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This Week’s Creature Feature: The Horseshoe Crab

A living fossil spawns again
       Horseshoe crabs are more closely related to scorpions than to our blue crab. They are living fossils virtually unchanged over 250 million years. They were on earth before the dinosaurs.
      The horseshoes spawn at the highest tide of the spring along the water’s edge wherever they live. Delaware Bay has the largest horseshoe population in the world. Each year a massive spawn occurs along its shores. In some areas, there are so many that you cannot walk along the shore. Frequently the spawning horseshoe crabs are flipped onto their backs by waves and cannot get back into the water, so they eventually die in the sun.
       As eggs are laid, they are covered by sand and debris. The embryos then develop and, at the next high tide, young crabs release themselves into the water.  The crabs, which live more than 40 years, do not reach sexual maturity until they are 10 to 14 years old.
        Horseshoe crab spawning is extremely important for shorebird migrations. Clouds of sandpiper-like birds time their migrations to coincide with the crab’s life cycle.
      One bird in particular, the red knot, relies on the Delaware crab spawn to fuel it to fly to breeding grounds in the Arctic Circle. These birds spend the winter at the far southern tip of South America and have an amazing 15,000-mile migration route. By the time they reach Delaware, they are thin and exhausted. During their time along the bay, they double their weight and are able to complete the journey north. Without the eggs, they cannot make it and will likely perish or at least stop short and will not reproduce.
      In the late 1990s, prices rose for eel and conch, which are caught in traps baited with horseshoe crabs. Harvests rose, and by 2002 the horseshoe crab population started to collapse.  Along with the crabs, the red knot population also collapsed, and the birds became critically endangered. In 2008, a limit to catching horseshoe crabs was put into effect in Delaware and harvesting was banned in New Jersey, but the population has not recovered.
      The population of red knots continues to shrink. This past year’s wintering population was 25,000, 5,000 fewer than the previous year. Other forces may have been involved, such as the severe weather of last summer, but the population has not shown any improvement since 2010.
       The best place to see the horseshoe crab shore invasion and the shorebirds that greet them is DuPont Nature Center at Mispillion Harbor near Slaughter Beach.
       Remember that the birds have flown thousands of miles to be there to feed and rest. Watch from a distance. Do not chase them or disturb them.
–Wayne Bierbaum