#4. Horseshoe Crabs: Tied up in Knots (Part 1)
In which the E@L ponders the question: Is the best use of horseshoe crabs for bait, pharmaceuticals, or ecotourism?
It was a moonlit night…
As we approached the island, a dark line at the water’s edge slowly resolved into a mass of bumpy, dark objects. On closer inspection, each of the bumps was a horseshoe crab. Horseshoe crabs were lined up so thickly on the shoreline that it was difficult getting from the boat to the shore without stepping on them. Large females, covered with algae, were swarmed over by smaller males eager to become their mates. On this evening, I was assisting Steve Doctor, a biologist with the Maryland Department of Natural Resources (MD-DNR), and his team of biologists to conduct a survey of horseshoe crab abundance. After finding few or no crabs at our first study sites, we landed at Skimmer Island, which is little more than a pile of dredge spoils in the midst of Assawoman Bay.
Horseshoe crabs lined up along the beach for mating (B. Stevens)
Here, at intervals along the shore, Steve lay down a 1-meter square frame made of PVC pipe and counted the crabs in the frame, while another member of the crew recorded the results. While he did this, a third team member tagged crabs for the US Fish and Wildlife Service. To do so, he drilled a small hole in the carapace and inserted a round plastic disc with a number and contact information. According to Steve, the Maryland population of horseshoe crabs is relatively stable. Deposition of dredged sand onto Skimmer Island ceased in 2014, however, and both the size of the island, and the number of horseshoe crabs counted there has declined ever since. As the sun set, and the full moon rose above the bay, we boarded our boat for the ride home, leaving the crabs to carry on their business of mass procreation as they have done for millions of years.
Biologist Steve Doctor (MD-DNR) counts Horseshoe Crabs near Ocean City, MD, during their annual mass mating ritual (Brad Stevens).
The Un-Crab
The American Horseshoe Crab Limulus polyphemus inhabits the shorelines of oceans and estuaries from Maine to Mexico. Their name is a bit of a misnomer because they are not actually crabs but are more closely related to spiders. Every spring, millions of these ancient, fossil-like creatures swarm the beaches during the highest tides under the full and new moons to mate and deposit their eggs in the sand along the shoreline. Female crabs are recognizable by their large size and their cover of attached slipper shells and algae. As they come ashore, they are surrounded by smaller males jockeying for position to pursue their amorous activities. The winner of this competition climbs onto the back of the female, in a process called amplexus, and proceeds to fertilize her eggs. The females then bury their eggs deep into the sand where they develop over the next few weeks. The larvae hatch and soon settle out in the nearshore region as miniature horseshoe crabs. Juveniles move offshore, where they live until reaching maturity at the age of 9-12 years old. Once mature, they migrate back to the beaches to start the cycle over again.
A male Horseshoe crab clings to a larger female that has been tagged by the US Fish and Wildlife Service (Brad Stevens)
Going to the birds
Healthy populations of horseshoe crabs are critical for maintaining ecological balance. They have important roles as bioturbators (i.e. plowing the seafloor), as predators of shellfish and other invertebrates, and as hosts to many organisms that live on their shells. Adult horseshoe crabs can live as long as 27 years. They are too big for most predators to eat, but sharks and loggerhead sea turtles prey extensively on them. But perhaps their most valuable ecological role is as a source of food for birds.
Horseshoe crab eggs are a critical food resource for migrating shorebirds, especially the threatened Red Knot Calidris canutus rufa. These tiny shorebirds migrate all the way from Brazil to the Arctic every spring for breeding. Over 90% of the population stops in Delaware Bay to refuel, timing their arrival to coincide with the spawning of horseshoe crabs. By the time they reach Delaware, they have used up most of their energy reserves and weigh only about 100 grams (3.5 ounces). During their stopover, they can double their weight by eating horseshoe crab eggs, which are rich in protein and fat needed for bird flight. Over 400,000 shorebirds visit Delaware Bay every spring and consume over 100 billion horseshoe crab eggs. You might think that would put a significant dent in the crab population, but most of the eggs are buried too deep for the birds, which can only reach the top 10% or so.
Red knots feeding on horseshoe crab eggs at Mispillion Harbor, DE (Gregory Breese, USFWS)
For many years horseshoe crabs were considered “trash” and were used for fertilizer and animal feed. Eventually, fishermen found them to be useful as bait for eels, conch, and whelk (but that’s another story). Female horseshoe crabs are valued so highly as bait that they are worth twice the value of male crabs. Unrestrained harvest of horseshoe crabs for bait decimated populations in the late 1990s, concurrent with a drastic decline in red knot populations.
Bleeding for Dollars
Besides bait, the most important use of horseshoe crab by humans is for blood extraction by the biomedical industry. Horseshoe crab blood (called hemolymph) is blue in color due to the presence of hemocyanin, which binds copper. Extracted hemolymph is used to produce Limulus Amoebocyte Lysate (LAL). LAL is the best available substance to test for the presence of endotoxins produced by bacteria and fungi in drugs, surgical utensils, and implants (including, probably the stent I just received).
I visited a laboratory near Ocean City where horseshoe crabs were being bled for LAL production. Dr. Tom Novitsky, a research scientist and pioneer of the LAL industry, described the process:
“To bleed the crab, a large gauge needle is inserted into the crab’s heart, and about 30% of the blood (called hemolymph) is drained out. After bleeding the crabs are returned to the ocean. It takes about a month or two for the crab to recover its full blood volume and cell count. The amount of LAL produced depends on the concentration of cells (lymphocytes), and can vary with sex, water temperature, and season. The blood from each crab produces about 4 grams of concentrated cells, which makes about 7 mL of LAL, and yields 70 tests, at 0.1 mL per test. However, there are several newer methods that use up to 10-fold less LAL per test.”
A horseshoe crab being bled for LAL production (Brad Stevens)
As production of coronavirus vaccines ramped up in 2020 and 2021, greater quantities of LAL were needed to test vaccines for contamination, leading to increased demand for horseshoe crab harvest. Three other species of Horseshoe crab live in Asia; one of those (Tachypleus tridentatus) is used to produce Tachypleus amoebocyte lysate (TAL). The total global market for lysate in 2021 was US$200M, including both LAL and TAL. The Asian species is also used as a food source, so few are returned to the ocean, and the species is considered endangered.
Numerous attempts have been made to create an artificial version of LAL, but most substitutes lack the specificity and sensitivity of LAL. One of these, recombinant factor C (rFc) may be equivalent to LAL but has not yet been approved by the FDA for general use. Likewise, research on artificial baits for conchs has not yet come up with a better product. And cultivation of horseshoe crabs is difficult due to their longevity and size so is unlikely to replace the use of wild crabs. A set of “Best Management Practices” has been established for bleeding crabs, including specifications regarding methods of capture, transport, holding, and release. For the immediate future, though, the best option for conserving horseshoe crabs is to improve the methods and efficiency with which LAL is produced and used.
The total harvest of horseshoe crabs from the Delaware Bay population (which also includes most crabs caught in Maryland) has been limited to 500,000 male horseshoe crabs since 2010. Crabs can only be caught from June to December. Although all crabs used for bait are killed, crabs used for medical purposes are returned to the ocean. Still, a portion of those crabs die as a result of blood removal, exposure to warm temperatures, and dehydration.
The big question is, what proportion of medical-use crabs survive? Information on the number of crabs bled and their mortality rate is closely guarded by the biomedical industry, however, making it difficult for biologists to understand the full impact of bleeding.
In Part 2 of this essay, we will discover how many crabs die from bait and LAL harvesting, how the crabs are managed, and what we can do to conserve them.