich in substances that are responsible for giving colours to the blood. The reasons why blood has other colours and animals that have them are listed below:

Blue blood

Some organisms have other protein types that will transport oxygen through their vessels. In the case of blue-blooded animals, this protein is hemocyanin. Hemocyanin holds copper instead of iron. Thus, the blood colour turns blue. Some of these organisms are: 

–       Octopuses;

–       Horseshoe Crabs;

–       Spiders;

–       Scorpions.

Green blood 

Green blood can be found in a variety of animals. The green colour is normally associated with chlorocruorin protein instead of haemoglobin. Chlorocruorin also holds iron in its composition, however, with a different chemical structure, for this reason, the blood of these organisms does not turn red. Some animals that have chlorocruorin as their blood pigment are polychaetes. 

However, there is also another reason for green-blooded animals to have green blood. Some lizard species from the skink family do not have chlorocruorin as the oxygen transporter protein. Their oxygen transportation is based on haemoglobin, such as human blood. Although they have high concentrations of biliverdin, a tetrapyrrole protein. 

The high concentration of biliverdin is responsible for giving a lime green colour to the blood of these animals. This protein is usually toxic to animals in high concentrations, which is different in respect to these lizards from New Guinea. Chlorocruorin is classified as metabolic waste, a result of the natural red blood cells breakdown. They do not excrete this substance and, as a result of this extremely high concentration, they also have green muscles, skin, and tongue.

Studies showed that some other organisms can hold quite significant biliverdin concentrations, Thus, biliverdin can be found on the body of hornworms, fish, frogs, and even some mammals. In some marine fish species, this protein gives a blue-green colouration to their blood.

Leeches similarly hold biliverdin in their bodies. Consequently, their blood also has a greenish tone.

Purple blood

Some marine animals have a purple tone in their blood. This purple shade is due to the presence of an oxygen transporter protein called hemerythrin. Hemerythrin is also rich in iron. However, the hemerythrin remaining chemical composition is different from the human haemoglobin. For this reason, the blood with hemerythrin does not become red. Some animals that have hemerythrin, and consequently, purple blood are:

–       Sipuncula worms, or peanut worms;

–       Brachiopods.

Hemerythrin can assist the organisms in fighting and beating infections. It also benefits the animals when regenerating if a part of their body is lost. Some scientists suggest that hemerythrin is a crucial substance during the regeneration and reconstruction process.

Yellow blood

The protein that gives the blood a yellow colour is vanabin. When the organisms have a high concentration of this protein in their blood, the colour becomes yellow. However, the vanabin is not an oxygen transporter protein, thus it does not assist in the oxygen distribution through the body. Some beetles and sea cucumbers have vanabin-rich blood.

Why do some fish have colourless blood? 

Some fish do not have any oxygen transporter protein in their blood. Thus, their blood is classified as white or clear blood. Some of them may not need it, while others cannot have it due to their blood density. Some of these organisms are the Icefish, a group of 16 extremophile species inhabiting cold waters.

The Ocellated icefish inhabits Antarctic waters. Thus, to live in extremely cold conditions, they are required to develop some adaptations. One of these adaptations is the absence of oxygen transporter protein in their blood. If they kept the blood with a high cell concentration, their blood becomes very thick and slow-moving in the vascular system. For this reason, fish that inhabit extraordinarily cold waters have fewer blood cells, and, consequently, clear blood.

How do these fish distribute oxygen to their tissues and organs without the transporter protein? Colder waters naturally have higher usable oxygen concentrations than warmers. For this reason, the oxygen is naturally distributed through the vessels to the tissues and organs without the support of any transporter cell or substance.

Which animals have no blood?

Some animal groups did not develop vascular systems based on vessels and blood. These animals are:

–       Jellyfish;

–       Corals;

–       Sponges.


Jellyfish are very simple organisms, without brain and vascular complexity. They have a very simple sensory system and the nutrients are obtained directly from the water or digestive cavity. 


Corals are usually very colourful organisms. However, they are typically sessile or semi-sessile colonies or individuals, which makes some people believe they are plants. Corals do not have blood. They obtain their nutrients from the zooplankton and symbiotic relationships with algae.

The Sea anemones are also corals. However, they have paralysing toxins on their tentacles to capture their prey. Thus, their tentacles keep the prey inside the mouth and then, they obtain their nutrients and other essential substances.


Sponges also do not have blood. They have simple layers of cells on their body. One specialized cell is responsible for the water movement through the sponge and for obtaining food and substances from the water. These cells are called choanocytes.


In this article, we answered the question “What colour is fish blood?” and learnt more about fish blood, their vascular system, and also about other blood colours.

If you have any thoughts or doubts, feel free to drop us in a comment below!

Frequently Asked Questions (FAQs): What colour is fish blood?

What are the blood colours?

Blood colour mostly depends on the oxygen transporter protein. Thus the blood can be red, blue, green, purple, and colourless, or white/clear.

What animals do have red blood?

Most vertebrates have red blood. Fish, amphibians, reptiles, birds and mammals, in general, have haemoglobin in their cells as oxygen transporter.

What animal blood is not red? 

Usually, the animals that do not have red blood are invertebrates. However, there are some vertebrates with other blood colours. They are the New Guinea Skink lizard with green blood and the Icefish species with clear blood.

Which fish blood is white?

There are around 16 Icefish species that do not have coloured blood. Their blood is called clear or white. They are members of the family Channichthyidae and are extremophiles, inhabiting extraordinary cold Antarctic waters.

What animal has purple blood?

Sipuncula or peanut worms. They are marine worms that have hemerythrin as the oxygen transporter protein in their blood.

Are all fish cold-blooded?

No, there is a unique single fully warm-blooded fish that circulates heated blood throughout its body. These fish are called Opah.


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Mancini, M. 2021. 5 Animals Whose Blood Isn’t Red. https://animals.howstuffworks.com/animal-facts/5-animals-whose-blood-isnt-red.htm

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