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From a bruised nose or a fingertip that has been pricked, our blood invariably exhibits a shade of red. However, this vibrant hue is not a universal characteristic across the animal kingdom. In some species, vital fluids manifest as purple, yellow, green, or even white. The question arises: what accounts for these diverse colours in this life-sustaining liquid?
To explore this intriguing subject, a deeper investigation into scientific findings was undertaken, with insights from experts in the field, including John Postlethwait, a developmental geneticist at the University of Oregon, and Zachary Cheviron, an ecophysiologist at the University of Montana. Herein lies what is understood regarding the science behind these varying shades.
Understanding Blues and Purples
The striking red hue of vertebrate blood is attributed to haemoglobin, a protein found in red blood cells that contains iron atoms. The primary role of haemoglobin is to transport oxygen, and it is the iron atoms, which bind with oxygen, that give our blood its red appearance. Blood appears a bright cherry red when oxygen is bound to the iron, while a reduced level of oxygen leads to a muddier brownish red.
However, not all organisms rely on haemoglobin. For instance, octopuses, squids, and crustaceans such as shrimp and crabs utilise a protein called haemocyanin for oxygen transport. In contrast to haemoglobin, haemocyanin contains copper atoms, which impart a blue colour when they bind with oxygen. Although haemocyanin is not as efficient at oxygen transport as haemoglobin, it excels in low-temperature environments, such as those found in the ocean’s depths. Furthermore, studies suggest that haemocyanin may have additional functions beyond oxygen transport, potentially playing a role in the immune system, aiding in wound healing, for instance.
Other marine creatures, such as brachiopods—similar to clams—and the peanut worm, which resides on the seafloor, exhibit violet blood. However, this colouration is not due to haemocyanin; rather, these organisms utilise a purple iron-rich pigment known as hemerithrin for oxygen distribution within their blood.
The Green Blood Family
Certain ascidians—marine animals with a soft, potato-like appearance—exhibit a yellow-green blood colour. These creatures do not produce haemoglobin, and there is ongoing debate regarding the source of their yellow hue. One hypothesis suggests that vanadium, an element that many ascidians accumulate in their blood, may be responsible.
Another theory posits that the diet of these organisms influences their blood colour. As ascidians metabolise their primary food source, marine phytoplankton, they produce carotenoids, which are pigments that can be yellow, red, or orange.
Various mechanisms can lead to entirely green blood in animals. Several species of New Guinea lizards employ haemoglobin for oxygen transport, akin to vertebrates. However, they then break down the haemoglobin into a naturally occurring green pigment called biliverdin. This same molecule, when present in elevated concentrations, causes the yellowish appearance associated with jaundice in humans. Unlike most vertebrates, which convert biliverdin into less vibrant compounds, these New Guinea lizards accumulate the pigment in their blood, allowing the green to dominate over the red of haemoglobin in both their bloodstream and tissues.
Unusual and Transparent
The Antarctic icefish, belonging to the family Channichthyidae, is the only known vertebrate that does not produce haemoglobin and lacks red blood cells. Their blood appears milky white and translucent. As Postlethwait observes, they indeed possess a somewhat ghostly appearance.
Instead of haemoglobin or any equivalent oxygen-binding protein, Antarctic icefish rely on oxygen that is dissolved directly in their bloodstream. They have larger hearts and capillaries compared to other Antarctic fish, enabling them to pump and circulate larger volumes of blood. Icefish are patient predators, employing a stalking method to catch their prey, allowing them to conserve energy and reduce oxygen requirements.
A recent study conducted by Postlethwait and his colleagues examined the genomes of various fish species to ascertain why the genes responsible for haemoglobin production have been lost from the DNA of these creatures. It remains a mystery whether Antarctic icefish first lost their red blood cells and subsequently their haemoglobin, or vice versa. What led to this loss is also an enigma. One hypothesis proposed by researchers is that the cold waters of the Antarctic Southern Ocean were sufficiently oxygen-rich to render the intermediate function of haemoglobin redundant.
Haemoglobin’s Extraordinary Adaptations
The peculiar colours exhibited by blood are merely the beginning of the remarkable properties of this vital fluid. Ongoing discoveries continue to emerge. In high-altitude ecosystems, many species must contend with oxygen scarcity, prompting adaptations in haemoglobin among mountain dwellers to enhance its efficiency.
For instance, the Peromyscus mouse has the ability to thrive at elevations of up to 14,000 feet (4,200 metres) above sea level—significantly higher than the altitude at which many humans begin to experience altitude sickness. This mouse has evolved a haemoglobin variant that forms exceptionally strong bonds with oxygen. Similarly, Andean hummingbirds and high-altitude Pelophylax nigromaculatus frogs exhibit analogous modifications in their haemoglobin.
Nevertheless, adaptations often come with trade-offs, explains Cheviron from the University of Montana. The high-altitude Peromyscus mouse, for example, faces challenges in releasing oxygen from its haemoglobin to its tissues. To mitigate the limited oxygen supply, it compensates by possessing a greater number of mitochondria that facilitate oxygen release in its muscle fibres.
The more scientists delve into research, the more they uncover exquisite variations of this nature in blood—cells that are “remarkably adapted by evolution,” as Postlethwait puts it, “which facilitate the wonderful diversity of life forms inhabiting our planet.”
The distinctive blood types found throughout the animal kingdom may also provide insights into human health. The evolutionary journey of haemoglobin in the diminutive Peromyscus mouse could inspire therapies for conditions resulting in hypoxia, according to Cheviron, including chronic obstructive pulmonary disease. Moreover, for the nearly two billion individuals grappling with anaemia, characterised by insufficient red blood cells or haemoglobin levels, the transparent blood of Antarctic animals may offer clues for new treatments. “Clearly, the Antarctic icefish have figured it out,” remarks Postlethwait.
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