Crab Season

Spring means an ancient ritual for an ancient creature—and its shorebird predators

Spring is upon us. A time when nature seems intent on impressing us. All around us, ancient cycles of life reveal themselves. The magnitude of the transformation is certainly impressive, as are all the delicate processes necessary to make it happen—pollination, seed dispersal, migration, mating—and perhaps most impressive, the singular beauty of a dogwood flower or the song of a warbler.

Here in Maryland, lost in this great spectacle thrust on us by nature is one of the most amazing natural processes in the world—and certainly one of the most ancient. One of the most beautiful? ThatÕs in the eye of the beholder.

The American horseshoe crab (Limulus polyphemus) can be found from Maine to the Yucatan Peninsula in Mexico, but more than 95 percent of the population inhabits waters of the mid-Atlantic seaboard, between North Carolina and New Jersey.

The horseshoe crab is MarylandÕs oldest living fossil, and has changed little over the last 250 million years. Horseshoe crabs were around long before the dinosaurs, and even before the Chesapeake and Delaware bays, where they now spawn. Of the four species of horseshoe crabs that have survived, only Limulus, the American horseshoe crab, lives outside of Southeast Asia.

It is a member of the phylum Arthropoda (the largest phylum of all living animals), but is more like a tick or a scorpion than a true crab. Horseshoe crabs are bottom-dwelling creatures that spend most of their time out in the deep waters of the open ocean, feeding on marine worms and shellfish. Lacking a jaw and unable to chew, the horseshoe crab eats with its feet. It uses its legs to grind food and guide it into its mouth.

Every spring, mature horseshoe crabs migrate inshore to spawn. Horseshoe crabs do not mate until they are eight or nine years old, but may live to be as old as 18. During the high tides of the new and full moons every spring, horseshoe crabs mate in an odd ritual that has been taking place here for perhaps 10,000 years.

The ideal beach for mating offers three things: protection from rough water, usually in a bay or cove; proximity to tidal flats, which can provide ample food for recently hatched juveniles; and well oxygenated sediment, to support the developing eggs.

The male crabs will wait along the shoreline for females making their way up the beach to lay their eggs. The male horseshoe crab literally attaches itself to the female crab, which is approximately 25 percent bigger than the male, with a specialized claw and then lets the female crab drag him up the beach. While she lays her eggs, he fertilizes them. The male will stay attached to the female for up to two weeks to make sure the eggs are fertilized. (TheyÕre also getting a free ride everywhere.)

The beaches of Delaware Bay see the most activity, and can host hundreds of thousands of horseshoe crabs in a single night. While the Chesapeake Bay numbers may be lower, it is still possible to get thousands of crabs on a Chesapeake Bay beach.

A single female can lay up to 100,000 eggs per year, which not only helps to continue a healthy horseshoe crab population, but also serves as an extremely important food source for migrating shore birds.

The Delaware Bay is the second-largest staging area in North America for migrating birds—a million birds may come to the bay in a two-week period looking for nourishment. It is believed that horseshoe crab eggs are the main source of nutrition (fat and protein) for migrating shorebirds.

Some birds, such as red knots and some sandpipers, feed almost exclusively on horseshoe crab eggs—up to 80 percent of their diet when migrating through our area. Over ninety percent of the total populations of these species stop over in the Delaware Bay, making horseshoe crabs essential to the health of the species. Many other species including ruddy turnstones and plovers also depend on the horseshoe crabs to a lesser degree. In the water, horseshoe crabs also provide a source of food for sea turtles and sharks as well as raccoons and foxes on land.

Due to a number of factors, including habitat loss and overfishing, horseshoe crab numbers have been in decline, which has had a significant impact on shorebirds. In an impressive dynamic between predator and prey, many shorebirds are dependent on an overabundance of horseshoe crab eggs for feeding. Equipped with short bills, the shorebirds cannot reach eggs laid by horseshoe crabs, which are usually buried too deep for their bills to reach. It takes another horseshoe crab to dig up the first cache of eggs while burying her own to make the eggs available to the birds.

When surface eggs declined by an average of 75 percent, red knots, sanderlings and semi-palmated sandpipers declined by 50 percent over approximately the same time.

Two industries that have had a direct impact on horseshoe crab populations are the eel and conch fisheries and the biomedical industry. There was a dramatic jump in the total horseshoe crab harvest from the early 1970s (approximately 12,000 pounds) to the late 1990s, when the harvest reached 6.1 million pounds. The fishery in Maryland is now limited to 750,000 pounds per year.

Eel and conch fishermen simply use the horseshoe crab for bait, but the biomedical industry has found an important compound in the copper-based blood of the horseshoe crab. Their blood is the only source of Limulus Amoebocyte Lysate (LAL), a compound used to detect and eliminate bacterial toxins. LAL is the worldwide standard for testing medical equipment and pharmecueticals. Crabs are not necessarily killed for their blood— the crabs are bled, 25 percent of their blood is taken, and then they are released. It is not well understood what effect this practice has on horseshoe crab populations or on individuals such as increased susceptibility to disease.

Along with limits on overharvesting in most states, the federal government has helped in the protection of the horseshoe crab by creating a horseshoe crab sanctuary off the mouth of the Delaware Bay estuary. The sanctuary was created with the express purpose of protecting spawning populations of horseshoe crabs.

On a night not long from now, with the light of a full moon, hundreds of thousands of prehistoric creatures, whose species has survived millions of years, will make their way up a beach. They ensure that there is a another generation of horseshoe crabs and support the delicate balance of nature, of which we are all a part. There is certainly something beautiful about that.