Whether it’s a tiny snail or a giant whale, depending on the species, most living creatures on Earth rely on the circulation of blood within their bodies to survive.
This precious fluid combats infections, delivers nutrients and gases to organs, and transports waste products like CO2 away from body tissues.
In some animal species, blood also serves as a special defense weapon. How do they manage to fend off enemies? There are also species of animals that have no blood flowing in their bodies; how do they survive?
What Determines Blood Color in Animals?
When we think of blood, we often picture it as red and rich in iron. However, in reality, the blood of many animal species comes in a variety of colors, not just red. This depends on the type of oxygen-carrying protein in the blood.
- For example, some species of crustaceans, squid, and octopuses have blue blood due to hemocyanin – a type of oxygen-carrying protein – that contains copper. In marine animals, hemocyanin is colorless but turns blue when it binds to oxygen, according to marine biologist Stephen Palumbi at Stanford University (USA).
Christopher Coates, a comparative immunologist at Swansea University in Wales, stated that hemocyanin appeared around 2.5 billion years ago and was initially used to neutralize oxygen for primitive organisms in Earth’s anaerobic or low-oxygen environments. Later, as the atmosphere became more oxygen-rich, this protein evolved to distribute oxygen throughout the organism’s body.
- Hemoglobin (found in humans) evolved much later, approximately 400 million years ago. Christopher Coates explained that this may have occurred because vertebrates have a more complex respiratory system than simpler organisms. Indeed, most blood in mammals, fish, reptiles, amphibians, and birds is red due to hemoglobin, a type of protein formed from heme (iron-containing molecules that bind with oxygen).
- Hemerythrin is another iron-containing protein that binds with oxygen and gives a pinkish-purple hue to the blood of certain mollusks, such as brachiopods and marine worms, according to the American Chemical Society.
Brachiopod with a pinkish-purple color.
On Earth, there is a fish known as the Crocodile Icefish, also referred to as “white blood fish” because it is completely devoid of blood pigments due to a genetic mutation that removed hemoglobin from its body. In the freezing habitat of up to -1 degrees Celsius in Antarctica, oxygen and gases readily permeate through the fish’s gills and skin.
Insects lack blood; instead, they possess a similar fluid called hemolymph that is yellow or green in color, which helps transport hormones and gases throughout their bodies, except for oxygen. Insects absorb oxygen directly through openings along their bodies or backs.
Blood as a Weapon
To ward off parasites, the New Guinea Green Tree Lizard (Prasinohaema prehensicauda) accumulates a large amount of a bile pigment called biliverdin. Biliverdin is formed when red blood cells break down and is retained in the plasma, the liquid that makes up most of the blood. In large quantities, it completely masks the red color of hemoglobin.
The bile pigment biliverdin helps the New Guinea Green Tree Lizard eliminate parasites in their blood, particularly those causing malaria (a common and debilitating disease in lizards). Additionally, biliverdin also causes their bodies to emit an unpleasant odor, deterring many predators.
New Guinea Green Tree Lizard. (Photo: Christopher Austin/The Conversation).
Due to biliverdin’s green color, the blood, bones, tongue, muscles, and mucous membranes of this species are similarly colored.
Humans and other animals also produce biliverdin, but we excrete it into the intestines and eliminate it from the body before it accumulates to toxic levels.
In the New Guinea Green Tree Lizard, the concentration of biliverdin is over 40 times higher than that of a person with jaundice. This level is extremely high and very toxic in humans, The Columbian reported.
“In other animals, the liver processes excess biliverdin “like an oil filter in a car, removing impurities to keep the engine running well.” But if a person had a biliverdin level similar to that of the New Guinea Green Tree Lizard, it could be fatal,” said Chris Austin, director of the Museum of Natural Science at Louisiana State University (USA).
The U.S. National Library of Medicine states that biliverdin is a water-soluble tetrapyrrolic compound formed from the breakdown of heme. Heme is broken down into biliverdin, CO2, and metallic iron by an enzyme called heme oxygenase. The accumulation of biliverdin, due to increased hemolysis or liver damage leading to impaired glucuronidation, results in jaundice.
Horned lizard squirting blood to trick predators into thinking it is dead.
Some animals can use their blood as a defense mechanism known as reflex bleeding or autohemorrhaging.
The horned lizard found in the southwestern United States and Mexico employs a unique escape tactic. When confronted by a predator, they rapidly squirt blood from their eyes, making the opponent think they are dead and halting the attack. Once the enemy is distracted, the horned lizard makes a quick getaway.
Some insect species, such as the Asian ladybug, possess “a very toxic, foul-smelling liquid.” When threatened, they can spray this “cocktail” from their eyes or leg joints to repel enemies.
Their relatives, the beetles, also spray a blood-like red liquid from their mouths for similar purposes.
Creatures Without Blood
Some animals lack blood or a circulatory system simply because they do not need them.
For instance, flatworms lack a circulatory system; gas exchange occurs directly through their skin. Oxygen moves straight to their tissues while nutrients are provided through diffusion from the gut.
Jellyfish and sponges also absorb oxygen through diffusion. For starfish and sea cucumbers, water functions similarly to blood, transporting nutrients and gases through a water vascular system instead of blood vessels.
