Out of the lapping waves they crawl, onto the moonlit beaches. Bent on sex and survival, tens of thousands of horseshoe crabs return by tens of thousands to the shores of Delaware Bay every spring.
Theirs is an ancient ritual, a slow-moving mating dance that has enabled the helmeted, spike-tailed creatures to endure for 350 million years, since the days of dinosaurs.
More closely related to spiders than to the blue crab, the Atlantic horseshoe crab is a familiar sight -- and well-known to vacationing families -- on beaches from Maine to Mexico's Yucatan peninsula. Delaware Bay is the epicenter, though, where the East Coast's largest population emerges from the water to lay and fertilize its eggs.
This spectacle of nature takes place over several weeks, beginning in late May. It usually peaks around the first full moon in June, which occurs this weekend.
Almost simultaneously, as if drawn by a dinner bell, about 1.5 million migrating shorebirds fly in from South America about the same time to gorge themselves on the billions of tiny eggs deposited on the beaches by the female horseshoes.
Meanwhile, thousands of people flock to the beaches to feast their eyes upon shorebirds and crabs. The twin event has become a major tourist attraction for Delaware and New Jersey, with some bay front towns staging horseshoe crab festivals and playing host to bus loads of bird watchers.
The crabs' arrival this year has excited more than the usual anticipation because the springtime orgy has flagged of late. Scientists worry that the species they know as Limulus polyphemus may be in trouble.
"Since 1990 we've noticed a significant decline in population," said Stewart Michels, a biologist with the Delaware Department of Natural Resources and Environmental Conservation. Scientists trawling the bay in boats have caught one-fourth as many horseshoe crabs the past two years as they did six years ago.
An annual one-day beach survey, conducted by volunteers around the crabs' spawning peak, has shown an even more alarming drop -- from 1.2 million in 1990 to less than 90,000 last year. Scientists say the beach count is too loosely done to be precise, but they don't quibble with the downward trend.
They aren't sure whether the decline is real, or simply the kind of variation often seen in nature. What is clear, however, is that the VTC horseshoe crabs' only real predator -- man -- has rediscovered the ancient animal, not as food but as bait and as a source of life-saving pharmaceuticals.
Often called "living fossils," the lineage of horseshoe crabs can be traced back about 20,000 years. Fossils recovered in places as dispersed as Bavaria and Colorado bear striking resemblance to today's horseshoe crab.
The four living species of horseshoes and their prehistoric predecessors all have distinctive three-piece bodies, with armored heads, triangular or hexagonal bodies and long spike tails -- not weapons, but tools to help a crab right itself if overturned.
In water, they skim along the bottom via five pairs of legs, with a boost from the beating of their leaflike gills. Though not as mobile as blue crabs or lobsters, horseshoe crabs do roam. Animals tagged by scientists around Port Norris, N.J., have been caught off Ocean City, Md., about 70 miles away.
By May, they have congregated in shallow water, awaiting the signal to hit the beaches.
The cue usually comes at high tide, often during full moon, when beaches are bathed in pale light. The greenish-brown female emerges with one or more smaller males clinging to her back. She crawls along water's edge, laying strands of dark eggs in shallow nests dug in the sand, then drags the male or males across the eggs so they can be fertilized.
But sea gulls, red knots, sandpipers and sanderlings swoop down and devour the eggs.
Each large female carries up to 100,000 eggs, Shuster notes, so at least some survive the avian frenzy. If they do survive, the eggs hatch in two weeks, and tiny juvenile horseshoe crabs emerge from the sand to head for the water on the next full moon.
Once hatched, horseshoe crabs spend nine to 12 years in the water before returning to shore to spawn for the first time. Though slow to mature, the animals' average life span is 19 years, giving them ample opportunity to make up for poor reproductive years. The crabs' slow development is one of its strengths, in some biologists' eyes, but they also acknowledge that trait could mask sudden declines in the animals' population.
"If the horseshoe crab population is reduced because of over-harvesting, we're not going to see the impact for five or six years," said Clark, the New Jersey zoologist. "That's what's got everybody nervous."
Horseshoe crabs have been heavily harvested before. They were scooped up by the millions from beaches in the late 1800s and early 1900s, to be ground up for use as fertilizer.