Horseshoe crabs quiet stars of COVID vaccine effort
When scientists come up with a vaccine for the global threat posed by COVID-19, they will have one the planet’s oldest, strangest-looking and least understood — and decidedly blue-blooded — residents to thank.
The 450-million-year-old horseshoe crab, the closest living relative to the prehistoric trilobite — and a denizen of Earth’s oceans since nearly 200 million years before dinosaurs showed up — is at the center of the race to make a safe and effective vaccine available to the planet’s 7.8 billion people.
You might know tanklike horseshoe crabs, commonly found in Long Island Sound and along its beaches, primarily as the heavily armored, exceedingly leggy creatures you sometimes find on Connecticut beaches in Old Saybrook, Guilford, West Haven, Milford, Stratford, Fairfield, Norwalk and Greenwich, among other places.
Perhaps you even once flipped one rightside-up to help it survive after its long, pointed tail failed to do the job.
But without horseshoe crabs’ deep blue blood, which is a crucial part of the biomedical research process to make sure new
breakthroughs are safe, a whole bunch of medical miracles just wouldn’t happen.
“It’s actually important for the tests of anything that goes into the human body at all,” including vaccines and other injectible pharmaceuticals, heart valves and other breakthroughs, said Dave Hudson, a research scientist at the The Maritime Aquarium at Norwalk, which recently opened its new “Horseshoe Crab Culture Lab,” with a window that lets the public watch as staff work and young horseshoe crabs molt their exoskeletons to grow.
One purpose of the Norwalk lab is to try to breed and grow horseshoe crabs to bolster their numbers in the Sound.
The Atlantic horseshoe crab, aka Limulus polyphemus, is found along the Atlantic and Gulf coasts of both the United States and Mexico, from Maine to the Yucatan. More closely related to spiders and scorpions than actual crabs, the misunderstood critters are one of four species of horseshoe crab in the world.
The other three species all are found in Southeast Asia: Tachypleus tridentatus, T. gigas, and Carcinoscorpius rotundicauda, which occupy coastal waters of Asia from India to Japan, including waters around the Dutch East Indies and the Philippine Islands.
Horseshoe crabs, which also are important sources of food for migratory birds, do better in some places than others — Delaware Bay seems to be the center of horseshoe crab life here in the United States.
The Connecticut and New York populations are considered to be “vulnerable,” or “of concern,” which is one step better than being threatened, according to researchers and a 2019 report by the Atlantic States Marine Fisheries Commission.
In addition to being used for biomedical research, horseshoe crabs are heavily fished as bait for the whelk (scungilli) and American eel fisheries, which many researchers consider to be a greater threat than any increased demand from the biomedical industry.
Biomedical companies “bleed” horseshoe crabs and then return them to the ocean, although about 15 percent of the horseshoe crabs taken to be bled don’t survive the process, researchers say.
“I have a contacts highly placed within the industry, who I respect,” who “basically said they’re talking about a couple of days production” for all of the Limulus amebocyte lysate, or LAL, that would be needed for production of enough vaccines to inoculate the world’s population, said Mark Botton, professor of biology and co-director of the Environmental Sciences Program at Fordham University.
The world’s human population currently is estimated at about 7.8 billion.
“I would think that this is not going to push (horseshoe crabs) over the edge,” said Botton, cochairman of the International Union for Conservation of Nature Horseshoe Crab Specialist Group and has been doing research on horseshoe crabs for many years. “Even if they required double the number of crabs, it’s still a relatively small number compared to the amount of horseshoe crabs that are taken for the bait-fishing industry.”
LAL produced by horseshoe crabs is used for the detection of bacterial endotoxins in medical applications. Immune proteins in horseshoe crab blood clot when exposed to bacteria, Botton said.
There currently are six companies along the Atlantic coast that extract horseshoe crab blood to produce LAL. They are located in Massachusetts, New Jersey, Maryland and Virginia, according to the state Department of Energy and Environmental Protection.
Jennifer Mattei, professor of biology at Sacred Heart University in Fairfield and the longtime leader of Project Limulus, a study and census of horseshoe crabs in Long Island Sound, said that while there are companies as near as Massachusetts that bleed horseshoe crabs to create LAL, Long Island Sound crabs “are not harvested or bled for that product.”
“But on the other hand, our population is in decline” and the Atlantic States Marine Fisheries Commission’s Horseshoe Crabs Advisory Commission “cited New York and Connecticut as having a poor management plan right now because the population is in decline.”
Horseshoe crab populations have been in decline in Long Island Sound for at least 15 years.
“They’re supposed to change the management rules and the harvest rules this year,” said Mattei, who has studied horseshoe crabs in the Sound for 22 years. “So we’re waiting to see what they’re going to do.”
Justin Davis, assistant director of the DEEP’s Marine Fisheries Division, said that while Connecticut horseshoe crabs currently are not used for biomedical purposes, “the available science suggests that their status ... is not great at this point.”
