July 18, 2022 — Between the local dump and highway, in a nondescript building that lacks any indication of who occupies it, one of the world’s largest pharmaceutical companies recently began harvesting the milky-blue blood of an ancient creature plucked from the beaches and bays around Cape Cod.

Charles River Laboratories is one of just four companies in the United States — and now the second on the Cape — licensed to harvest the blood of horseshoe crabs for a valuable component that’s used to identify harmful bacteria during the testing of new drugs.

The bleeding of crabs, combined with their use as bait and losses to their coastal habitat, has led them to be listed as “vulnerable” by the International Union for Conservation of Nature, which maintains the world’s most comprehensive list of threatened species. The group considers the crabs “endangered” in the Gulf of Maine and the Mid-Atlantic, areas that include Cape Cod, though officials in Massachusetts have cited surveys suggesting the region’s population has increased.

State officials also pointed to surveys done every spring for decades that show female horseshoe crabs at a near high in abundance. However, a similar state survey in the fall — which officials didn’t acknowledge until the Globe asked about it — shows the number of female crabs actually decreasing substantially over the past five years.

Since bled crabs are rarely tracked after they’re released, scientists and environmental advocates say it’s hard to know for sure how many crabs actually die, or are otherwise harmed, as a result of the bleeding, which drains large amounts of their blood.

Charles River officials told the Globe that just 4 percent of their crabs die before being returned to the wild; however, the South Carolina Department of Natural Resources has estimated more than 20 percent of female crabs die within two weeks of being bled by the company.

Charles River officials have defended the additional pressure on the species, contending that the good from preventing the contamination of drugs outweighs the environmental impact. But there are now synthetic substitutes that rival companies say could reduce and eventually eliminate the need to rely on the crabs’ blood, which has been used for testing drugs since the 1970s.

One pharmaceutical giant that has received government permission for a synthetic alternative, Eli Lilly, has used its substitute to test COVID-19 antibody drugs.

Officials at Charles River said they’re developing their own synthetic versions but it would take years before they were ready and widely adopted.

“The synthetics we’ve tested are not sensitive enough to ensure patient safety,” Girshick said. “We’re actually pushing for a synthetic ourselves, and spending a lot of money to get there.”

Until then, she said, the company will be harvesting the blood from horseshoe crabs.