Starting in the middle of May, female horseshoe crabs ready to spawn heave their tank-like bodies onto the moonlit sands of Delaware Bay. In what’s known as “spawning aggregation,” the crabs ready to lay eggs arrive en masse, often numbering more than 300,000 in a single spot on a single evening.

“They are shell-to-shell and on top of each other, five to six feet deep on a good night,” says Glenn Gauvry, founder of the Ecological Research and Development Group (ERDG) in Dover, Del. And if you want to see them, don’t blink or linger over dinner. These ladies are prompt, arriving just before the tide crests—and in even greater numbers during the full and new moons in May and June when the evening tides protect the buried nests from subsequent tidal cycles until the eggs hatch some two weeks later.

Of the four horseshoe crab specie¬¬s in the world, only one, Limulus polyphemus, calls the shores of the United States home—ranging from Maine to Florida and west to the Gulf of Mexico. And despite the fact that they’ve been overused as bait for eel and conch fishing, the horseshoe crab population is conservatively estimated at between 2.5 and 4.5 million in Delaware Bay alone.

In Japan, these creatures are believed to be the reborn spirits of Samurai warriors, helmets having evolved into hard shells. And while their reputation here hasn’t been so noble, Gauvry says that is changing. Virtually every man, woman, child, and domestic pet in the U.S.—anyone who has ever gotten a shot from a doctor--is intimately connected to the horseshoe crab. One may even have saved your life.

Discovered by Woods Hole scientists in the 1960s, an extract of horseshoe crab blood—called Limulus amebocyte lysate, or LAL--is used by the pharmaceutical and medical device industries to detect bacterial contamination in syringes, scalpels, and intravenous drugs. To produce this important extract, they collect a horseshoe crab, drain approximately one-third of its blood, and quickly return it to where they found it (ideally, that is).

While upwards of 600,000 horseshoe crabs are bled each year, they are not significantly harmed in the process. While bleeding seems to have no negative effect, collection results in an estimated three to ten percent mortality rate, and ERDG is working with the companies producing LAL to ensure a stable population. One of these companies, Lonza, has produced a synthetic test, which is currently undergoing FDA evaluation but won’t completely replace the current test using LAL, says Gauvry.

If you’re interested in viewing the spawning spectacle this month, you can find some hotspots on ERDG’s website: www.horseshoecrab.org. As to where he likes to watch, Gauvry’s lips are sealed: “They would kill me if I told you where their beaches are,” he says.