So ancient, they're timeless, horseshoe crabs would complement the scenery in futuristic science fiction or dinosaur movies. (They have appeared in the BBC-made TV series Walking With Dinosaurs.) The Japanese claim they look like a samurai helmet and have stories about horseshoe crabs embodying the reborn spirits of those legendary warriors.

When you look down at them on the beach, the Atlantic horseshoe crabs (Limulus polyphemus) appear to glide effortlessly. Their sleek, minimalist shells cruise the wet sand as though powered by some space-age energy source. The glass-fronted coastal display tank at the Virginia Science Museum in Virginia Beach offers a different impression: viewed from side-on, the crabs wave a cluster of busy segmented legs as they scrabble through shallow water, over rocks and other crabs. The contrast between the ultra-cool, gliding shell and the awkward waggling appendages is endearing - rather like ducks that appears so calm and serene on the surface but paddle like crazy to stay afloat. Or the Wizard of Oz - an ordinary human, as Dorothy discovers, operating some impressive machinery.

The crab's shell, like its body, is segmented. The curved, double-ridged forward section (the prosoma) houses seven light-sensitive eyes on the top. When the museum assistant turns over the crab in the demonstration pool and feeds it, five pair of legs and one of chelicera which surround the mouth wave eagerly. These appendages are multi-functional, as it turns out. The moving leg bases grind the food (mainly worms and shellfish); the chelicaerae push the food into its mouth. Move the legs back and forth to walk, up and down to chew. A stunning design concept.

"I was trained as an industrial designer," says Glenn Gauvry, "and with these guys, I’m, like, can I take classes with you?"

Gauvry is president of the Ecological Research and Development Group (ERDG), a non-profit wildlife protection organisation, and an ardent advocate for everything about horseshoe crabs, including their design. Through ERDG, Gauvry is involved in the full range of wildlife advocacy: consultation, research, education, community programs, website; and coordinates an annual count of horseshoe crabs during their peak spawning season.

Three of us have joined him on a night which would inspire a songwriter: full moon in June at high tide on a misty beach. We’ve driven out from Lewes ‘the first town in the first state’ - Delaware, past scattered suburban homes, classic farmhouses, fields of corn, strawberries and tomatoes, through reedy marshes singing with summer insects, to the grassy dunes and summer cottages of Prime Hook Beach on Delaware Bay. We are apparently the only people out on this beautiful balmy night, but we are not alone.

"… four, five, six," counts Gauvry, pacing the high tide line. Lyn drops the grid – a one metre square of white PVC pipe – Carolyn videos us for a TV documentary and we count crabs. "Three males; six females." Fourteen paces. "Woa, definitely another orgy here, three females and seven males." Six paces again. "Now it’s couples’ night, four females and four males." We continue along the tideline, counting at six and 14 paces, for a kilometre. It’s happening all along the eastern US coast, especially on Chesapeake and Delaware Bay beaches – volunteers out counting spawning horseshoe crabs.

"Believe me, you have not experienced life until you have become an ‘arthropod voyeur’", claims Bennie Williams, a fisheries biologist and horseshoe crab survey coordinator with the Maryland Department of Natural Resources. I wouldn’t argue. This is, as he says, "quite an experience".

Not everyone feels, or has felt this way.

Atlantic horseshoe crabs are a summer phenomenon along eastern US coastal bays from Florida to Maine. They spend the winter buried in seafloor sediment in the bays or offshore, on the continental shelf. Towards May and June, longer days prompt the light-sensitive crabs to migrate inshore and spawn, most often on full and new moon high tides. Each female can lay approximately 90,000 eggs in a season, 5,000 per golf ball sized clutch and four to five clutches per visit.

The crabs are numerous and repetitive beach visitors, remaining in the shallow bay waters between spawning forays. Inevitably, thousands of old, diseased, damaged or stranded crabs die. People with beachfront properties are understandably less than thrilled about a summer pervaded by the stench of dead and rotting crabs. Even today, says Gauvry, "if you talk to a lot of the people along here, the last thing they want to see is the population return to the numbers they remember, because they’re fearful their property values are going to decline." On Cape Cod, to prevent them disrupting and preying on shellfish beds, bounties were paid on horseshoe crabs until the 1960s and people were encouraged to toss the unfortunate crabs above the high tide line, to dry out and die. In many Atlantic states, the crabs were branded ‘pests’ and ground up for fertiliser. "It’s going to take an attitude shift," says Gauvry.

