Nature Notes: Horseshoe Crab
By Ed Robinson
If there is a creature of Harpswell waters with a bigger image problem than the horseshoe crab, it has my sympathy. Most of us know little to nothing about the prehistoric looking horseshoe crabs other than seeing their old exoskeletons on our beaches. Then there is the fact that these fascinating creatures are not crabs, not even crustaceans, despite their name. Let’s be honest, they are not the prettiest creature in town, appearing as if they just crawled out of Earth’s primordial seas over 450 million years ago. In fact, that is exactly what they did! But read on to learn just what a unique creature this is, including its role in saving millions of human lives.
The name horseshoe arose because of the shape of the creature’s shell. While they bear some resemblance to crabs under their shells, they are more closely related to ticks or spiders. The crabs found along the Gulf of Maine are one of four subspecies, known as Limulus polyphemus. They range from Maine down along the Atlantic coast and into the Gulf of Mexico. The other subspecies are mostly found around Southeast Asia. Some people in the Caribbean and Asia eat the horseshoe crab and also grind them up for fertilizer. Crabs were traditionally harvested in large numbers this time of year and used as bait, mostly for eels and conch. With no regulations regarding the harvest, it was clear the crabs were at great risk. Maine changed state regulations in 2004 to prevent harvesting the crabs between May 1 and October 30, protecting them while they are in shallow water. While an estimated one million crabs are still harvested for bait, their population has recovered somewhat, though the crabs are still exploited elsewhere.
Horseshoe crabs are classed as arthropods, along with lobsters. The earliest fossils of the crabs date back to the Ordovician period, 450 million years ago. It is interesting that the appearance of the creatures has changed little through that vast stretch of time. Like other arthropods, horseshoe crabs have a hard outer shell or carapace for protection of their fragile bodies. In what seems a wild evolutionary adaptation, the crabs have a total of nine eyes, one on either side of the head where you would expect them and the other seven placed around the body and having several functions, including seeing both visible and ultraviolet light. Scientists have determined that the rods and cones in their eyes are the largest of any known animal, giving them far higher sensitivity to nocturnal sources of light than our own eyes. No doubt this is helpful in deep water, or when foraging in muddy water.
Like lobsters, horseshoe crabs have six pairs of appendages under that shell. The first pair from the front are called chelicerae, used to place food in their mouth, which is located among their cluster of ten legs. The legs are also used for gripping a female crab during mating or walking along the ocean bed, and the base of those legs serves another role. Not only do they support the legs and muscles used in locomotion, they also serve as jaws to grind up food for digestion. Behind the legs are the gills, which not only allow breathing underwater but can be used for slow propulsion through the water in an inverted position. Emerging at the back of the shell is the telson, or tail, which looks like it might be dangerous but is in fact harmless. When crabs end up on their backs, they use the telson to flip over. Like other arthropods, the crabs can regenerate a lost leg or tail over time.
Our horseshoe crabs top out at about twenty inches long including the tail, and a weight of around four pounds. Females are 20 to 30 percent heavier than males, presumably to support egg production. The crabs like feeding on mud flats where they can find algae, clams, snails and marine worms. Crustaceans and small fish are also consumed when available. In the winter months the crabs move into deep water and are thought to feed only occasionally in the cold water.
I timed this article for May because it will soon be mating season for the crabs and they will be visible along our shores, numbering in the hundreds or more. When ocean temperatures warm to 50 degrees F or higher, horseshoe crabs use high tides to move from offshore waters in search of mates, mostly at night. The crabs are fussy about water conditions for breeding, and they need mud flats or sandy beaches so the females can prepare a depression for laying their eggs. There are just four areas in Maine that fit the bill: Casco Bay, the area around Damariscotta, the Bagaduce River basin and Downeast’s Taunton Bay. Biologist and good friend Tulle Frazer recommends a visit to the mud flats at Skolfield Shores Preserve or Thomas Point Beach to watch horseshoe crabs in action. Annual surveys of horseshoe crab populations started in 2001 identified both sites as important breeding habitats.
From experiments with electronic tags, researchers have established that the crabs return to the same spots each year to spawn, with females laying as many as 120,000 eggs over several sessions, and males depositing sperm to fertilize those eggs. Few of the eggs survive to hatch two weeks later because they serve as a nutritious food for many other creatures including shorebirds, fish and sea turtles. As with lobsters, these crabs shed their shells by molting, with larvae molting several times in their first year, then annually until they reach adulthood at about 10 years of age.
