Blood comes in a variety of colors across the animal kingdom, but purple blood is exceptionally rare. Only one family of sea creatures is known to have violet-hued blood flowing through their veins. If you’re curious about these unique animals with purple blood, read on to discover the fascinating science behind their curiously-colored circulatory fluid.
Here’s a quick answer to your question: The only animals found to have purple blood belong to a family of sea creatures called chordates – specifically sea squirts in the subphylum tunicata. Their blood gets its violet color from hemocyanin, a copper-containing protein.
In this comprehensive article, we’ll uncover what exactly makes the blood of these sea creatures appear purple, which species have purple blood, and how this strange blood color helps them survive in the ocean.
We’ll also look at some other animals with unusually-colored blood like green and blue, and compare the different blood pigments across species.
What Makes Blood Purple?
The surprising purple color of some animals’ blood is caused by a protein called hemocyanin. Hemocyanin contains copper and is fundamentally different from the iron-containing hemoglobin that makes human blood red.
Hemocyanin Causes Purple Blood
In most animals with hemoglobin-based blood, the iron in hemoglobin binds with oxygen to transport it throughout the body. But in mollusks like snails and cephalopods like squid and octopuses, the oxygen-binding protein hemocyanin contains copper instead of iron.
When copper binds with oxygen, it turns the blood a bright blue color. The blue blood appears purple when viewed through the animal’s semi-transparent tissues and skin. This unique purple blood allow mollusks and cephalopods to survive in low-oxygen environments like the ocean floor.
Some arthropods like horseshoe crabs also have hemocyanin-based blood. Their blood appears blue rather than purple because it is better-oxygenated in their air-filled book lungs. Horseshoe crab blood is being researched for medical applications due to its strong immune response.
Comparing Hemocyanin to Hemoglobin
Hemoglobin and hemocyanin have some key differences:
- Hemoglobin contains iron, hemocyanin contains copper
- Hemoglobin is red when oxygenated, hemocyanin is blue
- Hemoglobin is found in vertebrates, hemocyanin is found in some invertebrates
- Hemoglobin is more efficient at transporting oxygen
The table below summarizes the main differences between these two proteins:
| Property | Hemoglobin | Hemocyanin |
|---|---|---|
| Metal ion | Iron | Copper |
| Color when oxygenated | Bright red | Blue |
| Found in | Vertebrates | Some invertebrates like mollusks and arthropods |
| Oxygen transport efficiency | High | Lower than hemoglobin |
Which Animals Have Purple Blood?
Sea Squirts Have Violet-Colored Blood
Sea squirts, also known as tunicates, are the only animals known to have violet-colored blood coursing through their bodies. Their blood features a copper-containing protein called hemocyanin which is colorless when oxygenated yet turns violet-purple when deoxygenated.
The vibrant violet hue helps distinguish sea squirts within the animal kingdom. There are over 3,000 species of sea squirts identified so far inhabiting ocean floors worldwide. These creatures filter seawater for nutrients and are immobile, attaching themselves to submerged objects like coral reefs or even ship hulls.
The primitive sea squirt circulatory system uses a simple tubular heart to pump the violet blood, which lacks true capillaries and veins. Despite the strange blood color, sea squirts actually share common ancestry with vertebrates.
Sea squirt larvae exhibit spinal cord-like nerves and tails during early development, yet resort to a sessile lifestyle once fully grown. While human blood uses iron-rich hemoglobin proteins, the copper-infused hemocyanin is more efficient for oxygen transport in cold seawater habitats.
Purple Blood is Unique to Tunicates
Of all organisms, the curious chemistry producing violet blood pigmentation seems exclusive to the tunicates. No other major animal group manifests such an anomaly. Technically speaking, red blood cells in mammalian vertebrates also change colors depending on oxygenation status.
The iron in hemoglobin makes blood appear more “cherry” red when oxygen-rich yet darker red-purple when oxygen runs low. But this is temporary and only slightly alters the vivid red predominating normal blood.
By contrast, the blue copper proteins make tunicate blood permanently purple-violet in color regardless of oxygen content. And unlike the iron conveying oxygen through blood cells in vertebrate animals, the copper proteins simply dissolve in sea squirt plasma.
WhileVulcanoctopus hydrothermalis snails and some other invertebrates also rely on copper-rich hemocyanin rather than hemoglobin, they do not exhibit the trademark violet blood tint.
| Animal Group | Blood Pigment Type | Blood Color |
|---|---|---|
| Mammals | Hemoglobin | Red |
| Sea squirts | Hemocyanin | Violet |
Why Do Sea Squirts Have Purple Blood?
Hemocyanin Efficiency in Cold Water
Sea squirts, also known as tunicates, are marine invertebrates that have purple-colored blood due to the presence of hemocyanin, an oxygen-binding protein that contains copper. Hemocyanin helps sea squirts efficiently transport oxygen through their blood in cold ocean environments (1).
