Untamed Lowcountry

This crab's blood could save your life: 6 things to know about Lowcountry horseshoe crabs

Horseshoe crabs inundate shores of Hilton Head

Montana resident Sara Sewall Johnson shot this video on May 2, 2017 of thousands of horseshoe crabs mating on the southern shores of Hilton Head Island.
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Montana resident Sara Sewall Johnson shot this video on May 2, 2017 of thousands of horseshoe crabs mating on the southern shores of Hilton Head Island.

This is a crucial time of year for horseshoe crabs.

Every spring, these ancient invertebrates come ashore in droves to spawn on Lowcountry beaches. Activity peaks in April and May, especially at night during the high tides of the full and new moons.

Here are six things to know about this fascinating creature:

1) They are basically living fossils -- even older than dinosaurs.

Horseshoe crabs date back at least 450 million years and haven't changed much in looks since. Today there are only four species left: three in Asia, plus the Atlantic horseshoe crab (Limulus polyphemus), found along the Atlantic coast down to the Gulf of Mexico.

2) They really aren't crabs.

Although their soft parts are protected by a crab-like exoskeleton (carapace), horseshoe crabs aren't actually crabs. In fact, they're more closely related to spiders and scorpions.

3) Their unique structure is all for a purpose.

On top of their carapace is a pair of large compound eyes, used to locate mates. Their undersides are a wriggling mass of appendages and flap-like structures -- pincers for grasping food; legs for walking, swimming, burrowing, and food-handling; and leaf-like gills for breathing.

They use their long tails for steering and righting themselves if they get flipped over by the tides.

Though cumbersome on land, horseshoe crabs are surprisingly graceful in the water. Sometimes they even swim upside-down. They spend most of their time scuttling over bottom sediments in bays and estuaries, foraging for clams and worms.

4) They have an interesting mating routine.

Once day length, temperature and tides are right, mature males gather in shallow water, waiting for females to show up. Using special hooks on their legs, males try to attach themselves to the back of a female's carapace.

Once paired up, a couple heads for the beach.

But mating may not be a private affair.

Sometimes an entourage of unattached "satellite" males - half a dozen or more - try to come along for the ride.

5) Their eggs don't have great survival rates.

After digging a hole in the sand, the female releases several thousand eggs. Those eggs will be fertilized externally. Meanwhile, the satellite males jockey for positions around her, while her original partner holds fast.

Some satellite males may fertilize as many eggs as the male who is attached.

The female repeats this process several times during the season, producing as many as 80,000 eggs.

Only a small fraction of the eggs survive.

Many are eaten by shorebirds, including the Red Knot, which depends heavily on horseshoe crab eggs to fuel its 9,300 mile-migration from South America to the Arctic.

Surviving eggs hatch into larvae resembling tiny horseshoe crabs without tails. As they grow and develop, they'll shed their exoskeletons many times. It takes at least eight years for males to reach maturity; even longer for females. Adults can reach venerable ages of over 20 years and weigh over 10 pounds.

6) They are vital to biomedical research.

Aside from spicing up our beach walks during spawning season, horseshoe crabs are vital to biomedical research.

Studies of how their retina and brain process visual information may help treat human visual disorders such as retinitis pigmentosa.

Their blood -- milky blue from the copper it contains -- yields a clotting agent used to test for bacterial contamination in medical equipment and drugs.

A quart of horseshoe crab blood costs about $15,000.

Each year, over 60,0000 horseshoe crabs are harvested and bled and then returned to the ocean. Most survive, but 10 to 30 percent may not. Bleeding may cause females to become weakened or disoriented, interfering with spawning, at least for a while.

Fortunately, researchers are working on a synthetic substitute for the key ingredient of horseshoe crab blood.

Meanwhile, most of us owe considerable thanks to horseshoe crabs for the service they provide.

Vicky McMillan, a retired biologist formerly at Colgate University, lives on Hilton Head Island.

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