Each May and June, millions of Atlantic horseshoe crabs come ashore along the East Coast to spawn. The height is usually when there is a full moon or new moon, creating the highest tides of the month. The result is those familiar helmet-shaped shells of Limulus polyphemus and trillions of their greenish eggs. At least 11 different species of shorebird rely on horseshoe crab eggs as their primary food source for about 2-3 weeks during their migration. Migrating to their breeding grounds in the Arctic, some come from as far away as the southern tip of South America. Recent declines in the horseshoe crab population have triggered similar and more drastic declines in shorebird populations.
Red knots feeding near horseshoe crab. Photo by Gregory Breese/USFWS
Horseshoe crabs are often called "living fossils" as they are thought to have changed very little in the last 450 million years. Scientists are still not sure if they were land-based arachnids that moved into the sea or an ancestor that stayed put in the oceans. Recent genetic research favors arachnids, placing them alongside spiders and scorpions. The horseshoe crabs spawn from Maine to the Yucatan Peninsula, but our own Delaware Bay is the center of activity. The warm water and mild surf with sandy shores must be appealing because some of the crabs have come from more than 60 miles out from the continental shelf off the mouth of the bay. The blue blood of the Atlantic Horseshoe crab is used in medical testing of equipment and vaccinations to detect bacterial contamination in minute quantities, and the crabs have always been harvested as bait. The Atlantic horseshoe crab is currently considered “Near Threatened” by the IUCN. While not currently listed as a threatened species by the State of New Jersey, there is currently a moratorium on the harvest of horseshoe crabs within the state. With this law, it is illegal to remove a horseshoe crab, dead or alive, from its habitat in the wild.