Horseshoe crabs have been around for 300 to 450 million years but are now declining rapidly due to the loss of coastal breeding habitat, overharvesting for bait and fertilizer, medical use, hunting and harvesting as a delicacy.
They also play a critical role in the future of the endangered Red Knot shorebird. These two species’ fates have been intertwined for millennia. As the birds migrate from the southern tip of Chile to the Arctic to breed, their most important stop-over is the Delaware Bay, which has the highest concentrations of horseshoe crabs on the East Coast. These 4-ounce birds time their arrival to the full-moon breeding cycle of the crabs and depend on being able to feed on the protein-rich crab eggs before continuing the last leg of their 9,000-mile journey. Without this chance to rebuild their fat supplies, many birds never reach their breeding grounds.
Because of overharvesting of the crabs in the 1990s, when the allowed take escalated from 100,000 to 2 million a year, the numbers of crabs declined by as much as 80 percent. Without this critical food resource, Red Knot numbers plummeted by an estimated 60 percent, making them eligible for federal threatened species status. Even with the problem clearly defined by scientists, it still took years to get the Atlantic States Marine Fisheries Commission to lower the crab harvest numbers. The state DEC regulates crab harvesting in New York. Crab fishermen are allocated 300,000 yearly by ASMFC but voluntarily lowered that to 150,000 and ban harvesting during peak spawning periods in May and June out of concern for shorebirds. Since 2007, DEC and Cornell Cooperative Extension have worked together to monitor horseshoe crab spawning activity on beaches during May and June.
Loss of their refueling stop is not the only peril Red Knots face. In eastern South America and the Caribbean, they are shot for food and sport. A further challenge is climate change for both birds and crabs. Shoreline habitat is most threatened by coastal sea level rise. Additionally, warming water may shift the crabs’ breeding season, which would cause the Red Knots to arrive at the wrong time for this critical last feeding stop. In addition to Red Knots, other birds, turtles, and fish eat the eggs as an important part of their diet.
Horseshoe crabs have unique features. Their common name comes from the shape of their shell, resembling a horseshoe. That shell is their exoskeleton. As they grow their shell does not enlarge so they shed that shell in order to grow a new one. If you find a shell on the beach, you can check if it was a molt by pressing just under the front of the shell. If it separates from the top, the crab molted to grow a new shell. They have 10 legs they use for walking on the ocean bottom and nine eyes scattered over the body and several more light receptors near the tail. Their tail is called a telson. It’s long and pointed, but it is not dangerous, poisonous or used to sting. They use the telson to flip themselves over if they happen to be pushed on their backs.
They are not true crabs in the crustacean family but related to sea spiders. There are four species of crabs worldwide. The three other species are found in the Indian Ocean and Pacific Ocean along the coast of Asia. The Asian crabs face similar threats and are also in decline.
Horseshoe crabs have become extremely important to medicine. Their blue, copper-based, blood has antibacterial properties and contains a chemical that is very effective and fast reacting to bacteria. It is utilized for medical testing of vaccines for contamination. Due to the limitations of harvesting, the blood can sell for up to $15,000 a quart. This has led to the harvesting of a quarter of a million horseshoe crabs. In attempts to mitigate overharvesting, scientists began to only take 30 percent of the crabs’ blood and then release them. While they are believed to be able to survive, we don’t know what happens when they are released, but it is believed that around 10-30 percent do not survive and the impact on their ability to breed is unknown. A synthetic version is being developed which may help relieve the pressure on the crabs.
June is mating season for horseshoe crabs, so it’s the best time to see these fascinating creatures. They come up on the beaches around the full moon and highest tides to breed and lay eggs. The females may be surrounded by several males and she’s easy to spot because she’s much bigger.
It would be a terrible shame, and a shame on us, if these ancient creatures that are older than the dinosaurs were to become extinct mainly because they are being chopped up alive for bait. Their loss would have a tremendous impact on the coastal food web, leaving the Red Knot and dozens of other species without a critical food resource and likely causing the extinction of the Red Knot as well.