Can blood really be blue? Yes, it can. Many familiar animals have blue blood. Shrimps have blue blood, and so do crabs. Humans and other four-legged vertebrates, however, have red blood.
Why do different animals have different coloured blood? The answer lies in the chemicals it contains. Human and vertebrate blood contains haemoglobin, a coordination complex made of iron (Fe) and an organic compound called porphyrin. Blue blood, on the other hand, contains haemocyanin, another coordination complex made of copper (Cu) and porphyrin.
Whether it is the iron in haemoglobin or the copper in haemocyanin, both serve essentially the same purpose: carrying oxygen from the air through the bloodstream to every part of the body.
Blood is extremely valuable. In many medical conditions, blood from healthy people is transfused into patients. Humans also make use of the blood of other animals for various purposes. Today, there is a great demand for blue blood around the world. But where does it come from?
Blue blood is collected from the bodies of certain crabs. This blood, and the substances it contains, are used in many fields. Before discussing those uses, let us first learn a little about crabs.
Crabs appeared on Earth about 300 million years ago, long before the dinosaurs. They possess an extraordinary ability to survive in harsh environments. Around 6,000 species of crabs are known today. These mysterious crustaceans belong to the phylum Arthropoda. They come in many different shapes and sizes, and their body structure, movement, and way of life are quite different from those of most other animals.
Their bodies are protected by a thick external skeleton, or exoskeleton, made of chitin and minerals. This hard covering shields their soft internal organs. Most crabs have ten legs. The front pair serves many purposes, including catching prey, defending themselves, and handling food. The remaining eight legs are used for walking and swimming. Rather than moving straight ahead, crabs usually walk sideways.
Crabs are found all over the world—in salty seawater, freshwater rivers and ponds, and even on dry land, depending on the species.
Crab is a favourite food for many people. Its meat is rich in protein, omega-3 fatty acids, riboflavin, and selenium. Crab farming and export are important industries in many countries.
But crabs are valued for more than just their meat. Humans also make use of their blue blood. A small amount of blood is drawn from a species known as the horseshoe crab, after which the animal is released back into the sea. Unfortunately, not all of them recover. At least 20 percent die from blood loss or injuries sustained during the process. As a result, their numbers are steadily declining.
Why is horseshoe crab blood so valuable?
Their blue blood contains a remarkable substance called amebocyte, a type of protein. Amebocytes form the horseshoe crab's primary defence against infection. When harmful bacteria enter the animal's body and release toxins, the amebocytes immediately react by forming a gel around the toxins. This process neutralises the bacterial toxins and prevents them from spreading.
This extraordinary protein is of immense importance to humans. Amebocytes collected from the horseshoe crab (Limulus polyphemus) are used in a laboratory test called the Limulus Amebocyte Lysate (LAL) test, which detects bacterial contamination in medicines, medical equipment, and injectable drugs. The protein also plays a crucial role in the manufacture and safety testing of many medicines and injections.
Horseshoe crabs are therefore not only fascinating survivors from Earth's distant past but also vital to modern medicine. They also play an essential role in maintaining the biodiversity of marine ecosystems. If their population continues to decline, it will not only harm the oceans but could also have serious consequences for human health.
