Why the U.S. Still Bleeds Horseshoe Crabs Despite a Lab-Made Alternative

On boat decks along the Atlantic coast in the cool dark of night, horseshoe crabs are hauled from the sea. The ancient creatures have a smooth, round shell, like an upside-down plate, and a long spindle of a tail dragging behind them as they scuttle along the deck. When they reach shore, they’re driven to bleeding facilities, where they are wiped with alcohol pads and iodine. A needle is inserted into their pericardial membrane — a soft spot on the crab’s belly just below their curled legs — to collect their sky-blue blood.

The crabs go back to the sea, usually alive, while their blood is processed and used to test for bacterial contamination in medicines, vaccines and medical devices. This essential testing keeps people safe from potentially deadly infections. 

Since the beginning of the Covid pandemic, the number of horseshoe crabs bled for biomedical purposes has basically doubled to about a million crabs per year, Jay Bolden, senior director at Eli Lilly and Company, tells Sentient.

But a synthetic alternative to this blood-based test has been available for more than 20 years, widely used in Europe. The alternative was added to the U.S. Pharmacopeia, an independent book of pharmaceutical standards, in 2025. But the U.S. pharmaceutical industry continues to rely heavily on horseshoe crab blood, despite concerns about supply chain issues, ecological impact and ethics.

The U.S.’s lag in adopting an alternative is simply “technological inertia,” Kristoffer Whitney, an associate professor at the Rochester Institute of Technology who studies horseshoe crabs, tells Sentient. “The only thing stopping us from removing horseshoe crabs from the pharmaceutical pipeline is the will of the pharmaceutical companies themselves.”

The debate over horseshoe crab blood reflects broader questions about how regulatory systems evaluate and adopt non-animal alternatives.

How Horseshoe Crab Blood Became a Pharmaceutical Standard

The medical value of horseshoe crab blood comes from a clotting reaction triggered by bacterial toxins known as endotoxins. Scientists isolate an enzyme from the blood, known as limulus amoebocyte lysate (LAL), which reacts when endotoxins are present.

Whitney tells Sentient, “LAL is great for detecting endotoxins.” Endotoxins are toxic substances produced by bacteria that can make people sick. 

Whitney traced the test’s origins to mid-20th-century laboratory observations in Massachusetts, where researchers first noticed horseshoe crab blood clotting when it encountered bacteria. 

By the 1970s, limulus amoebocyte lysate began replacing older safety tests that relied on injecting rabbits with pharmaceutical samples to see whether they developed fevers. “There was strong industry incentive to switch from the rabbit test to the LAL test,” says Whitney. “The rabbit test was expensive. You had to maintain colonies of rabbits, you had to have specialized technicians dealing with the rabbits, and it took days to get a result.”

The horseshoe crab enzyme quickly became the global standard because it was faster, cheaper and more precise than the rabbit test. It also significantly reduced the number of rabbits used in pharmaceutical safety testing.

Why Companies Are Switching — and Why Many Are Not

Despite its advantages, limulus amoebocyte lysate still depends on wild animals. Horseshoe crabs are collected from coastal waters, bled, and then returned to the sea with a mark on the side of their shell, signaling they have already been bled so they should not be taken from the water again within the calendar year.

Recombinant factor C (rFC) offers an alternative to this method. rFC uses a version of the same endotoxin-detecting reaction as the enzyme from horseshoe crab blood. Developed in the 1990s and commercially available since the mid-2000s, rFC replicates the clotting response without using horseshoe crab blood.

Many of the companies in the pharmaceutical supply chain that sell limulus amoebocyte lysate also produce synthetic alternatives.

Producers of synthetic alternatives have been arguing for years that LAL’s dependence on horseshoe crabs could create supply chain problems, Whitney says, if the safety of intravenous vaccines and medications depends on the supply of a wild animal.

However, adoption of alternatives has been uneven.

Eli Lilly and Company was one of the first U.S. pharmaceutical companies to make the switch to rFC. Senior director Jay Bolden says one of the reasons the company made that change was concern about how sustainable the horseshoe crab enzyme is.

“We want to ensure the safe supply of quality medicine to our patients, and if we rely on a wild animal for that, there are some real supply chain risks. We are just inherently in a better place if we don’t have to rely on a wild animal,” says Bolden.

Eli Lilly began formally transitioning in 2016. “We basically drew a line in the sand and said all of our new medicines going forward are going to get this,” Bolden says. The company is now “approximately 80% converted,” he says, though “that last 20% has been stuck for a while.”

Bolden says that the last 20% has been hard to eliminate because of global regulatory differences, as some countries still require or prioritize LAL testing.

