(Describer) A beach appears.

(narrator) Every year, East Coast beaches play host to one of the world's wildest parties. It's all sex and gluttony on the shore. But these spring breakers are of the ten-legged variety. Millions of prehistoric creatures converge here. What's behind this epic fiesta?

(Describer) Waves lap over rows of horseshoe crabs along a shoreline. The title appears: "Overview." Logo: "PBS Terra."

[upbeat music]

The waters of Delaware Bay look unassuming, but as the temperatures warm in spring, something curious happens. Strange creatures emerge from the depths. These ancient beings are horseshoe crabs. Horseshoe crabs spend most of their lives in the deep ocean, but every May and June, millions of these creatures drag themselves ashore on sandy beaches up and down the Atlantic coast. Some will travel 60 miles or more just to get here. While aerial views of these creatures are pretty cool, we want to take you even closer. So we teamed up with our friends at Deep Look to show you this.

(Describer) The view zooms revealing the claws on a horseshoe crab's underside.

Well, they might look like crabs, they're actually more closely related to spiders and other arachnids than to crabs or lobsters. Nowhere is the concentration of horseshoe crabs greater than here in Delaware Bay. As many as 30 million crabs, a population greater than the size of Shanghai, China, descend on the bay.

(Describer) Crabs cover a low tide mudflat.

But what are they doing here? These beaches are the ultimate horseshoe crab hookup spot. Male crabs arrive just ahead of the females. They form a wall that females must plow through to reach suitable nesting sites. They jostle until one lucky male successfully grabs onto the female's shell and pulls himself aboard. Each female will carry upwards of 80,000 eggs. She buries them in the sand, which her suitors fertilize.

(Describer) "Dave Smith, Ph.D."

(speaker) The females burrow into the sand so she is almost completely covered up when she's making those nests. And the males are on the surface of the sand and surrounding it. They like to spawn high on the beach. The theory is that that was a strategy to avoid aquatic predators.

(narrator) To do that, the crabs need to arrive when the tides are highest around the new and full moon in May and June.

(Describer) A full moon rises in a night sky.

Only a handful of the thousands of eggs a female lays will even survive a year. Fewer yet will survive to sexual maturity.

(Dave) You just have to buy a lot of lottery tickets to have a chance of winning.

(narrator) It's a strategy that served them well for a long, long time. They're older than dinosaurs.

(Dave) If you saw a fossil of horseshoe crab from hundreds of millions of years ago, you would recognize that as a modern day horseshoe crab. So it's a living fossil.

(narrator) There was a time, however, when their survival was in question. In the early 1900s, horseshoe crabs were harvested by the millions and used as fertilizer.

(Describer) Crab piles in a photo.

That practice ended, but today, they're harvested for bait, and for a different use.

(Describer) In a lab.

Their blue blood contains an ingredient critical to testing the safety of vaccines and other drugs. To prevent over harvesting, a yearly survey is conducted around Delaware Bay.

(speaker) Two males.

(narrator) Researchers and citizen scientists team up to walk the beaches and count the crabs. Zero, zero.

(Nivette) The horseshoe crab surveys are the way for us to learn more about the population.

(Describer) "Nivette Pérez-Pérez"

Three, four males, several females. There will be a recorder that will be the person in charge of taking notes and collecting the data. There will be a person rolling the quadrant, which is the method in which we randomize which areas of the beach we collect counts. One, two, three, four, five, six.

(Describer) "Marianne Walch"

(speaker) Every time I see these crabs, I just go, "This is so freaking cool."

[laughs]

Horseshoe crabs need big stretches of sandy beach area. It's an environment that's slowly disappearing around here with all of the development, so it is a big concern.

(narrator) For now, the population in Delaware Bay appears to be healthy and growing. That's a good thing, not only for the crabs, but for the other spring breakers. Up to a million migratory birds visit these shores on their way to arctic nesting grounds. It's not just coincidence the crabs and the birds are here together. For the crabs, it's mating time. But for the birds, it's the product of this mating that they're here for, eggs. These tiny green gems are horseshoe crab eggs. They litter the beaches during mating season. The eggs are an essential all-you-can-eat buffet for migratory birds. Some of these birds travel thousands of miles to get here, and their survival literally depends on this food.

(Describer) "Henrietta Bellman"

(speaker) Shore bird migration is timed such that birds arrive in the Delaware Bay during the peak of horseshoe crab spawning.

(narrator) The threatened red knot is one such traveler. It flies from the southern tip of South America all the way to the Arctic. A journey of nearly 9,000 miles.

(speaker) When they arrive in these stopover locations on the Delaware Bay, they'll be very skinny and you can almost visibly see their breastbone.

(narrator) These nutritious eggs are the fuel needed to complete their migration.

(Henrietta) By the time they leave, they've blown up like a balloon that you could pop with a pin. When we think of migration, we think of these really incredible international events that wouldn't be happening right here in little old Delaware.

(Describer) From above, sunlight glistens on rippling bay water.

(narrator) But like any good party, the morning after can be a real drag.

(Describer) In time lapse, crabs return to sea. Dave Smith.

(Dave) They have the ability, surprisingly, to be stranded after a tide is retreated, hunker down, stay moist, wait for the next tide.

(Describer) "Diane McFaul Hindman, Delaware Bay Resident"

(speaker) The sun really takes a toll on them, so sometimes you think they're dead until you give them a little push. And then when they start moving, it's like, "Oh, good, I could save this one!" It's almost addictive. You say, "I'm just gonna flip over ten more," and then you say, "Well, I can do another ten." I really do take great pleasure in saving a big mama knowing that she will be able to lay eggs again, because when you see the how the birds rely on them, you know, it's all part of the circle of life. Accessibility provided by the U.S. Department of Education.

[upbeat music]

(Describer) Dave and Diane walk on the beach. Credits appear. Host/Co-Writer: Joe Hanson, Ph.D. Producer/Editor: Michael Werner Director of Photography: Michael Werner Executive Producer: Rachel Raney Credits continue. Produced by PBS North Carolina for PBS Digital Studios. Anne Ray Foundation. Thank you to Margaret A. Cargill Philanthropies for Supporting PBS. Accessibility provided by the US Department of Education. Logo: PBS.