Horseshoe Crab Could Survive Mass Extinction But Can They Survive Covid?

Adithya Joseph

12th January, 2021


The Atlantic horseshoe crab’s unique blue-colored blood can detect endotoxins and it has become an essential part of testing the safety of drugs and vaccines. Hence it has a crucial role in the fight against the COVID-19 virus. Conservationists are worried about the dependence of the biomedical industry on these crabs. Even though synthetic alternatives have been discovered, a significant shift in usage has not yet been observed.


The Atlantic horseshoe crab or Limulus polyphemus is one of the oldest living things on the planet. They have been around for the last 450 million years and have largely remained unchanged, hence they are known as the living fossil. They have outlived the dinosaurs and survived 5 mass extinctions- including one that nearly wiped out life on earth. The horseshoe crab gets its name from the horseshoe-shaped exoskeleton that protects its interior. They are ancient marine arthropods that belong to the class Merostomata, which means legs attached to the mouth. They also have 10 eyes (or light-sensing organs) distributed throughout their body. Studies of the crab’s compound eyes have led to Nobel prize-winning research on the neurophysiology of vision in the year 1967. 

What makes them unique?


Horseshoe crabs use hemocyanin to carry oxygen through their blood. The hemocyanin contains copper which makes their blood blue. Instead of producing antibiotics like human blood to fight infection, the horseshoe crab immune system releases proteins that can surround and kill bacteria. This blue, copper-based blood contains a substance called Limulus amebocyte lysate (LAL), which clots in the presence of bacterial toxins. The horseshoe crab’s armored body and the LAL in its blood have protected it from mass extinctions and predation for hundreds of millions of years.

The commercial importance of Horseshoe Crabs


Their milky-blue blood is the only known natural source of Limulus amebocyte lysate (LAL). Their blood is exquisitely sensitive to toxins from bacteria. Endotoxin testing LAL test ensures that sterile pharmaceutical products like vaccines, sterilizing solutions, and prescription drugs are free from bacterial toxins and hence are safe for human use. Endotoxins are bacterial structural components that are released when such a cell is lysed. These components are toxic if administered to humans, causing a pyrogenic response. For this reason, drugs and medical devices that are either injected or implanted must be tested for their endotoxin content.

Amebocytes have a unique ability to produce an instantaneous, visible reaction to endotoxins. This has driven commercial demand from pharmaceutical and biomedical companies to use the Limulus amebocyte lysate (LAL) test, which has become the method of choice for endotoxin detection to test for the safety of the drug and medical devices. The blue blood of these crabs is expensive. One liter of their blood coats about 21 lakh rupees. With the wide usage of horseshoe blood in medicine and research, industries have developed to extract the blood out of the horseshoe crabs. The horseshoe crabs lay their eggs usually during May or June. During high tide, the females come to the surface of the beach, dig numerous holes, and lay about 80,000 eggs in two batches.  During the warm months especially at night with full moons, horseshoe crabs emerge from the sea to spawn where they are captured by lab workers in hundreds or thousands and are taken to the lab to culture their blood. By puncturing the heart with a needle, about one-third of each horseshoe crab’s total blood volume is extracted. Then they are returned to the ocean. To extract about one liter of LAL, about 300 crabs are needed. Each year from about 6 lakh carbs blood is extracted. While most animals survive this process, numerous researches have shown that among the ones that are released back to the shores, only 30% survive. It has been shown that bleeding reduces breeding potential. Most of the crabs are captured from the Delaware Bay, a northeast seaboard in America where they come to lay their eggs. 

Horseshoe Crabs’ unique blood and COVID vaccine 


As scientists around the globe are trying to make vaccines for coronavirus, billions of doses will have to be tested. Conservationists fear that this will drastically increase the demand for LAL and hence lead to the massive killing of these crabs. In 1997, scientists at the National University of Singapore developed a laboratory-synthesized recombinant Factor C (rFC) which could have been used as an animal-free endotoxin detection technology. They cloned the DNA of a factor C molecule and synthesized rFC, the synthetic alternative to the LAL test. This synthetic substitute for LAL known as Recombinant Factor C (rFC) has been available for nearly two decades and has recently been approved in Europe. However, according to Pharmacopeia (USP), the organization that makes decisions about drug safety in the USA, announced that the rFC test requires more research before it can be approved. Companies that want to export medicines or vaccines to the USA will have to continue testing them using LAL from horseshoe crab blood.




The biomedical applications of horseshoe crab blood create a huge demand for it. They are also harvested as bait. The effects of climate change like increasingly frequent and intense storms and rising sea levels diminish the availability of suitable spawning habitat for the horseshoe crab. Horseshoe crabs are a keystone species, and they are an essential part of the ecosystem. A lot of ocean animals including sea turtles feed on the eggs. Their eggs are an important food source for at least 11 species of migratory shorebirds. An endangered species- the red knot relies only on these eggs as their food before migrating to the arctic to lay eggs. All these factors are leading to the decline of these crab populations. Hence proper conservation of this species is required with immediate effect.




  1. The Atlantic Horseshoe Crab and COVID-19 from 

  2. Horseshoe crab blood is key to making a COVID-19 vaccine-but the ecosystem may suffer. Wechsler& Fadek from

  3. Ventana al Conocimiento (Knowledge Window) Scientific journalism Estimated reading time Time 4 to read, Ventana al Conocimiento (Knowledge Window) Scientific journalism, Window), & journalism from 

About the Author

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Currently pursuing Biotechnology. Interested in learning more about human genetics and molecular biology. Science enthusiast and interested in reading.