A creature more ancient than dinosaurs by over 200 million years, the Atlantic horseshoe crab is an organism treasured by conservationists, biomedical researchers, and even Northeastern’s very own “Dang, I love fossils” club. With five pairs of claws and able to grow up to two feet long, the horseshoe crab is an incredibly useful animal in many areas. These horseshoe crabs serve as a huge factor in maintaining the ecosystems of the Atlantic coast. Their eggs are used to nourish the red knot bird, a threatened species. The Atlantic horseshoe crab is also a common form of fishing bait used to attract eels and conchs. Additionally, and perhaps most importantly, it was discovered in the 1960s that this prehistoric creature contains a powerful substance for scientific research — its baby-blue blood.
The Atlantic horseshoe crab’s blood is used as a reliable determinant of toxin levels in scientific technologies such as vaccines, antibodies, gene editing drugs, and implanted medical tools. These crabs are the sole source of a substance called limulus amebocyte lysate (LAL), a toxin-sensitive tool used in biomedical research. In the presence of endotoxins, a specific type of harmful substance, the blood will start to clot in a mechanism of self-defense. Much controversy exists surrounding the use of these crabs, primarily for biotechnology companies and the often lethal “bleeding process” the crabs undergo before being returned back to the wild or fisheries. Now, scientists are looking for alternatives.
These crabs populate the eastern coastlines of the U.S. and the Gulf of Mexico. In 2021, over 700,000 crabs were drained of their blood — the highest count since the early 2000s. With the increasing need for both the crabs themselves and their blood, safer methods of extracting the blood have been developed to ensure the crabs’ survival. However, according to the Atlantic States Marine Fisheries Commission, the bleeding process is still lethal for about 112,000 crabs, amounting to 15%.
“The bleeding process is lethal for about 112,000 crabs, amounting to 15%.”
The crabs are shipped off to various biomedical facilities for the process. A needle is inserted into the crab’s chest cavity and an absolute maximum of 400 milliliters of blood can be drawn at once. The mixture is handled in temperate environments to avoid unwanted clotting. Unfortunately, it takes dozens of crabs to generate one vial of blood. A gallon of blood is worth a whopping $60,000. Depending on the state, the crabs are then transported back to their native habitats in South Carolina or New Jersey, or sold as bait in Massachusetts.
Once harvested and maintained, the blood is incorporated into the LAL test, which is a research procedure that allows scientists to determine the toxin concentration of a drug to determine whether it is safe for human use. The LAL test specifically detects levels of endotoxin, a component of bacterial membranes that humans are highly sensitive to. Upon endotoxin detection, the blue blood will clot corresponding to the endotoxin concentration, with more clotting representing higher levels and thus indicating that a drug is unsafe for human use.
Since endotoxin elimination remains critical for drug development and discovery as the numbers of healthy Atlantic horseshoe crabs diminish, alternatives to the LAL test are needed. Two such potential options are the Recombinant Factor C (rFC) test and the monocyte activation test (MAT). One of the benefits of rFC is that it utilizes a non-animal substance to detect endotoxins. The substance is a synthetically generated clone of Singaporean horseshoe crab blood and has the same mechanism of action as the blue blood in the LAL test. Considered an untraditional method, the rFC method is not popular, but according to an article published in Frontiers in Marine Science, it is more sensitive and specific. MAT, on the other hand, uses human white blood cells extracted from blood samples and is typically used secondary to LAL. MAT is limited because it is unable to detect certain cytotoxic substances, so further development is needed on this technique.
At present, LAL tests dominate the industry in terms of endotoxin detection, so a major driving force is needed to inspire tests like rFC to become more popular. A decline in flourishing horseshoe crab populations and disruptions to coastline ecosystems are major drawbacks in the use of these crabs as the sole source of toxin testing. Both MAT and especially rFC serve as promising options for LAL alternatives. The increased use of one of these alternate methods could greatly improve the quality of life of the Atlantic horseshoe crab and increase population numbers along the entire East Coast. It would also hopefully revitalize the threatened red knot bird population, which depends on the crabs.