Unlike the iron-containing hemoglobin in human blood that turns red, the copper-containing hemocyanin turns blood blue or purple when oxygenated. At depths with high water pressure and low temperatures, hemocyanin binds oxygen more readily than hemoglobin.
The purple hue provides sea squirts with better oxygenation for respiration in frigid, hypoxic conditions.
Studies show that the hemocyanin oxygen binding affinity in sea squirts increases substantially as habitat temperature decreases (2). For example, at 4°C binding affinity can be 10 times greater than at 20°C. This allows efficient oxygen uptake even when very little O2 is dissolved in cold water.
Some species like Ciona intestinalis have evolved complexes of multiple hemocyanin isoforms to optimize oxygen binding over a range of temperatures and oxygen partial pressures in the changeable intertidal zone (3).
The purple oxygen-carrying pigment is thus perfectly adapted to function for sea squirt survival in cold, oxygen-poor marine climates.
Camouflage and Predator Defense
In addition to respiratory efficiency, the violet-hued hemocyanin serves a secondary purpose for disguise and protection from predators. Sea squirts use tunicates containing cellulose and proteins to attach themselves to surfaces like rocks and piers (4).
The translucent tunic is often tinted purple from the blood circulating within. This provides camouflaging coloration to match the hue of coastal marine vegetation and blend into the environment. Protective mimicry makes it harder for predators like fish, crabs, and sea stars to distinguish the sea squirts as prey.
If predators try attacking, some sea squirts can explosively contract their bodies to spray sticky, noxious purple blood through siphons as a defense mechanism (5). This distracts predators with an disorienting, inky cloud while allowing the sea squirts to escape.
The peculiar purple blood is thus a key adaptation enabling crypsis and survival in the dynamic, competitive ecology of ocean communities. Next time you see aHint of purple in a tide pool, it’s likely from one of these fascinating filter-feeding invertebrates!
| Sea Squirt Species | Predator Defense Strategy |
|---|---|
| Ciona intestinalis | Ink cloud of purple blood |
| Pyura chilensis | Foul toxins in blood |
| Styela plicata | Noxious chemicals |
Research shows there are over 3,000 diverse species of sea squirts around the world, occupying every ocean (6). Though they may appear simple, these creatures are highly adapted with purple blood at the heart of their survival.
The next time you spot a dash of violet in the sea, take a closer look at the fascinating science behind it!
References:
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3684273/
- https://jeb.biologists.org/content/204/22/4059
- https://pubmed.ncbi.nlm.nih.gov/9155004/
- https://askabiologist.asu.edu/explore/what-are-sea-squirts
- https://www.amnh.org/exhibitions/permanent/ocean-life/simple-animals/sea-squirts
- https://www.britannica.com/animal/sea-squirt
Other Animals with Unusual Blood Colors
Green Blood in Some Worms
Certain worms and other invertebrates actually have green blood flowing through their bodies! This green coloration comes from biliverdin, a green pigment that is related to bilirubin, the substance that gives human blood its red hue.
Some species of worms in the Pacific Ocean have blood that contains extremely high levels of biliverdin and literally glows bright green.
Scientists think these creatures evolved to have green blood because the biliverdin pigment carries some useful benefits. It functions as an antioxidant and helps bind toxic heavy metals. Additionally, since green wavelengths of light penetrate ocean water deeper than other colors, having green blood might provide additional camouflage.
Blue Blood in Horseshoe Crabs
The blood of horseshoe crabs is blue in color, instead of the familiar red hue found in vertebrates. Their blood contains hemocyanin, a copper-containing protein that turns blue when oxygenated. Horseshoe crab blood is exceptionally sensitive to toxins and bacteria.
In fact, a substance made from horseshoe crab blood called Limulus amoebocyte lysate is used by pharmaceutical and medical device companies to test their products for contamination.
Colorful Blood Across the Animal Kingdom
From bright green blood in some worms to the bluish color in horseshoe crab blood, it is clear that blood comes in all shades across the diverse animal kingdom. Scientists have identified important functions and benefits for these oddly-colored types of blood, including detoxification, camouflage, and detecting hazards.
Additionally, these animals produce unique proteins and pigments not found in human blood. Continuing to study these biochemical oddities found across species has the exciting potential to unlock new medical insights that may one day benefit humans as well.
As research on these fascinating creatures continues, perhaps even more surprisingly-colored blood remains to be discovered in animals throughout the world!
Conclusion
The violet hue of sea squirt blood is a rare and fascinating anomaly in the animal world. Their purple plasma comes from hemocyanin, a blood pigment that makes oxygen transport efficient in cold marine environments.
While humans rely on iron-rich hemoglobin for red blood, sea squirts and other invertebrates use copper-based hemocyanin instead.
So if you ever spot a sea creature with a purple tint, it likely belongs to the special chordate group known as tunicates. Beyond these unique organisms, blood comes in an array of colors across animals and their adaptations. But the purple blood of sea squirts remains an awe-inspiring rarity.
