Horseshoe Crabs Support Wildlife Along Delaware Bay
In late May this year I spent two days watching shorebirds feeding along the beaches of Delaware Bay during their migration from the tip of South America, Brazil and the Texas coast on their way to the Arctic tundra. One shorebird, a sandpiper called the red knot, is known to fly 9,000 miles enroute. Delaware Bay, bounded on the west by Delaware and New Jersey on the east provides the final feeding site to refuel and gain fat stores needed to fly non-stop the additional 1,000 miles north to the tundra.
While my goal during this trip was to observe the spectacle of many thousands of shorebirds like sandpipers and plovers feeding, more importantly, I came to appreciate how the balance of nature can easily tip leading to the collapse of animal populations.
As explained in the book, “Life Along the Delaware Bay” (2012), given to me by my friend, Ranjit, horseshoe crabs provide food in the form of eggs to support shorebirds on their journey north. A million of these crabs lay up to 80,000 eggs each over several days in the beach sand nightly at high tide during late May. The eggs become exposed for consumption when other horseshoe crabs dig to lay their eggs and by gentle wave action. The horseshoe crab therefore is a “keystone” species because it supports other animal species like gulls, fish and sea turtles.
The horseshoe crab has remained relatively unchanged in appearance during 450 million years of evolution. The olive green body and head are the size of a large dinner plate. A long, rigid, pointed tail helps upright the crab when overturned in water. During full and new moon phases tides are highest which triggers the mating urge. A female leaves the water dragging a male up the sandy, pebbly beach to lay several thousand eggs which are fertilized by the male’s free swimming sperm.
Breathing by gills sandwiched between flaps resembling pages of a book permit survival for up to four days out of water when right side up. My friends and I discovered hundreds of horseshoe crabs flipped on their back on the dry sand at low tide. Realizing they were harmless we flipped at least 100 crabs with our hands and feet. Quickly, they started walking down the beach to the muddy bay shore. Apparently, “flipping crabs” is a pastime for volunteers at local nature centers.
During the late 1990s and early 2000s, wildlife managers discovered that shorebird populations declined precipitously, particularly the red knot – a robin-sized, rusty-colored sandpiper. This decline coincided with the time fishermen started using horseshoe crabs as bait in traps to catch eels, conch and minnows for sale in Asia.
Since there were fewer horseshoe crabs, there were fewer eggs available for shorebirds to eat, resulting in reduced breeding and a declining population.
Fortunately, by 1998, 15 Atlantic Coast states established horseshoe crab harvest quotas in order to stabilize the decline in horseshoe population. Since the horseshoe crab requires nine years to become sexually mature, populations will remain unchanged until the present population matures and breeds. Many crabs can live 20 years.
An earlier assault on horseshoe crabs occurred in the 1970s by the pharmaceutical and medical prosthetic industry. A derivative of blood cells in horseshoe crabs called Limulus Amoebocyte Lysate (LAL) will form a clot or gel when in contact with bacterial toxins. Therefore, vaccines, injectable medications and implanted prosthetic devices are tested for the presence of bacteria and toxins with LAL. Synthesis of LAL in the laboratory so far has been unsuccessful necessitating continued harvest of horseshoe crabs. Attempts are made to return horseshoe crabs to the bay after partial blood collection but mortality is 15 percent.
Communities along Delaware Bay celebrate importance of the horseshoe crab in the balance of nature, with nature walks, kayak cruises and craft shows at the Horseshoe Crab/Shorebird Festival each Memorial Day weekend.
By practicing conservation which is wise use of our natural resources, man, birds and commercial fishermen can share the benefits of the horseshoe crab into the future.