Davis, who is one of three Connecticut representatives on the Atlantic States Marine Fisheries Commission’s Horseshoe Crabs Advisory Commission, and also a member of the commissioner’s Horseshoe Crab Management Board, said the most recent stock assessment suggests that the Connecticut stock “is in kind of a depressed state.”
A 2019 ASMFC assessment found that the New York fishery, which includes New York, Connecticut and northern New Jersey, was the only one of four geographical horseshoe crab fisheries rated “poor.” The Northeast (Maine, Massachusetts and Rhode Island) fishery and the Delaware Bay fishery both were found to be “neutral” and the Southeast fishery was rated “good.”
“Unfortunately, with this species we don’t have the
good scientific models that we have for some of the other species,” said Davis. “It’s one of those species where we know that it’s threatened, but we don’t really know ... why it’s threatened.”
Since 2001, Connecticut’s annual fishing quota for horseshoe crabs has been set at 48,689 crabs, while the actual annual harvest has varied between 15,000 and 30,000. In 2018, more than 99 percent of Connecticut’s harvest was taken by just 12 license holders, according to DEEP figures.
Over the last 18 years, the number of horseshoe crabs harvested in Connecticut has ranged from 12,175 in 2001 to a high of 32,535 in 2008. From 2013 through 2018 the number was about 20,000 per year.
Fishing generally takes place on land when the animals come up on beaches to spawn, Davis said. “Our annual quota is much smaller than New York’s and we don’t harvest the entire quota,” he said.
The horseshoe crab fishing season in Connecticut runs from May 22 to July 7, although horseshoe crab fishing is banned in Milford, Stratford, West Haven and Westbrook.
The Connecticut Audubon Society in February urged state officials to ban the harvest of horseshoe crabs and increase law enforcement efforts to curtail illegal horseshoe crab harvesting.
Shorebirds, in particular the Red Knot, time their migration to their northern breeding grounds to coincide with the horseshoe crabs’ egg-laying.
The drop in the number of horseshoe crabs in Long Island Sound, as well as in Delaware Bay and other locations along the coast, is believed to have led to a decline in the population of Red Knots, which recently were listed as threatened under the federal Endangered Species Act.
One of the purposes of Mattei’s research is to try to understand the critters’ movement patterns in the Sound, including “whether or not we share the population with New York or whether we have a separate population in Connecticut.”
Over the years, they have tagged more than 90,000 crabs “and we have a huge database now” that shows “that they move all around,” Mattei said.
“Crabs may be on our beach, and then two years later they’ll be on the North Shore of Long Island,” she said. Fifty percent to 60 percent of the horseshoe crabs appear to be “home bodies” that don’t travel much, but the rest appear to move mostly from east to west, and sometimes cross the Sound, Mattei said.
“So there’s one population, one genetic pop
ulation, in the Sound,” she said.
It takes a horseshoe crab about 10-12 years to mature, which is when the females start laying eggs, and they live for probably another 10-12 years beyond that, Mattei said. “The oldest can be 20-25 years old.
“The problem is, particularly in the Sound, that not many make it to adulthood,” she said.
While each female horseshoe crab may produce up to 88,000 mature eggs, “only a fraction of the eggs laid make it to adulthood,” Mattei said.
Decades of research, harvests
Horseshoe crabs’ value for medical research was originally discovered in the late 1960s or early 1970s at Wood’s Hole Marine Biological Laboratory on Cape Cod, Mattei said. “They spilled some blood on a countertop and it immediately coagulated.”
Prior to that, researchers used rabbits to assay the purity of new medical breakthroughs, she said. “This sped up the process. That’s why everybody’s talking about it ... they can do an immediate test with the horseshoe crabs.”
Horseshoe crab spawn during the late spring on coastal beaches along the Atlantic and Gulf of Mexico coasts. They lay their eggs in nests buried in the sand, generally in areas within bays and coves that are protected from the surf.
New Jersey has banned the harvesting of horseshoe crabs and in Delaware Bay, which is bordered by New Jersey on the north and Delaware on the south, only males may be taken, said Mattei.
Mattei said that the three Asian species of horseshoe, which are not protected the way Atlantic horseshoe crabs are, all are in decline, in part because of heavy fishing by China both for medical purpose and for food. In China, they eat the eggs,” she said.
Botton said horseshoe crab eggs also are eaten as food in parts of Thailand, Vietnam and Cambodia, as well as Southern China.
Down in Delaware Bay, however, it’s the shorebirds that are eating the eggs.
That’s one reason why it’s not enough just for horseshoe crabs to not be threatened. “It’s one of the species that needs to be abundant,” Mattei said. “In Delaware Bay ... shorebirds eat the eggs. ... It helps fuel their flight to the northern areas where they lay their eggs.”
“The population in the Sound is not big enough for that ... and that’s one of the reasons why there’s not a lot of shorebirds,” Mattei said. “It’s not going to go extinct, but they are not functioning well in Long Island Sound.”