Few creatures wear the mantle of deep time as visibly as Limulus polyphemus, better known as the Atlantic horseshoe crab. To walk through the gravelly shores of the Delaware Bay or the back-bay shallows near Ocean City, New Jersey during the high spring tides of June is to witness a gathering unchanged since the Triassic.
Here, the ancient arthropods—who have existed for roughly 445 million years—assemble for their great spawning. Under the gravitational pull of the full moon, king tides cue the helmet-shaped crabs to emerge from the depths of the Atlantic Ocean. The females, robust and broad-carapaced at nearly two feet in length, plow into the damp sand at the water’s edge. They then deposit thousands of eggs beneath the slurry of the surf.Each tiny, colorful orb is barely larger than a mustard seed.
The process works like a precise biological clock, and at dawn, the cycle shifts away from the horseshoe crabs and to the daytime feeders. As the crabs deposit millions of these fatty and nutritious eggs, thousands of migratory shorebirds arrive from the sky. Many of these birds have flown thousands of miles up from the southern tip of Patagonia, only to touch down upon these precise Northeast shorelines. Among them is the Rufa red knot (Calidris canutus rufa), a master of the air executing an annual 9,000-mile odyssey to its Arctic breeding grounds.
Throughout their long journey, red knots can remain airborne for up to a week straight, burning stored energy and losing nearly half of their body mass in the process. The tiny horseshoe crab eggs are an immediate, vital fuel source, allowing the knots to double their weight in a matter of days.
Over the past eight years, New Jersey photographer Susan Allen has captured these spawnings. “The quiet Delaware Bayshore becomes globally significant to the survival of many species each spring,” Allen tells Popular Science. “Hopefully this natural wonder will continue to happen.”
Yet, this ancient convergence faces immediate threats. Climate change is warming bay waters and intensifying storms. In some years, the warmer water has prompted horseshoe crabs to spawn earlier in the season, throwing off the timing that red knots depend on when they arrive to feed on crab eggs.
At the same time, horseshoe crabs have faced mounting pressure from commercial harvest. They are widely used as inexpensive bait in whelk and eel fisheries, and are also collected for the pharmaceutical industry. During the 1990s, harvest numbers surged: in just five years, annual take rose from about 100,000 crabs to 2.5 million.
But against these modern pressures, the endurance of this bird-arthropod partnership remains a profound marvel of prehistoric connection, forged over hundreds of millions of years. The bay is still coming alive as ancient crabs meet the arriving birds in the middle of their long migration.
