If you’ve gotten your Covid-19 vaccine, give a nod of thanks to the horseshoe crab (Limulus polyphemus). This animal is truly a “blue blood” — its blood contains copper instead of the iron that gives human blood its red color. And that blood, some of it harvested here on the Outer Cape, is currently going for $60,000 per gallon.
Horseshoe crab blood is the key ingredient in Limulus Amebocyte Lysate (LAL), which is required to test vaccines and medical equipment for the presence of bacterial contaminants that could be introduced during the manufacturing process.
We don’t have exact numbers on the local horseshoe crab population, though anecdotal reports and a decade of beach spawning counts being done by the state Div. of Marine Fisheries (DMF) and Mass Audubon at Wellfleet Bay indicate they are in decline.
We do know horseshoe crabs are not really crabs. They are more akin to spiders. Their appearance on Earth over 400 million years ago predated the dinosaurs, but, unlike the dinosaurs, horseshoe crabs have persisted. They are considered living fossils — they’ve evolved, but natural selection has stabilized around the traits we still see.
They have nine eyes and a number of light receptors that help them to navigate their benthic (sea floor) habitat. As they crawl along the bottom, they eat worms, small shellfish, and crustaceans. Their long tails, known as telsons, help them to flip back over if they get up-ended when waves wash them onto the beach.
Horseshoe crabs inhabit the Atlantic and Gulf coasts of the U.S. and Mexico; three related species can be found in Southeast Asia in the Indian and Pacific oceans. Their numbers are dwindling in all these areas.
When Crabs Mate
They seem numerous here right now, as they gather on bayside beaches for their annual mating ritual. It is intimately connected to the global ecosystem.
Males, often more than one at a time, trail behind the larger females and externally fertilize her 60,000 to 120,000 eggs, which she then buries near the high tide line. The small green eggs are a food source for birds, fish, and other wildlife, and critical for migrating birds like red knot (Calidris canutus rufa), which time their migration to coincide with the spawning of horseshoe crabs along Delaware Bay, where they refuel before completing their journey from the tip of South America to the Arctic circle. Because of the importance of horseshoe crab eggs to the survival of red knots, New Jersey imposed a harvest ban.
After the eggs that survive hatch, the juveniles burrow down into the sand, then return to the bay. We don’t see many of them — they come to shore again only to mate, once they are mature. Immature horseshoe crabs shed their outer shells 16 or 17 times before they become adults. Those seen scattered in the wrack line throughout the summer and fall are often mistaken for dead horseshoe crabs, but they are just the remains of the animals’ outgrown exoskeletons.
The Mass. DMF issues permits for harvesting horseshoe crabs for bait in the whelk and eel fisheries, where the catch limit is 400 per day. But, in another indicator of their population decline, only about 3 percent of the quota has been caught so far this season, and the harvest ends on June 30. The limit for biomedical harvest is higher, 1,000 per day, but the biomedical industry is not required to report harvest statistics.
When gathered for medical purposes, the crabs are bled, then rules require they be returned to their respective estuaries or embayments. Scientific studies indicate, however, that mortality of bled horseshoe crabs may be as high as 30 percent.
Loss of habitat is also an issue for these ancient animals. Sea walls and other types of coastal revetments have caused the loss of the upper intertidal areas they need for mating and laying eggs. Where there is no beach at high tide, the habitat for horseshoe crab reproduction is gone.
Due to decreasing numbers of these animals statewide, the DMF went beyond the harvest limit to impose full and new moon closures of the fishery — that’s when it is believed that most of the spawning occurs. The Cape Cod National Seashore also prohibits their harvest within its boundaries.
But even with these harvest restrictions, the Cape has not seen a rebound in horseshoe crab numbers.
The shellfishermen of Wellfleet, who are out on the water every day, recall times when horseshoe crabs would be crawling all over their grants and on the local oyster reefs. In 2012, the Wellfleet Shellfish Advisory Board, with support from the larger shellfishing community, petitioned the DMF for a moratorium on harvest in Wellfleet Harbor. Their petition was denied.
In Pleasant Bay, which is limited to biomedical harvest, the number of horseshoe crabs has not declined as dramatically, but the crabs are mostly males. The larger females, preferred for bleeding, don’t always survive their trip to the lab and back, or may be too depleted to reproduce.
A Synthetic Alternative
There is a synthetic alternative to LAL called Recombinant Factor C (rFC), available since 2003 for testing of vaccines and medical equipment.
This laboratory-made product does not rely on a supply of horseshoe crabs. A review of 10 studies, published in the online journal PLOS Biology in 2018, concluded that our dependence on horseshoe crab blood for vaccine safety is “ecologically unsustainable.” The studies also showed that commercially available rFC detects endotoxins with highly reliable results, equivalent to or better than LAL.
Recombinant Factor C is currently being used in Asia and Europe. But the U.S. has not yet approved its use. Pharmaceutical companies have so far argued they must balance the time and investment in pursuing new solutions against the fact that they have something that works. The endotoxin tests of the kind horseshoe crab blood is used in number 70 million annually and estimates put the relevant market at $1 billion annually by 2024, according to a report in The Guardian.
If and when rFC is approved here, there may be hope for a future for these creatures, among the most ancient arthropods that visit our shores.
The DMF is tagging horseshoe crabs. If you see a white tag on a horseshoe crab, look for the contact information on it and report your sighting to the U.S. Fish and Wildlife Service at www.fws.gov/crabtag or call 888-546-8537. You could be helping ensure testing of the next batch of Covid-19 vaccine.