Horseshoe Crabs: Over the Moon

By Ben Scott

horseshoe crab

The moon shines brightly. You take a deep breath of fresh ocean air as you feel the warm summer night breeze pass through your hair. You sense the soft sand against your bare feet and your lover’s fingers intertwined with yours as you walk together down the beach. As you turn to glance at the gentle moonlit waves lapping against the shore, something on the edge of the water catches your eye. What could it be? Is it a giant piece of red sea glass? A beautiful shell?

As you move closer and closer to the water, you start to hear sounds you can’t recognize. Your curiosity gets the better of you, and though a little frightened, you continue on and see it: thousands of strange creatures, with spidery legs and large bowl-shaped shells gathered together in the shallows of the ocean, dancing in the moonlight. Horseshoe crabs.

A few times every year, during the high tides of the warmer months, tens of thousands of horseshoe crabs crawl out of the water off the Atlantic coast. Though the bizarre sight might seem to many at first to be a massive crustacean orgy, there is actually no intercourse in the ritual. Female horseshoe crabs dig small holes in the sand and lay their eggs, while males (often four or five per female) attempt to fertilize the eggs as they emerge. The act is called “spawning.”

spawning horse shoe crabs

You may ask, why should you care about some random mating crabs on the beach? Well, besides the sheer strangeness and beauty of the act, there are three reasons:

  1. The first reason is that the ritual is extremely rare. It only happens a few times a year in North America and Southeast Asia. The crabs hatch from the eggs after two weeks, and the larvae make their way to the water, spending the next 10 years on the sea floor until they reach adulthood and return to spawn themselves.
  2. The second reason is that this mating ritual has been taking place for over 450 million years. Yes, 450 million. For reference, the oldest evidence of dinosaurs dates to around 230-240 million years ago. Horseshoe crabs are in the top ten oldest creatures still living today. In witnessing the yearly horseshoe crab spawning, you are seeing one of the oldest rituals still practiced on our planet—quite literally a glance into the Paleozoic era.
  3. So, horseshoe crabs are extremely rare, they show up en masse a few times per year, but what else? Well, if all that wasn’t enough for you, how about this? Horseshoe crabs save lives. Every year, hundreds of thousands of horseshoe crabs are removed from the ocean and bled using steel syringes. Their blue blood is a miracle substance: With a massive sensitivity to bacteria, horseshoe crab blood is used in the modern medical industry to test cleanliness and contamination of intravenous drips, shots, and other items that may come in contact with the human body, including pharmaceuticals. These tests—using the crab blood to determine the cleanliness of a solution—have become commonplace in the medical industry.

When the procedure is done, the crabs are released back into the water, though 20-30% of them die during the procedure. This, coupled with the practice of using the crabs as bait for fishing, has led to a massive decrease in the last few decades in the population of horseshoe crabs, along with other animals that thrive off of their eggs.

Thankfully, scientific progress now allows for synthetic horseshoe crab blood as a legitimate replacement for the natural miracle blood. Though synthetic blood has not been adopted worldwide, its proven success will pave the way for the ending of the harmful practice of bleeding, and perhaps an increase in the population of horseshoe crabs.

So, does a rare gathering of alien-like prehistoric beings responsible for massive progress in the medical and archaeological fields—creatures that have likely saved millions of human lives—interest you? Then get out to the beach and see these wonders in action. The best time to see horseshoe crabs this year is around the new moon, from June 1 to June 5. Horseshoe crabs spawn on protected beaches along the Atlantic Ocean and Gulf of Mexico. Slaughter Beach and Pickering Beach on the Delaware side of Delaware Bay are prime locations to see large numbers of horseshoe crabs spawning.

Horseshoe crabs have survived mass extinctions, continental shifts, and wildly changing climates, and hopefully they will continue to crawl up from the seafloor and bask in the moonlight for millions of years to come.

flip over horseshoe crab

TIP If you’re walking down the beach and see a horseshoe crab on its back, gently grab its shell on one side and flip it back onto its feet. Don’t worry! It won’t hurt you, and the simple act may save its life.

For more about horseshoe crabs, listen to the podcast at

This article appeared in the Beachcombing Magazine May/June 2019 issue.

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