Living Fossils on Our Shores
The horseshoe crab has been called a living fossil and has remained virtually unchanged over 250 million years, predating the dinosaurs. These arthropods are not true crabs (crustaceans) and are more closely related to a scorpion than a blue crab.
Horseshoe crabs feed on debris in sandy or shallow water estuaries or saltwater shorelines. They are most active at night and have seven eyes to see in the dark. Their eyes are several hundred times more sensitive to light than ours. They have external gills found in a tail section and a spine that is primarily used to right themselves and not for protection.
The horseshoe crabs spawn at the highest tides of the spring along the water’s edge. Delaware Bay has the largest population of these crabs in the world, possibly due to the shallow average water depth. In some areas of the Delaware Bay, so many spawn at once that you cannot walk along the shore. The Chesapeake Bay has a smaller population but they still have a spring spawn.
The eggs are laid along the high-water mark and waves cover them with sand and debris. The embryos will then develop in the moist sand and at the next high tide, the young crabs release themselves into the water and spread out over the surface for a few days to a few weeks when they grow too heavy to swim. As the crabs mature, they molt and develop a larger shell. They molt six times in their first year and can live to be more than 40 years old. They do not reach sexual maturity until they are 10 to 14 years old.
Horseshoe crab’s blood is used by the medical industry to test for bacterial toxins. About 500,000 crabs a year are caught for this use. They are released after having their blood taken but only about 75 percent survive. Horseshoe crabs are also used as eel and conch bait. In the late 1990s, when eel and conch prices went up, more crabs were harvested and by 2002 their population started to collapse. 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 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. The red knot relies on the Delaware spawn to fuel their migration to their breeding grounds in the Arctic Circle, a 15,000-mile trip. The birds spend the winter at the southern tip of South America and are very thin and exhausted when they arrive in Delaware on the hunt for horseshoe crab eggs. They will double their weight and are able to complete the journey north after this rest stop. Without the eggs, they would likely perish or at least stop short and will not reproduce.
Seeing thousands of crabs and thousands of birds along a beach is amazing. But they spawn in the shallow at their own peril. Waves are the crabs’ nemesis. Frequently the spawning horseshoe crabs are flipped onto their backs by waves and cannot get back into the water and die in the sun. I have been to a private beach where several thousand were dead and drying out, belly up in the sand. If you see a horseshoe crab that has been flipped on its back, tap the shell with your foot to see if it is alive. If alive, then pick it up by the sides of the shell and release it upright into the water. Lifting it by the tail can injure the animal.
Remember that the crabs you find on the shore are at least 10 years old and will continue coming to the shore for 15 more years.