Insects and arthropods are some of the oldest animals on the planet; they were some of the first to colonize land, and easily the first to take to the air. Over the course of their evolution, many insects and arthropods have changed significantly to adapt to their changing environment, but one arthropod remains relatively unchanged since it appeared in the fossil record 450 million years ago: the horseshoe crab.
Horseshoe crabs are not true crabs; they’re not even crustaceans. They belong to their own order, Xiphosura, and their closest living relatives are arachnids like scorpions and spiders. The four living species of horseshoe crabs look nearly identical to their ancient relatives. To put their time on this earth in perspective, consider Pangea, the supercontinent that broke apart 175 million years ago to form the continents we know today. Pangea formed 335 million years ago; 65 million years after the horseshoe crab showed up. The horseshoe crab has gone relatively unchanged in its hundreds of millions of years on this earth, and considering they’ve managed to survive 5 mass extinctions and two ice ages, I’d say they’re doing something right.
Like all arthropods, horseshoe crabs possess an innate immunity; their immune system is simple and unlike ours, cannot build an immune memory to specific pathogens. Instead, their body will attack any foreign invader by clotting. Essentially, cells called granular amoebocytes (which are the horseshoe crab’s white blood cell equivalent) will coagulate or clot around a foreign invader.
The clotting ability of horseshoe crab blood is a key component in medical testing. Before any product or device that comes in contact with human blood can be made widely available on the market, it is crucial that the product in question is free of not only bacteria, but endotoxins.
Endotoxins, also called lipopolysaccharides, are components of the bacterial cell wall. When bacteria are killed through heat or chemical sterilization, endotoxins are released when the cell wall disintegrates. So while sterilizing equipment will kill any bacteria, endotoxins may still be present. It is imperative for medical practitioners to know whether or not an endotoxin is present in equipment or injectable products, as they are capable of causing serious disease or illness like septic shock, meningitis or salmonella.
In the mid-50’s, researchers found that horseshoe crab blood will clot in the presence of endotoxins, which led to the development of the Limulus amoebocyte lysate (LAL) test. Limulus is the genus name of the Atlantic horseshoe crab; in Japan and China, the testing equivalent is the TAL test, using horseshoe crabs from the Tachypleus genus.
The granular amoebocytes in horseshoe crab blood are able to detect endotoxins present in 1 part per trillion in less than 90 seconds, making it the most sensitive endotoxin detection method we’ve discovered so far. Before developing the LAL test, medical professionals would inject innumerable rabbits with a substance going out for production, like a vaccine. The test (known as the rabbit pyrogen test) was simple; if the rabbit became sick or died, then we knew the product was contaminated.
The LAL test was approved for drug testing by the FDA in 1970, and since then has become a medical industry standard. Those COVID vaccines? Every batch underwent LAL testing before being released to the public. The development of the LAL test revolutionized medical testing, making the process faster and more efficient. Not to mention the number of rabbits it saved. But what consequence has this revolutionary method had on horseshoe crabs? As you can imagine, the outcome isn’t a good one.
The crabs are captured when they emerge from the ocean to spawn. Being large, slow, and virtually harmless, they are easy to collect. Once transported to a collection facility, the crabs are bled alive. During collection, horseshoe crabs may lose up to 30% of their blood, which is blue thanks to copper-based oxygen-carrying molecules. The substance is considered one of the most expensive in the world at $60,000 per gallon and every year, around 600,000 crabs are collected for the blood harvest. It sounds like something out of a science fiction novel.
According to the biomedical industry, horseshoe crab mortality is low enough to consider the practice sustainable, but the Atlantic States Marine Fisheries Commission disagrees. Studies on horseshoe crab mortality following the blood harvest suggests that industry estimates are nearly 10x lower than actual mortality rates; in addition, the biomedical industry does not consider the crabs’ lowered fitness once they are returned to the ocean. Crabs that are bled exhibit sluggishness, disorientation, and decreased spawning numbers, which contribute to their alarming decline in numbers. In 2016, the IUCN listed the Atlantic horseshoe crab as vulnerable, but sadly, the blood harvest continues unabated. But there is hope: In 1995, researchers at the University of Singapore were able to isolate the gene responsible for the clotting mechanism, known as factor C. The process to adopt this new testing strategy has been slow, but in 2018, the first drug tested and approved using recombinant factor C (rFC) hit the markets, and companies have been feeling pressure from conservation agencies to make the switch permanent. Europe, China and Japan all recognize rFC as a viable alternative to LAL, and at least one US pharmaceutical company, Eli Lilly, has committed to using rFC instead of LAL. Advocates like the Horseshoe crab recovery coalition are confident that horseshoe crab numbers can recover as soon as 2030 if other companies commit to the new testing method. After surviving the first 5 mass extinctions, there is hope that they can survive the current one too.