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GUY RAZ, host:

We're back with ALL THINGS CONSIDERED from NPR News. I'm Guy Raz.

Now, for most of us, the state of the moon won't have much effect on how our evenings go tonight. But on a recent night on the shore of the Delaware Bay, a full moon was a signal to start a really wild beach party. It's a party that's been taking place for millions of years.

And for Louisa Jonas of WYPR in Baltimore, it's one of the highlights of the season. Here she is with the first of two reports from the water's edge.

LOUISA JONAS: I want you to imagine you're stepping back in time, say, 400 million years. We're talking before the dinosaurs, before flying insects even. Now, imagine you've morphed into a female horseshoe crab.

You spend most of your time trudging along the bottom of the ocean. But tonight will be your night to remember. You're swimming to the shore, which you only do for a few weeks a year. Along the way, you meet that special someone, and he clasps onto the back of your shell. You and your mate crawl into the beach, where you'll spawn at high tide under the light of the full moon. And you're not alone. Thousands of other horseshoe crabs are piling on top of one another. Your glistening shells cover the beach for miles. And because you're female, you're in for a treat. The mate attached to your shell is not your only tryst. Tonight, you will mate with up to 12 or 13 males, all at the same time.

(Soundbite of song, "Sea of Love")

Ms. CAT POWER (Musician): (Singing) Come with me my love to the sea, the sea of love. I want to…

JONAS: Fast forward to the 21st century A.D., it's three nights before the full moon. As a horseshoe crab, you haven't changed much. Dinosaurs may have grown wings and turned into birds. You haven't really needed to evolve. A master generalist, you've survived asteroids, volcanoes and ice ages. Scientists call you a living fossil. And like your ancestors, you know where to find a few willing companions this evening.

The Delaware Bay shore is the site of the largest horseshoe crab spawning in the world. And tonight, you have some human visitors.

Ms. JORDAN RUTTER(ph): My name is Jordan Rutter, and I'm from Silver Spring, Maryland.

Mr. KEITH RUTTER(ph): I'm Keith Rutter.

Ms. ANN RUTTER(ph): I'm Ann Rutter.

Ms. PAM RUTTER(ph): I'm Pam Rutter, and I'm from Silver Spring too.

Mr. RUTTER: We just love the whole phenomena of how once a year or, you know, for a season, the horseshoe crabs do this incredible mating on the beach. I don't know how anybody can watch this and not get excited about, you know, just nature and science and how things work in the world.

JONAS: But the Rutters didn't travel to Pickering Beach, Delaware, just to watch. They came here to count. In the 1990s, the horseshoe crab population in the bay plummeted. So tonight, at the world's epicenter of horseshoe crab sex, volunteers in headlamps and waders will tally up the amorous crabs.

Mr. K. RUTTER: If the untrained observer - if you didn't know about horseshoe crabs and you just walked up on this beach right now, and you looked out along a shoreline, you would just think they were shiny stones - thousands of shiny stones. And it looks like you could walk the whole length from this beach stepping from stone to stone. They're that thick. But when you stop and think that they're horseshoe crabs and how many thousands of years - millions of years that they've been doing the same ritual, you know, you can't help but touch it.

JONAS: At first glance, it's hard to get sentimental about a horseshoe crab. Horseshoe crabs are weird. They're like armadillos of the sea. Their shells look like armored helmets and their tails resemble swords. Then, consider their eyes. They have 10 of them, including one on that spiked tail that helps them tell time. And there's no such thing as safe sex here.

About 10 percent of crabs die upside down when they can't right themselves during spawning.

Stew Michels is a fisheries scientist from Delaware Division of Fish and Wildlife. He's leading tonight's survey. Stew admires a single pile of crabs.

Mr. STEW MICHELS (Fisheries Scientist, Delaware Division of Fish and Wildlife): Okay. What we have is a congregation of spawning horseshoe crabs. There's number of males on top surrounding a female, who's buried into about 10 centimeters in depth. Attached to her rear is a single male, and he can fertilize the eggs exclusively if there's no other males around. However, there's lots of males on this beach right now, and they're all competing to reproduce with the female. And they will also - and fertilize a portion of her eggs tonight.

JONAS: In one sitting, each female lays about 4,000 tiny, green eggs that look like clumps of pesto in the sand. In a few weeks, only the luckiest of eggs will hatch and billions of eraser-sized horseshoe crab babies will wash into the bay.

At exactly 9:04, high tide on Pickering Beach tonight, the teams get to work. Jordan, the eldest Rutter daughter, thrusts her French-tip manicured fingers into the foamy surf. She gropes the spawning crabs in a one square meter grid, distinguishing males from females.

(Soundbite of waves)

Ms. J. RUTTER: I counted four females and then I counted 13, plus the one that you threw out.

JONAS: It's not so easy to determine who's who in the breaking waves, with the crab sometimes three layers deep. But the Rutters move fast. They have to cover one kilometer of beach before the tide recedes and the creatures disappear back into the water. Pam says she and Keith have been bringing their daughters to Delaware to count spawning horseshoe crabs since the girls were small.

Ms. P. RUTTER: You feel like you're part of the bigger world here. You know, it's pitch dark right now, and all of this is happening. And it's just - it's pretty amazing. Look at them.

(Soundbite of laughter)

JONAS: By sunrise, most of the crabs will have vanished. But if it's been a successful spawn, so many eggs will cover the sand that the shore for miles will shine emerald green.

Recent surveys suggest that the horseshoe crab population may be stabilizing, and everyone here hopes this latest count will show an increase in spawning. The crabs themselves are certainly doing what they can; even after the tide recedes, taking most of the females with it, a few determined, solitary males still patrol the beach, scanning the deserted sand for a mate.

(Soundbite of waves)

For NPR News, this is Louisa Jonas.

RAZ: Tomorrow, Louisa explores how this annual crab orgy is key to the fate of a small, cinnamon-colored shorebird. And if you go to our Web site, you can see pictures of the crabs and birds, and hear more about the efforts to conserve them.

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