For more than 400 million years, horseshoe crabs — 10-eyed, blue-blooded, dome-shaped sea creatures — have inhabited our planet, crawling ashore in the spring and summer to mate and lay thousands of eggs that look a bit like grains. millet
They did this before Pangea broke up into the continents we know today. They were doing this the year an Everest-sized, dinosaur-killing asteroid smashed into what is now Mexico.
And they continue to do so as humans transform the planet, replacing forests with farms, covering beaches with buildings, and flooding the air and water with pollution. Even in the most urbanized parts of the coastlines of North America and Asia, these animals still appear each year to spawn the next generation.
One evening during a heatwave in June, I went with some friends to Plumb Beach in South Brooklyn. It is a beautiful place. The water is calm and speckled with sandbars and patches of tall grass. It is also a city beach. Plastic takeout containers poked out of the sand. There is a large tire in the shallows. Traffic on the Belt Parkway competes with the sound of the waves.
However, during the months of May and June, horseshoe crabs can be seen here like magic around the full moon and new moon.
Females, which are noticeably larger than males, release pheromones on their way to shore. A lucky male will use a pair of special claws to grab her back and release sperm while laying eggs in the sand. Other “satellite” males will follow the pair and try – often with successful results – to get their sperm into the mix as well.
That evening the orgy of these ancient creatures was on full display. Just before sunset, dozens of crabs—which aren’t technically crabs but arthropods more closely related to spiders—start coming ashore. Most of them pair up. Every few meters I saw a woman dragging a dinner-plate-sized man with a salad-plate-sized one. Some crabs were in piles. Others were in conga lines: rows of five or more people connected to each other.
Horseshoe crabs were covered in colorful combinations of other sea creatures – barnacles, oysters, snails and even crustaceans – as if they each had their own style. Sometimes called walking hotels, these creatures are a foundation of so much life. They are ecosystems unto themselves.
Not all of that they support. The many eggs that horseshoe crabs lay on the beach—which can number in the tens of thousands per female—are food for more than 20 species of migratory birds.
None, perhaps, depend on them more than the red knot. Rotund sandpipers, which sport rust-colored chests in summer, are among the world’s most impressive travelers. In the Eastern Hemisphere, some red knots migrate more than 9,000 miles from the tip of South America to the Arctic Circle. And they rely on horseshoe crab eggs for this journey. In the spring, red knots travel north in line with spawning along the East Coast (and especially in Delaware Bay).
Red gulls will use Plumb Beach for fuel, although I saw a different group of eggs that evening: laughing gulls, which are essentially advanced seagulls. Although they are also seagulls – a colloquial term that people use for all gull species found near the sea – they have black heads and sound like cackles. I saw dozens of them traveling down the beach in a pack, eating eggs and laughing.
Horseshoe crabs also support the lives of billions of people. Their blood, made blue by copper-based proteins that transport oxygen, is a medical wonder. It coagulates rapidly in the presence of bacterial toxins, making it invaluable for sterility testing. Scientists use horseshoe crab blood to ensure that medical devices, intravenous drugs and vaccines — including those for Covid-19 — are free of harmful bacteria and safe to use.
I find them quite remarkable: these animals, which are basically walking with helmets, have been supporting life around them for millions of years.
Then we have.
In an impenetrable fraction of the horseshoe crab timeline, humans have snuffed out countless lives. We’ve ripped up forests and grasslands and overfished the oceans, and we’re now slowly cooking the planet with carbon dioxide. Humans are breaking down the support beams of ecosystems.
We are even challenging the resilience of horse crabs. Over the past few decades, and especially in the 1990s, millions of these crabs have been harvested and bled. Later they often die. Fishermen also catch horseshoe crabs and use them as bait for eels and conch. Overharvesting can cause their populations — and the populations of some birds that depend on them — to crash. The red knot is federally threatened under the U.S. Endangered Species Act, primarily due to declining horseshoe crab populations.
When you see these animals on the beach, they don’t seem particularly intelligent or skilled. Males will confuse other males for mates. Sometimes they even pair with stones. I saw many of them stuck upside down, like June bugs, their legs moving in the air in an almost mechanical rhythm.
Then again, these animals have outlived almost every other organism on Earth. They are obviously doing something right. More profoundly, they helped sustain life around them. I might even suggest that we should all aspire to be more horse crab-like. We should look after these sea spiders and try to strengthen the ecosystem instead of destroying them.
