Horseshoe crabs have been around for 450 million years. They’ve survived several mass extinctions and have outlived the dinosaurs. Arthropods more related to spiders than crabs, these primitive, resilient sea creatures are coastal dwellers, preferring the ocean bottom, but can swim if they need to. They’re most common along the Atlantic coast in North America. Their circulatory system, while similar to ours, is much more primitive. We have red iron-containing hemoglobin, and three types of cells in our blood; red cells to carry oxygen, white cells to fight infection and platelets for clotting. Horseshoe crabs have blue copper-containing hemocyanin, and two types of cells; amoebocytes for clotting and cyanocytes for oxygen transport.

Why does this matter? If you’ve ever had a vaccine, IV fluids, or any injectable medicine, you owe a debt to horseshoe crabs. Let me explain.

It was the mid-1800s when technology advanced enough that people began to get injections of various liquids. No one understood why sometimes patients got high fevers after injections. By the 1920s researchers discovered that solutions were sometimes contaminated with bacteria, causing fevers. A test was developed to prove these liquids were germ-free; it involved injecting rabbits with the solution to see if they got febrile (the Rabbit Pyrogen Test). So, this became the standard of care into the 1950s.

In the 1950s researchers at Woods Hole Marine Biological Laboratory in Massachusetts noticed that horseshoe crabs‘ blood coagulated and clumped in an odd way. Further studies led them to the realization that there was something inside the amoebocyte cells that would clot almost instantly in the presence of contamination, changing the blue blood from a liquid to a thick jelly-like consistency. They isolated and purified this substance, limulus amebocyte lysate (LAL). As it turned out, it was an exquisitely sensitive way to test for endotoxins (contamination) in medical liquids. It was, in fact, better than the rabbit test. By the 1990s, LAL was the FDA- approved way to test medicines for purity. Each year, around eighty million tests are performed using LAL from horseshoe crabs’ blood.

So how is LAL harvested? There are five companies who do this, but their details (along with sales and revenue) are proprietary. We do know that up to a million horseshoe crabs are caught yearly, during spawning time, when they emerge on the beaches to lay their eggs. Up to 30% of their blue blood is drained, and then they are returned to the sea. It is hard to know their survival rate after this procedure; estimates are that up to 30% of crabs die each year in this process. And many of the survivors are too weak to reproduce thereafter. For the past 25 years, the horseshoe crab population has seemed to be holding stable, but now conservationists are beginning to worry. The population is clearly declining.

We aren’t the only one who depend on the lowly horseshoe crab. Red knots, charming migratory shorebirds, have one of the longest migration paths in the bird world. From Tierra del Fuego to the Canadian Arctic, they fly over 9,000 miles. Their northward migration is synchronized with horseshoe crab spawning. On their way north to their Arctic nesting grounds, they stop along the coast and bays of the Carolinas, Virginia, and Delaware to rest and feast on the clumps of blue eggs that horseshoe crabs deposit. Because of the decline of the horseshoe crab population, these birds aren’t finding enough eggs. Additionally, the largest LAL producing company, on the South Carolina coast, moves captured horseshoe crabs to holding ponds before processing. These ponds aren’t at the shore, and the red knots only know how to look for eggs on the shore, so they can’t find these thousands of horseshoe crabs. Because of this, these birds are currently a threatened population and need all the help they can get. Their population has declined by over 90% in the past 40 years. And they are only one of five shorebirds that depend on horseshoe crab eggs for survival.

So, for several reasons, there has been a push to develop a synthetic alternative to horseshoe crab blood for toxin testing. There is, in fact, a good replacement. In the 1990s several companies developed a recombinant DNA substitute for horseshoe crab LAL. Recombinant Factor C (rFC) has been tested in Europe, is as good as LAL from horseshoe crabs, and has been approved for use by the European Pharmacopoeia. In 2020, during Covid, when the volume of LAL testing skyrocketed, Eli Lilly started testing their Covid antibody with rFC rather than LAL, and found it was just as good. Because Lilly is an international company, they began to use rFC in European manufacturing.

But there’s been a stand-off for the last five years. The five companies that make LAL said the horseshoe crab population is up to the task, and there was no reason to change. They felt the horseshoe crab population was stable, and the LAL test was natural and sustainable. And the regulatory body, US Pharmacopeia (USP) abandoned plans to approve rFC. Conservationists argued that the LAL producers were stalling, to protect their sales revenue, trying to avoid the cost of switching to rFC. LAL producers and US Pharmacopeia insisted they were going through due diligence to make sure the test was good enough. Finally, last July the US Pharmacopeia approved rFC for bacterial testing. It became official just last month, May 2025.

There are now coalitions of biomedical and conservationist watchdog groups tracking as drug companies migrate from LAL to rFC, and publishing a Sustainability Scorecard. How are companies doing? Eli Lilly and Glaxo Smith Kline have great initial scorecard results. Companies are quickly switching from horseshoe crab-based LAL to synthetic rFC, especially with the increased popularity (and need for increased testing volume) of GLP-1 inhibitors for weight loss and diabetes. And these coalitions are watching closely for rebound in the horseshoe crab population.

So, the next time you get an injection or IV fluids, think of the horseshoe crabs’ blue blood and thank them for their decades of service. Their lives are about to get better.