Tom Horton

Tom Horton has written about Chesapeake Bay for more than 40 years, including eight books. He lives in Salisbury, where he is also a professor of Environmental Studies at Salisbury University.

Billion-year-old show still draws crowds

The moonlight mating of horseshoe crabs

This spring, re-enter a more elemental time. Rising moon, May or June, a third of a billion years ago. Sunset gleams in the lap of saltwater on sandy shore. Today’s continents have not formed. Birds and trees, even dinosaurs are 100 million years or more in the future.

The tide swells, stars emerge, and just offshore a dark spike of a tail punctures the surface, followed by dozens, thousands. It’s a scene set since the oldest mountains were forming: horseshoe crabs, an impossibly ancient species, each spring emerging from the deeps, massing by the millions, bulldozing their way onto beaches to lay their eggs.

It’s one of earth’s primordial and longest-running spectacles, the apotheosis of “sustainability” we humans have only begun to ponder for our own species. It peaks every May and June, on the nights of the new and full moons, along the Chesapeake and Delaware Bay on beaches easily accessible to the public.

“I am still amazed how many people remain unaware of this amazing phenomenon,” says Delaware state fisheries expert Stewart Michels.

Michels said Slaughter Beach in Delaware is a prime viewing spot. The DuPont Nature Center is a great place to learn more about the ancient crabs. Delaware is the world epicenter of spawning, with tens of millions of animals converging and hundreds of billions of eggs being laid in the sand.

According to Maryland biologist Steve Doctor, the Chesapeake and Maryland coastal bays have significant populations. On the ocean side, the Ocean City beach near the inlet is reliable, but the crabs come ashore from Ocean City to the Virginia Capes. In the Chesapeake, you will find good beaches along the Eastern Shore’s Chester and Wye Rivers, and on the western shore at North Beach and Chesapeake Beach in Calvert County.

The horseshoe crab is like nothing else on today’s planet, closer to scorpions and spiders than to true crabs. It is a literal blue blood — copper-based, versus the iron that makes our own blood red. Scientifically, it is in a class of its own (Merostomata), and is also the lone member of the order of Xiphosurans, or sword-tailed animals.

It has an optic system that features 10 “eyes” that can sense the ultraviolet as well as light and dark, with the ability to increase night vision sensitivity by a million times as dark falls.

The living fossil can tolerate high levels of radiation, freezing temperatures, days out of water and extremely low levels of oxygen in the water.

Most of the year, the old crabs range unnoticed along bay bottoms and on the continental shelves, feeding on worms and mollusks. From April through August, peaking in from May through June, they clamber ashore to spawn, a ritual that underwrites the equally spectacular spring migration of hundreds of thousands of shorebirds through the mid-Atlantic coastal fringes.

The birds come for the rubbery, gray-green eggs, which are tiny — several fit atop a pencil eraser — and not very nutritious. But this is offset by sheer abundance.

A migrating red knot, exhausted from flying 5,000 miles from South America, may peck in the sand for 14 hours a day for two weeks, ingesting 18,000 eggs a day, doubling its body weight before heading another 4,500 miles to the Canadian Arctic.

Each day, the rising tide sets the table. The spike tails one first sees offshore are the smaller male horseshoes, swimming back and forth to intercept the big, egg laden females as they move up the beach with rising water.

Special clasper claws let the males hang onto the hind edge of the females’ helmet-like shells. She pulls him up the wet beach, excavates a hole and lays her eggs; then drags him over the hole to fertilize them. It’s a process she may repeat multiple times in a summer.

The links between crab and egg abundance and the energetic needs of migrating birds triggered alarms in the 1990s about overharvesting. Fishermen were taking crabs by the hundreds of thousands for cheap bait to catch eels and whelks — and were targeting pregnant females in particular. Both crab and bird numbers declined sharply.

More recently, federal and state fisheries agencies have dramatically reduced harvesting, especially of females. Populations of crabs and birds are mostly stable or even increasing, but remain well below levels of 25 years ago.

I’ve taken students to see the spectacles of crabs and birds for years. A good resource is “Extraordinary Horseshoe Crabs,” a small but informative book by Julie Dunlap (Carolrhoda Books, Minneapolis).

Other good beach bets are Tower Shores, a Delaware coastal bay beach just south of Dewey Beach, and Pickering Beach on Delaware Bay a little north of Dover.

Full moons are May 3 and June 2; the new moon is May 17. The night high tides are best, and the beaches mentioned here are open at night. Water temperatures are cold this year, so think about the two later dates, especially on the Chesapeake side. Even so, you can usually find some spawning around high tides from May through August.

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Tom Horton

Tom Horton has written about Chesapeake Bay for more than 40 years, including eight books. He lives in Salisbury, where he is also a professor of Environmental Studies at Salisbury University.

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