Hundreds of thousands of horseshoe crabs, often referred to as “living fossils,” are flocking to Delaware Bay to spawn during the high tides of May and June.
Horseshoe crabs coming ashore to spawn. (Video: AFP)
Scientists and volunteers are gathering on a protected beach along Delaware Bay to survey the horseshoe crab population as they spawn in the hundreds of thousands along the East Coast of the United States from late spring to early summer. The research team sets up a measuring frame on the sand to count the individuals inside, providing estimates of their numbers while also helping the animals that have been turned upside down by the rising tides. The involvement of citizen scientists helps raise community awareness and expands the government’s dataset, according to environmental scientist Taylor Beck, who is part of the survey project.
With a shell resembling a helmet, a tail that looks like a sharp spike, and five pairs of legs connected to its mouth, the horseshoe crab (Limulidae) may not appear cute. However, their bright blue blood clots when exposed to a toxic substance known as bacterial endotoxin, making horseshoe crabs essential for testing the safety of biomedical products since the 1970s, replacing tests on rabbits. “They’re not scary at all. They’re just going about their day, trying to produce more horseshoe crabs,” said Laurel Sullivan, a community education staff member specializing in invertebrates.
For 450 million years, horseshoe crabs have wandered the oceans of Earth. Now, this “living fossil” is classified as vulnerable in the Americas and endangered in Asia due to habitat loss and overharvesting for use as food, bait, and in the pharmaceutical industry. Despite their English name meaning “king crab,” they are actually more closely related to spiders and scorpions, with four subspecies, one living along the East Coast and Gulf of North America, while the other three are found in Southeast Asia.
Onshore, female horseshoe crabs lay egg clusters of 5,000 the size of golf balls for males to fertilize.
The Atlantic horseshoe crab has ten eyes and feeds by crushing prey such as worms and clams between its legs before bringing the food to its mouth. Males are much smaller than females. They crawl in groups of about 15 during spawning. Males mate with females as they move onto the shore. Once on land, females lay clusters of 5,000 eggs the size of golf balls for males to fertilize. Millions of tiny green eggs are scattered across the beach, becoming a vital food source for migratory birds.
Nivette Perez-Perez, a community science manager at the Delaware Bay Estuary Center, points to a massive strip of eggs stretching nearly the entire beach at the James Farm Ecological Preserve. Many orange-billed gulls swoop down to eat the eggs. Spawning is very dangerous for horseshoe crabs as they are most vulnerable when on land. When the tide comes in, some unfortunate ones get flipped over. About 10% of horseshoe crabs die each year from being exposed to the sun on their backs. In 1998, Glenn Gauvry, founder of the Ecosystem Research and Development Organization, encouraged the public to help by flipping the living crabs back over, increasing the survival chances of the species.
Each year, approximately 500,000 horseshoe crabs are harvested and have their blood drawn to extract a substance called Limulus Amebocyte Lysate, which plays a crucial role in testing for a type of bacteria that can contaminate drugs, syringes, and medical devices. The estimated mortality rate for horseshoe crabs during this process is 15%, with the survivors being released back into the sea.
