The Importance of Horseshoe Crab Habitats
These arthropods are found all along the Eastern seaboard, but their numbers are less abundant in New England and New York. Why?
No one knows for certain how the American horseshoe crab (Limulus polyphemus) survived cataclysmic events like the Great Dying, which claimed close to 95% of marine life and famously killed off the dinosaur population. But the fact remains that the horseshoe crab, a helmet-shaped arthropod whose lineage dates back 445 million years, has withstood a lot of evolutionary pressure.
Today, its unique blood plays a central part in vaccine contamination control. Horseshoe crabs are also important in the food chain and provide bait for commercial American eel and conch fisheries along some parts of the Eastern seaboard, including the Mid-Atlantic and New England.
But the horseshoe crab is not resistant to outside forces. A complimentary lecture at the New England Aquarium's Simons IMAX® Theatre will discuss how the American horseshoe crab experiences challenges in and out of their natural habitat and review the significance of their remarkable blue blood in public health.
While the American horseshoe crab is not in peril, its prevalence does vary depending upon where you look. Regional trends reported by the Atlantic States Marine Fisheries Commission (ASMFC), a multi-state group created in 1940 to promote and protect Atlantic coastal fishery resources, suggest that the stock of horseshoe crabs have increased in the Delaware Bay (from 1988-2008) and Southeast (from 1993-2009). Since then, the abundance of crabs has remained stable or continued to increase in parts of the Southeast and Delaware Bay, the ASMFC reported (PDF).
But the data paints a different picture for New England and New York. Stock assessments in New England from 1978 to 2008 and New York from 1987 to 2008 show a decline in the abundance of horseshoe crabs. Since 2009, the trend has not reversed itself, and in the case of New England it appears to have gotten worse, according to the ASMFC.
Conservation laws are one reason—and perhaps the primary reason—why the numbers are up in some geographic regions but down in others. In Maryland and Delaware, for instance, fishermen are prohibited from harvesting female horseshoe crabs and Delaware has established sanctuaries for horseshoe crabs during the spawning season. The Delaware Bay region also has arguably the most robust and reliable tracking data related to horseshoe crabs, which has helped identify trends and inform management.
New Jersey has placed a temporary moratorium on the harvest of horseshoe crabs. The moratorium was issued after overfishing of horseshoe crabs for bait in the Mid-Atlantic and Northeast was blamed, indirectly, for a decline in the number of small migratory birds, including red knots. The birds drop in at Delaware Beach for several weeks every spring to feed on horseshoe crab eggs. Some have traveled over 5,000 miles from their South American wintering grounds enroute to their Arctic breeding grounds.
In South Carolina, harvesting horseshoe crabs for baiting is prohibited, the stringent conservation laws are among the earliest to be enacted, and the hand-harvesting of crabs is only allowed for biomedical use, and there are quotas for that. Charles River, whose LAL manufacturing facility is based in South Carolina, is committed to employing only fishermen licensed by the Department of Natural Resources to hand-harvest horseshoe crabs from the coastal waters of South Carolina. In South Carolina, horseshoe crabs can only be used for LAL production and marine biological research, and not as bait for the eel and whelk industries.
In contrast, harvesting of horseshoe crabs for bait appears to be thriving in New England, with Massachusetts accounting for about 85% of the commercial horseshoe crab bait landings in 2012, according to ASMFC. The horseshoe crab fishery supplies bait for the American eel, conch (whelk) and, to a lesser degree, catfish fisheries, according to ASMFC. New England states also support the harvesting of crabs for biomedical purposes, but unlike South Carolina, fishermen are not prohibited from selling the crabs for bait after bleeding.
Glenn Gauvry, director of the Delaware-based non-profit Ecological Research and Development Group (ERDG), whose primary focus is the conservation of the world's four horseshoe crab species, says the moratorium in New Jersey has, ironically, put added pressure on the New England and New York horseshoe crab populations, where baiting is still allowed.
"We were opposed to NJ's moratorium on harvesting because we felt that it was going to put pressure on the smaller populations in the North, and it has done just that," says Gauvry. "By shutting down NJ [horseshoe crab] fisheries, it has shifted the attention out of NJ to New England states where it is more difficult to monitor the harvest."
Jennifer Mattei, a professor at Sacred Heart University (SHU) in Fairfield, Conn., who has been studying horseshoe crabs for nearly two decades, says habitat quality is most important for their survival; increasing levels of disturbance pollution and even sea level rise have also put pressure on New York's and New England's horseshoe crab populations. This is because the crabs reside along shorelines less pristine, more urbanized, and more developed and crowded than habitats further south.
These urbanized estuaries and coasts have high nutrient loads, large fluctuations in algal and bacteria, increased levels of pollutants (including heavy metals and pesticides), hypoxia and relatively low pH.
Mattei's conclusions are based, in part, on her longstanding efforts to document the movements and mating practices of horseshoe crabs that live and reproduce along the Connecticut beaches of Long Island Sound (LIS)—a tidal estuary of the Atlantic that stretches from the New York's East River in The Bronx to Block Island Sound in Rhode Island. Since 1999, Project Limulus has tagged over 90,000 horseshoe crabs—largely with the help of volunteers that Mattei and her colleagues recruit every year to help conduct the census.
Initially, Mattei noticed that more and more females were coming ashore without mates and as time went on reproducing well below their maximum rate. Her findings, published in 2010 and in 2015 contrasted with studies in Delaware Bay where horseshoe crab population density is higher and clustered mating behavior much higher, suggesting that population density is an important condition that determines mating behavior and that low population density may lead to decreased mate finding ability and lost opportunities for spawning.
Mattei and her co-author, Mark Beekey, Associate Professor at SHU, have also turned their attention to assessing the demographics and habitat requirements of juvenile horseshoe crabs, which have been largely absent from more than half of estuarine habitats that she has surveyed. "We don't know where they are," says Mattei. "Some people say they are out at sea somewhere, some say they might be below the surface."
What Mattei can say is that fewer and fewer newly-molted horseshoe crabs are being tagged every year on the beaches that Mattei conducts her census—suggesting the female crab population is aging—and Mattei wants to determine whether the quality of the habitat is to blame. Mattei thinks the probable causes are overharvest of adults (both legal and illegal) and polluted or hypoxic estuaries.
The ongoing study will try and identify the marshes and intertidal flats where juvenile horseshoe crabs are residing, feeding and growing during the first few years of their lives. Data from these habitats on sediment type, wave action, depth, temperature, vegetation type, water quality and food availability will then be used to determine if there are specific characteristics that optimize juvenile horseshoe crab growth and survival.
Finding better ways of managing and increasing horseshoe crab populations remains an ongoing concern,though. One potential solution that some groups have raised is the use of artificial bait for eel in conch. This practice, if adopted widely, could alleviate harvesting pressure of horseshoe crabs in New York and New England. However, habitat conservation and restoration is of utmost importance.
"Living Fossils and Blue Blood: The Story of the Horseshoe Crab and Human Health" will be held Aug. 13 at 7:30 p.m. at New England Aquarium's Simons IMAX® Theatre, Central Wharf, Boston, MA. A 6:30 p.m. reception precedes the event. Speakers include John Dubczak, General Manager of Charles River's Endotoxin and Microbial Detection Division and Kathryn Tuxbury, Associate Veterinarian at the New England Aquarium.
How to cite:
McEnery, Regina. The Importance of Horseshoe Crab Habitats. Eureka blog. August 05, 2015. Available: https://eureka.criver.com/the-importance-of-horseshoe-crab-habitats/