Why modern medicine needs the horseshoe crab

Remaining virtually unchanged since they first crawled the earth over 450 million years ago, these ancient creatures (sometimes referred to as living fossils) have outlived almost every other species—mostly thanks to their incredibly robust immune systems. The unique way their blood clots in the presence of endotoxins makes them incredibly useful for detecting harmful bacteria.  

In this article, we take a closer look at horseshoe crabs and what they do for us. 

What are horseshoe crabs?

The horseshoe crab is a species of arthropod that, despite its name, is technically more closely related to spiders and scorpions than to crabs. There are four species belonging to the horseshoe crab family: the Atlantic, the Mangrove, the Indo-Pacific and the Chinese horseshoe crab. 

Limulus polyphemus aka the Atlantic horseshoe crab
Limulus polyphemus aka the Atlantic horseshoe crab

What makes them so special?

Quite simply - their blood. Surprisingly, the mesmerising blue colour—due to its high copper content—is not the most fascinating feature of horseshoe crab blood. What is of greatest interest to scientists is the ‘amebocyte’ cells it contains. These specialised cells protect horseshoe crabs from bacteria and viruses by forming a gel and clotting around these invaders, preventing them from spreading and multiplying within the crab’s system. This immune defence mechanism is not unusual for animals. What is unique about the amebocytes in horseshoe crab blood is their extreme sensitivity to deadly endotoxins (molecules from the cell walls of certain bacteria) and the rapid and vigorous clotting that occurs as soon as these enter into their blood streams. 

When this high sensitivity and rapid clotting process was first observed in 1968, scientists immediately noted the potential for application in human health. So began the development of the Limulus amebocyte lysate (LAL) test for the detection of endotoxins produced by harmful bacteria.

The trademark baby-blue blood of the horseshoe crab is even more fascinating than it looks
The trademark baby-blue blood of the horseshoe crab is even more fascinating than it looks

The Limulus amebocyte lysate (LAL) test

The LAL test is named for the creature whose observed immune behaviour led to the discovery of the test, the Atlantic horseshoe crab (scientific name Limulus polyphemus).

The LAL production process involved collecting horseshoe crabs from the beach and taking them to one of only five production labs globally. There, they are drained of 30% of their blood before being returned to the water. The harvested blood is then processed, purified and freeze dried to produce LAL. 

Prior to the LAL test being introduced, testing for endotoxins involved injecting a group of rabbits with a particular sample and monitoring their condition closely over the next four hours. Since the rabbits’ reaction to endotoxins is similar to our human reaction, a consequent fever in the rabbits would mean the sample in question was tainted by endotoxins. 

At the time, this rabbit test method was considered a highly effective, albeit very time consuming and costly method for the detection of endotoxins. It soon became clear that the LAL test offered a much cheaper, easier, faster and more effective solution, and the test was adopted as the global standard for screening bacterial contamination. 

Testing with LAL requires the technician to simply add the LAL to the sample and observe the reaction. Even at concentrations of 1 part per trillion, the endotoxins will be evident almost instantly through the formation of a jelly-like clot in the sample.

The LAL test is incredibly important in revealing the presence of endotoxins in pharmaceuticals and on medical implants/prosthetics. Every drug certified (including medical prosthetics and implants) by the U.S. Food and Drug Administration (USFDA) must first pass the LAL test. If you have ever received an injection of any kind, you have the horseshoe crab to thank for its safety!

The future of LAL

Manufacturers of LAL report the measured mortality rates of the 500,000 annually bled crabs to be only 3%. However, recent independent studies have shown that number to be closer to 15–30%, equating to 75,000–150,000 dead horseshoe crabs annually. With LAL so rare and highly prized (one litre can cost up to $15,000 USD!), it’s no wonder their mortality rates are downplayed by the manufacturers.

With their conservation status currently rated as ‘vulnerable’, the Atlantic horseshoe crab is just one level away from ‘endangered’, with ‘critically endangered’, ‘extinct in the wild’ and ‘extinct’ following shortly if some action is not taken soon. The fate of this species is at risk if we do not re-evaluate current LAL harvesting practices, and it’s not only humans who will suffer when this species dies out. Many shore birds, fish and turtles relying on a diet of the horseshoe crab eggs will also suffer because of their extinction. 

Threatened Red Knot birds are among the many animals to feast on the protein rich horseshoe crab eggs
Threatened Red Knot birds are among the many animals to feast on the protein rich horseshoe crab eggs

It’s not all bad news though. In 1995, researchers at The National University of Singapore were able to identify and isolated the gene responsible for the sensitive clotting that characterises LAL. This gene, known as ‘factor C’, was reproduced synthetically in yeast, resulting in the creation of ‘recombinant factor C’ (rFC). Synthetic LAL differs from organic LAL in that instead of clotting in reaction to the presence of endotoxins, synthetic LAL causes endotoxins to emit fluorescent colours. 

Although its discovery was a major breakthrough, regulatory and safety concerns regarding this LAL alternative have slowed mainstream adoption of synthetic LAL. Europe did not recognise rFC as an alternative to LAL until 2015 and the USFDA only approved the first drug tested with rFC in 2018. In 2020, The United States Pharmacopeia, which sets the scientific standard for drugs in the U.S., declined to recognise rFC as an equivalent to LAL, insisting its safety has yet to be proven. This despite the fact that their European, Japanese and Chinese counterparts have all recognised and adopted the use of synthetic LAL. 

As industry ideals and opinions are slow to change, it is likely that the biomedical field will continue to quench their insatiable thirst for the blue blood of these ancient animals for the foreseeable future. This is hardly sustainable, and we can only hope that we do not reach the point where the choice is made for us through the extinction of the horseshoe crab.

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