Once a Spawn a Time: Horseshoe Crabs Mob the Beach

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Horseshoe crabs may look scary, but when it's springtime in Delaware Bay, millions of these arthropods show they're lovers, not fighters. They lay masses of blue-green eggs up on the shore. At just the right time, they pop and release the larvae within to the sea.


These delicate, otherworldly creatures are just starting their lives —twitching and twirling inside their translucent homes.

They’re baby horseshoe crabs, preparing for the moment they can break free.

It all started two weeks ago, when the tides were at their highest.

Horseshoe crabs emerge from the briny deep.


And head for the shore with only one thing on their minds.

A springtime spawning spree.

Atlantic horseshoe crabs gather, by the millions, along the East Coast of North America — from Maine to the Gulf of Mexico.

The crowds here in Delaware are some of the biggest.

The males show up first, hanging out on the beach and in the shallows of the bay.

They scramble to latch on to the backs of passing females.

The females can be the size of dinner plates, way bigger than the males.


They take advantage of the fleeting high tides to haul their precious cargo as high up the beach as possible.

The females muscle their way up the beach, with the males in tow.

The greatest danger for these horseshoe crabs is flipping over, exposing their vulnerable underside.

And it only takes a small wave to do it.

Under the hood, horseshoe crabs’ legs end in pincers.

But mature males have something special on their front legs …

This “clasper” that looks like a little boxing glove with a hooked finger.

It’s perfect for gripping the back of a female’s shell.

To flip back over, the animals use their long, spiky tail, called a telson.

It looks a little scary, but horseshoe crabs don’t sting.

Horseshoe crabs have been making these high-tide treks under the glow of the full moon since before the dinosaurs.

That’s more than 400 million years ago.

Not everyone finds a date.

Especially those younger males.

But they hike up the beach anyway in hopes of getting in on the action.

This female is dragging around two admirers.

When a female finds a good spot, she digs and digs into the wet sand.

The female lays roughly 4,000 eggs in one go.

The male that clung to her all this time is in the best position to fertilize the most eggs.

All those single dudes crowd around too, vying to fertilize the rest.

As the party winds down, the grown-ups start heading back to sea,

But they’ll return for more of these high-tide soirees throughout the season.

They leave their fertilized eggs behind, buried in the damp sand.

Each one is smaller than a pea.

Over the next couple of weeks the embryos inside develop.

By the time the high tides return — these larvae are ready to hatch.

The jostling of the waves stimulates them to break out of their shell.

This is their chance.

They have just a few hours to scramble into the turbulent surf, before the high tide recedes.

They’ll mature beneath the waves for roughly a decade before they’re ready to return as adults themselves.

… Where they’ll take their turn in this ancient dance of the moon, tides and sand.

Hey Deep Peeps.

The party doesn’t stop here.

Notice those cool aerial drone shots?

That’s because we collaborated on this episode with our friends at "Overview," an awe-inspiring new nature show on PBS Terra.

Where Deep Look zooms in, "Overview" zooms out.

Watch and tell them Deep Look sent you!