As thousands of ancient, armoured creatures shuffle along the crowded beach, it looks like a scene from the TV show Robot Wars, or some sort of dinosaur invasion. But this aggregation of Roomba-like animals with sharp tails is in fact the world’s largest horseshoe crab spawning event.
Despite their name, these strange animals aren’t crabs. In fact, they’re more closely related to spiders and scorpions than crabs or lobsters.
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These curious critters are famous for their bright blue blood, which is used in developing vaccines. They have a special compound in their blood which clots when it detects bacterial toxins, so medical professionals use it to test that vaccines or other drugs aren’t contaminated.
Each year in April, the ‘crabs’ start arriving in Delaware Bay, the hub of the action. Spawning peaks on new and full moons in May and June when the tide is high.
"When the female is ready to lay her eggs, she crawls up to the high water line on the beach with a male attached to her," says the U.S. Fish & Wildlife Service (FWS) on its website. "The male clasps onto the female’s shell with a modified pair of claws. The female drags him around during the spawning process."
But it’s not just one-on-one. "In addition to the attached male, several other males may also attempt to fertilise the female’s eggs by arranging themselves on and around the spawning couple during the egg-laying process," says FWS.
Hence the armoured chaos. The beach is a mass of shells getting rolled around – and even flipped over – by the waves surging against the shore.
Finding themselves upside down isn’t a problem for these ancient animals. Their seemingly menacing tail (called a telson) might look like a weapon but it’s not a stinger. It’s actually for flipping themselves right-way-up when they’ve been upended.
The design clearly works. These strange-looking creatures have looked like this for around 450 million years (give or take a hundred million years, depending on how your source interprets the fossil record).
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“Despite existing for hundreds of millions of years, horseshoe crabs are nearly identical to their ancient relatives,” says Florida Fish and Wildlife Conservation Commission (FWC) on its website. “This is because their body structure is extremely effective for survival, think, if it ain’t broke, don’t fix it!.”
“Some people think horseshoe crabs are dangerous animals because they have sharp tails, but they are totally harmless,” says FWC. “Really, horseshoe crabs are just clumsy and they use their tail to flip themselves back over if they get overturned by a wave.”
Together, the male and female crawl to the beach where she lays her eggs – around 4,000 in one clutch – and he fertilises them. She might lay several clutches each night for several nights. In total, she could produce more than 100,000 little eggs (to account for the high mortality rate of the offspring). They take between two and four weeks to hatch.
A shorebird called the red knot loves to gobble up these little eggs, arriving in their droves to stock up on energy before continuing on their migration from South America to the Arctic.
Although horseshoe crabs look hardy with their tough carapace, overfishing and habitat loss is a real issue for these marine arthropods. Coastal erosion can threaten these spawning areas because the horseshoe crabs need the sandy area to be just right.
Top image credit: Selwa Baroody/Getty Images
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