Horseshoe crabs are marine animals rightly considered living fossils. Their ancient lineage, distinctive appearance, and remarkable resilience have fascinated scientists for centuries. Yet one of their most extraordinary features lies within — their bright blue blood.
Why Their Blood Is Blue
The blue hue of horseshoe crab blood comes from a special protein called hemocyanin. Like hemoglobin in humans, hemocyanin transports oxygen — but instead of iron, it contains copper, which turns the blood blue when oxygenated.
Interestingly, unlike human blood, which visibly changes color when oxygen is lost, horseshoe crab blood remains visually unchanged during oxygen depletion.
Not the Only Blue-Blooded Species
Horseshoe crabs aren’t the only creatures with blue blood. The same oxygen-carrying protein, hemocyanin, is also found in:
- mollusks (such as octopuses, squids, and snails),
- arthropods (including scorpions, lobsters, and some crab species).
Fighting Infections With Blood
Horseshoe crab blood has a second, even more valuable property. It contains specialized cells that instantly react to endotoxins — harmful substances released by certain bacteria. When in contact with these toxins, the cells rapidly coagulate, forming a clot to isolate the threat.
This unique biological defense is the basis for the pharmaceutical LAL test (Limulus Amebocyte Lysate), a critical method used to test the purity of vaccines and medical products. It can detect even microscopic traces of bacterial toxins that could be deadly if injected into the human body.
"Thanks to the horseshoe crab's blue blood, we’ve been able to prevent countless infections and sepsis cases during mass vaccinations," experts say.
Millions of Lives Saved
The use of the LAL test has become essential in modern medicine. Every batch of injectable medication, including vaccines, is tested using this method. It is estimated that this ancient marine species has helped save millions of human lives.
Rising Hope Through Synthetic Alternatives
Given the limited population of horseshoe crabs and the importance of their blood, researchers began seeking alternatives in the 1990s. A synthetic molecule mimicking the crab’s toxin-detecting function has since been developed and is gradually being adopted in pharmaceutical manufacturing.
This advancement helps reduce the need for harvesting horseshoe crab blood and ensures more sustainable practices for both science and nature.