The company’s motivation, he says, was primarily ethical rather than financial. Bolden is an avid birder and learned that a declining horseshoe crab population was affecting seabirds that rely on their eggs for food. He wanted to move away from a method that he saw as negatively affecting the ecosystem. 

But the switch has had unexpected benefits, Bolden says: rFC is less susceptible to false positives than the horseshoe crab enzyme. And the cost of switching to rFC was lower than anticipated for Eli Lilly.

But other stakeholders in the pharmaceutical industry have concerns about the synthetic option.

Allen Burgenson, global subject matter expert for testing solutions at healthcare manufacturing company Lonza, argues that synthetic alternatives are not equivalent in performance, speaking on behalf of himself and not Lonza.

“There is a notion that rFC is just as good as LAL, and it’s not,” he says. “Our studies show that rFC underpredicts by 50% or more” in some products.

That claim is disputed by other scientists and companies working on recombinant methods, who point to successful regulatory approvals and widespread adoption in parts of Europe. 

Regulation, Inertia and the Slow Pace of Change

The divide over testing methods is shaped as much by regulatory bureaucracy as it is by science or ethics. 

Jeffrey Brown, a senior scientist for pharmaceuticals and medical devices with the advocacy group PETA Science Consortium International, says that the persistence of horseshoe crab-based testing reflects broader structural issues in how the U.S. regulates medicine. 

“There is no legal motivation,” he says, to adopt alternatives.

In contrast, in 1986, the EU passed a law requiring the replacement of animal methods with non-animal alternatives when available. He says that’s why rFC has been much more widely adopted in Europe than in the U.S.

In contrast, he says, “The U.S. has nothing like that.”

Without a clear mandate, companies face higher barriers to changing methods. “The cost of bringing a new drug to market is in the billions of dollars,” Brown says. And firms are often reluctant to introduce changes that could complicate regulatory approval. “You don’t want a new test to be the reason the drug development process fails, costs more or takes longer,” he says.

Brown says that if Congress were to pass a directive similar to the EU’s, the switch to synthetic testing would be much simpler. 

Regulatory change moves incredibly slowly. Although rFC was first commercially produced in the early 2000s, for many years any company using rFC had to prove its efficacy against limulus amoebocyte lysate. It wasn’t until May 2025 that the U.S. Pharmacopeia adopted a chapter about rFC’s use, officially making it an acceptable standard for testing. 

“Cynical observers say that group has too many ties to the LAL industry and it wasn’t in their interest to replace LALs,” says Whitney.

“It’s like steering the Titanic, you can’t turn it very quickly,” says Brown. “But that’s been an excuse for so many years, and at some point, you have to say, yes, you can’t move fast, but I don’t see any movement at all.” 

Ecosystem Impacts and Competing Claims

While pharmaceutical companies are focused on the efficacy of their testing methods, scientists are concerned about the impact that bleeding horseshoe crabs may have on the coastal ecosystem.

Horseshoe crabs first evolved more than twice as long ago as the first dinosaurs. They have remained largely unchanged for hundreds of millions of years. Three species of the crab are found in Asia, and one is found along the North American Atlantic coast. In Asia, one of those species is endangered after years of being harvested for food, bait and medical testing.

Even if horseshoe crabs are returned to the ocean alive, scientists and advocates note that the bleeding process can have biological impacts, including stress and reduced reproductive capacity. 

“If you pull horseshoe crabs out of the water, drain them of a portion of their blood and put them back into the ocean, it interferes with some stuff,” he says. “It makes horseshoe crabs reproduce less frequently.” This could affect coastal food webs, he says: horseshoe crab eggs are an important food source for migratory shorebirds.

However, Burgenson, who, in addition to working for Lonza, serves on the Horseshoe Crab Specialist Group of the International Union for the Conservation of Nature, tells Sentient that collection practices are tightly regulated and that mortality rates are low under current standards. He points to seasonal limits, handling protocols and monitoring by state agencies. He says that in his area, both he and the Maryland Department of Natural Resources go out a few times a year to check that horseshoe crab fishing operations are following regulation protocols.

A System in Transition

Both industry and regulatory experts agree on one point: synthetic alternatives exist and are increasingly viable.

Brown described the situation as a broader lag in scientific modernization.

“This is an area where we know the U.S. is already falling behind,” he says “It’s not just an animal ethics question, or a science question or an ecology question, it’s a little microcosm of how science gets ahead in other places, and it’s not happening in the U.S.”

For companies like Eli Lilly, the transition is already underway, though incomplete. For others, the system remains largely unchanged.

“Somebody had to be first,” says Bolden. He is hopeful that the updated Pharmacopeia will inspire more companies to move toward rFC and away from animal-derived methods.

Until then, the United States drug safety system largely depends on a marine species that has survived for hundreds of millions of years. Perhaps the system won’t depend on these armored pancakes for much longer.