It’s going to take something because, as it turns out, horseshoe crabs are valuable. Over a million shorebirds a year depend on horseshoe crab eggs to fuel their migration from South America to the Arctic. Eggs and larvae are a seasonal food source for striped bass, white perch and other fish, and loggerhead turtles feed on the crabs. Adult crabs, especially females, are the preferred bait in commercial conch and eel fisheries. The horseshoe crab’s large eyes and optic nerve makes it a good subject for eye research. Chitin from their shells is used in the surgical sutures and wound dressings for burn victims. And Limulus amebocyte lysate, or LAL, made from their blood, is used in a simple, relatively inexpensive and extremely sensitive test to ensure pharmaceutical products are free of bacterial toxins that can cause fever, shock and death in humans and animals.

The design that has basically endured for upwards of 300 million years must be effective. Everyone wants a piece of the crab.

Meanwhile, on the gentle, sandy slope of Prime Hook Beach, the crabs continue, as they have on millions, maybe billions of other nights, to do what they can to ensure the survival of their species.

The first pair of legs behind the feeding appendages on the male horseshoe crab are modified to clasp a spawning female, clamping firmly onto her serrated middle section (the opisthosoma), just in front of the tail. These claspers are one way of differentiating adult males from females. The females also tend to be larger, and they’re more likely to be found at the bottom of a spawning stack. It’s not unusual for a male to clasp a promising female and maintain the grip throughout the spawning season, wearing a groove in her shell. They glide up to the high tide line, looking from above like remote-controlled robots, and spawn in pairs and clusters.

"The crab doesn’t dig very well without water," Gauvry explains. "She relies on the wave motion going over the top of her." We stand in the sloshing waves and, like the horseshoe crab, feel ourselves sink as the undertow pulls sand out from under our feet. "She’s pushing, ploughing forward, using her legs to make a slurry. She needs the full moon high tide so her eggs are less likely to be churned up by other high tides. Also, it produces a greater distribution of nests. As you’re walking down the beach, you can see the indentations where the eggs are buried. When there’s an abundance of horseshoe crabs spawning on top of each other, many eggs will be churned up, spread out. That’s food for the birds. Those eggs will never be horseshoe crabs."

Horseshoe crabs are unlikely to spawn in rough surf but even the gentle slosh tonight not only spreads out the top layer of churned up eggs, it tips over crabs. An overturned crab can swivel and dig its tail, or telson into the sand, bend its segmented body, and sometimes right itself. Or sometimes not, and an overturned crab out of water is vulnerable to predators and dessication. Those with broken or damaged tails can’t flip themselves; neither can females with a persistent male attached. They lie quietly, perhaps resigned, or wave their legs in another hopeful but doomed attempt. An estimated 10 percent of the spawning population die this way each season. We're flipping over dozens tonight. It’s easy. The appendages are for walking, bumping into food and putting it into mouths, not for attacking. The crabs are completely non-threatening. It feels good to watch a mature female glide back into the bay, rescued to spawn again on the next high tide.

"I was going to all these hearings on horseshoe crabs," says Gauvry. "The watermen and birders were always arguing with each other: if the birds didn’t eat the eggs, there’d be more crabs; the birds only eat non-viable eggs; well, there’s more stranded, dying upside down, than I harvest for bait. Then somebody would stand up and say, well, why don’t you just flip them all over? and everyone would laugh. After a while I thought, you know, it’s not that stupid an idea. You have millions of crabs spawning, 10 percent is hundreds of thousands. That’s substantial."

So Just flip ‘em! ™ was born.

"I don’t see it as a major contribution to horseshoe crab population as much as changing attitudes," says Gauvry. "People will walk down this beach totally indifferent to these unique animals dying in front of them. Just flip ‘em! ™ is geared more to getting you to pause and give a helping hand, make a connection, maybe learn something. They’re losing their spawning habitat, we’re harvesting several million a year, they’re slow to mature (9-11 years) – it starts to make a difference. Kids are the best ones to educate. Once they realise they can’t get hurt by these things, they like talking to adults, saying 'don't pick them up by the tail; it'll get broken or damaged'. If everyone living along the beach starts to look at their backyard in a more special light, recognise that it's the spawning site of a rather unique species, then it becomes a kind of paradigm shift in their way of thinking."

Kids don’t just talk, they also enter poems and drawings of horseshoe crabs in ERDG’s Just flip ‘em! ™ competition.

Meanwhile, in an approach to industry, ERDG is trialling a strategy that could reduce the number of crabs harvested for bait by 75 percent. Instead of baiting a trap with an entire crab, conch fishers in Virginia are using a quarter of a crab in ERDG-manufactured bait bags.

At the end of our kilometre of counting, we’re exhilarated. We've seen spawning crabs, dented crabs ("When they moult, the old body is the mould for the new," Gauvry explains. "So more than likely she was injured at the dent site before she moulted."), crabs cluttered with slipper shells and crabs decorated in streaming banners of algae. "This is the best night so far this season!" says Gauvry. "I’m glad they were all there for you."