Even before the pandemic shocked our healthcare system, there was growing concern about hospital infections and drug resistant bacteria like E. coli, Salmonella and Legionella. These bacteria carry endotoxins in their cell membranes that can trigger sepsis in humans, a condition that kills up to 11 million people annually. Horseshoe crabs have unusual features that have resulted in them playing a vital role. First, their blood uses a copper-containing compound called hemocyanin to carry oxygen. That copper results in blood that is light blue when exposed to the air. Within the blood are special amebocytes that play a role similar to our white blood cells in fighting pathogens, forming a clot-like gel around bacteria to prevent infections. This discovery by Frederik Bang and Jack Levin led to a new safety test approved by the Food & Drug Administration in 1977.
Specialized biomedical labs began to draw blood from crabs and extract the amebocytes. Once purified and processed into a solution called Limulus Amoebocyte Lysate (LAL), the crab product detects dangerous bacteria before surgical tools are used, an artificial joint is inserted, or a Covid-19 vaccine is injected into your body. The bacterial endotoxins mentioned above sometimes survive all attempts to produce sterile drugs and medical devices. In the presence of the endotoxins, LAL changes from a liquid to a clotted gel and alerts technicians to the risk of infection.
So valuable is the blood of the crab that more than 600,000 of them are harvested each year so that some of that blood can be extracted by needle for processing and purification. As happens all too often, growing demand by humans for a naturally-sourced product can lead to excess use and the risk of destroying the source of that product. The pandemic has raised the price of crab blood to as high as $60,000 per gallon! Sadly, there is no quota on the harvest of blood from the crabs. The biomedical labs argue that by taking just one third of the crab’s blood there will be no long-term effects. But the entire harvest, shipping and handling process is stressful for the crabs and there is little research to document what happens to the crabs once they are released back into the sea. Limited data indicates that as many as 30 percent of the crabs die after their return to the ocean, and that females suffer impacts in their ability to spawn for some period of time.
It would be nice to think that scientists could develop new technology that would replace crab blood in LAL. In fact, Japanese scientists did just that in 1986, and a Korean scientist patented a new process to make it, with availability starting in 2003. The biosynthetic compound known as recombinant factor C (rFC) has been produced by the French company bioMerieux and Swiss company Lonza where I ran the biotech business 20 years ago. Unfortunately, medical practice is very slow to change, especially for marketed products because the cost to modify things like endotoxin tests is high and there is little incentive to make that investment. To date the FDA has approved only one new drug that uses the rFC endotoxin test (Eli Lilly’s migraine drug Emgality). In 2020, the agency that rates the safety of products used in drugs and devices, U.S. Pharmacopeia, refused to rate rFC as the equal of LAL, meaning that healthcare companies will be reluctant to switch to rFC for the near term. Part of that may be due to one of the leading bleeding companies actively campaigning against the use of LAL made with rFC, despite research showing that rFC works.
With rising human population, increasing access to more sophisticated health procedures, and the growing threat from antibiotic resistant bacteria, the need for LAL will continue to increase. In 2012 the International Union for Conservation of Nature (IUCN) created a subcommittee to monitor the population of horseshoe crabs and labeled them as “vulnerable.” At the time, IUCN scientists predicted that horseshoe crab numbers might fall as much as 30 percent by the year 2052. The erosion of beach habitat due to the rise in ocean levels, and the continued development along our coastline are ongoing factors in the health of the species.
In April 2020, Kepley BioSystems made an announcement that could significantly alter the need for horseshoe crab harvesting. Working with the Joint School for Nanoscience and Nanoengineering, they developed a method for using aquaculture to grow and maintain healthy horseshoe crabs. They went on to develop special lab techniques for extracting crab blood in a manner that is much easier on the crabs, cutting the death rate to zero. They calculate that through their methods they could produce the same amount of LAL now extracted from 500,000 harvested crabs with only 45,000 crabs grown in a special aquaculture facility.
In an interesting twist, states like New Jersey and South Carolina have now halted the harvests of horseshoe crabs to protect a declining shorebird, the red knot, which feasts upon protein-rich crab eggs during migration flights through those states. Conservation groups have also mounted efforts to protect horseshoe crab populations from exploitation. Along with the Ocean Conservancy, the Ecological Research & Development Group (ERDG) has made the crab the focus of its fundraising, research and advocacy efforts. They have worked with fisherman in the Caribbean to use bait bags and reduce the need for horseshoe crab bait. They are working with shoreline landowners to develop beach sanctuaries for known crab breeding sites. Finally, they encourage beach goers to help crabs when they are found lying on their backs. ERDG’s slogan is “Just flip ‘em!” in hopes that a struggling crab might find its way back to the sea and survival. All of us should be able to help with that!