Are Horseshoe Crabs Endangered? Conservation Status and Threats

Out of the four extant horseshoe crab species left on the planet today, only the tri-spine horseshoe crab found along the coast of India, Southeast Asia, China, and Japan, is classified as endangered by the International Union for Conservation of Nature. The American horseshoe crab, which lives on the east coast of the United States and the Gulf of Mexico, is listed as vulnerable.

The IUCN has deemed the American horseshoe crab "moderately depleted," giving it a recovery score of 69% ("while [recovery] is viable throughout much of its range, declines due to development, harvesting, climate change and sea level rise prevent it from being functional save for in a few spatial units"). It estimates a reduction of more than half of the current tri-spine horseshoe population over 60 years in a significant portion of the species’ range.

Threats

These “living fossils” have existed nearly unchanged for an estimated 445 million years, well before the dinosaurs. While horseshoe crabs have proven themselves resilient, experts believe that these populations are decreasing due to threats from overharvesting for bait, food, and biomedical testing, as well as habitat loss.

Habitat Loss

Unfortunately, impacts from coastal development and climate change have already encroached on areas required by horseshoe crabs to lay their eggs.

As sea levels continue to rise (approximately 0.13 inches per year, according to NASA), mature horseshoe crabs will have to spawn further upland to reproduce above the tide line successfully. The growth and development of larval horseshoe crabs are also temperature dependent, meaning that changes in water temperature and salinity can affect their survival.

Overharvesting

Horseshoe crabs may be graceful and self-sufficient underwater, but once they come ashore in large numbers to breed, they’re all too easy to harvest.

They move slowly, don’t bite or pinch, and return to the same beaches each year. Not only are they used as bait for eel pots and whelks (a common name applied to several families of edible sea snails), their eggs and meat are sometimes eaten in parts of Asia.

Biomedical Testing

The blue blood of horseshoe crabs contains a rare clotting agent called Limulus amebocyte lysate that’s critical for the safe development of biomedical devices and injectable drugs like vaccines. The blood extract, which coagulates when exposed to harmful endotoxins, has been widely used since the 1970s.

About 500,000 American horseshoe crabs are collected from wild marine populations in the United States each year to be bled and returned to the wild. The mortality rate for bled and released crabs is highly debated in the scientific community—though estimates put it somewhere between 8.3% and 29.8%.

What We Can Do

As they have for hundreds of millions of years, horseshoe crabs are critical in sustaining their native ecosystems. Their eggs provide food for hundreds of species of shorebirds, for example, since their spawning often coincides with migration patterns. (It’s a delicate balance, as a female horseshoe crab may lay over 10,000 eggs throughout several nights.) Their shells also support multiple organisms, such as barnacles, mussels, and sponges, which attach and live on their shells.

Outside of their biodiversity roles in nature and benefits to human health in biomedical science, horseshoe crabs also represent a valuable ancient link to evolutionary history. 

Protecting Habitat

Places such as Pangatalan Island in the Philippines have seen successful horseshoe crab population recoveries thanks to efforts to restore reefs and native plants. After the Sulubaaï Environmental Foundation began a coral restoration project in the marine protected area in 2017, endangered horseshoe crabs returned to the site. By 2022, a National Geographic photographer captured proof that the crabs were thriving there. 

Breeding Programs

Aquaculturing horseshoe crabs has been presented as an alternative to capturing them in the wild. Some studies suggest that taking smaller amounts of blood more frequently would be more beneficial than the current system (in which the crabs are drained of between 25% and 40% of their blood). However, there are obvious ethical concerns about the effect on animal suffering.

Artificial Bait

In 2013, scientists from the University of Delaware set out to reduce the number of horseshoe crabs harvested for bait. By identifying the chemical scents that attracted eel and whelk to horseshoe crab tissue underwater, they developed an artificial bait made from the compounds found in brown seaweeds and kelp, ground horseshoe crab, and a combination of food-grade chemicals.

According to the press release, the mixture was just as productive in catching eel using one-eighth of the amount of crab compared to the Delaware bait limit, and substituting the crab with one-sixteenth of invasive Asian shore crab was just as successful.

Alternative Endotoxin Testing Methods

Synthetic alternatives to horseshoe crab blood have been available since 2003. The most widespread option, Recombinant Factor C (rFC), is produced in cell culture using cloned genes.

The debate continues on whether the manufactured rFC alternative is effective enough to replace natural horseshoe crab blood in endotoxin testing. The Food and Drug Administration does accept medicines tested with rFC, but it requires companies to jump through more costly and time-consuming hoops, making them less likely to use the product.