Carmela Cuomo thought she had the secret within reach, hidden in a shallow black tank at the NOAA marine fisheries laboratory in Milford, Conn. The horseshoe crabs she had plucked from New Haven Harbor in 2000 trundled about their springtime ritual, digging pits in the sand, laying their eggs and fertilizing them. She was trying to understand what formula of light, food and chemistry induced these 500-million-year-old creatures to breed. But the next year, before she could figure it out, the crabs stopped mating, and the secret eluded her.

Cuomo, an environmental scientist at the University of New Haven, continued to search for the answer for 10 years, in the tanks at Milford, at labs at her university, and in a set of aquariums in her own basement. Now, finally, she has begun to unlock the mystery.

Having the answer would have major practical implications. No one, except by accident, has been able to get horseshoe crabs to mate in captivity. If scientists could figure out how to breed them, the ability might take pressure off the wild populations along the U.S. Atlantic coast and in East Asia. The pharmaceutical and medical products industries value the armored arthropods because a clotting extract from their blood is the world standard for detecting deadly gram-negative bacteria. Their eggs are also a vital food source for migrating shorebirds. And a huge fishing industry uses them for bait.

When Cuomo’s crabs failed to mate in 2001, she fiddled with mimicking the tides, altered the angle of her artificial beaches, and changed their food. Each year she shifted her parameters, but nothing worked. Then, in 2007, at an international conference on horseshoe crabs, Cuomo heard an elderly Japanese researcher talk about raising crabs in mud taken from the beach where the eggs were laid. Cuomo realized what had been missing from her breeding experiment: natal sand. The one year she had managed to get her crabs to breed, unlike any other year, she had taken both the crabs and the sand for her tanks from the same spot. She tried again, and the crabs canoodled—not only in the traditional late spring season, but ongoing into October. She has repeated the process, with the same success.

Now, driven by her innate curiosity, Cuomo is moving on to other aspects of the mystery: What’s in the sand that matters? How do the crabs sense it? And can she help save a species?