Horseshoe crab

There are four species of horseshoe crab still alive, but only one (the Atlantic horseshoe crab) lives in the Western Hemisphere. They can be found all along the Atlantic Coast from Nova Scotia, Canada down to the Yucatan Peninsula in Mexico, but the majority live in Delaware Bay. The largest concentration of breeding horseshoe crabs anywhere in the world can be found around the Bay each spring. The Bay is particularly ideal because of its relatively shallow depth, allowing the water to warm quickly after winter and stimulate horseshoe crab spawning. When not onshore for spawning season, horseshoe crabs live offshore in deeper waters. Some have been found at depths of over 600 feet, but studies indicate they prefer depths of less than 100 ft.

Horseshoe crabs feed on marine worms, small clams, mussels, crustaceans, snails, and slugs, but they are largely scavengers.

During the full moon of late May or early June each year, horseshoe crabs undergo a “local migration.” They travel from deeper waters off the coast to the shores of sandy beaches in order to reproduce. Each female digs a hole at the water’s edge and lays between 3,000 and 10,000 eggs. While she does this, several males hold on to her while releasing sperm. The female then buries the cluster of eggs under a few inches of sand. By the end of the spawning season, each female will have laid about 80,000 eggs.

These eggs take about two weeks to hatch, and the newly-hatched horseshoe crabs immediately return to the water. They will swim for about six days before settling on the bottom in the shallow water near the beach. About 20 days after hatching, the young will molt their hard outer shell, or exoskeleton, for the first time. They will molt several times by the end of their first year, but will still be only about half an inch wide. By age three or four, however, the now larger horseshoe crab will only molt about once a year, usually during the summer. In all, horseshoe crabs will molt at least 15 times before reaching sexual maturity at age 9 or 10, at which point they will begin the annual migration to the shore to spawn.

Horseshoe crabs are believed to live about 20 years in the wild.

Horseshoe crabs are highly valued by conch and eel fisherman, who use them as bait. Additionally, horseshoe crab blood contains a substance called Limulus amebocyte lysate (also known as LAL), which clots in the presence of bacteria. For this reason, it is valuable in the pharmaceutical industry, which uses it to test for contamination of drugs and medical equipment. Though crabs used for this purpose are returned to the ocean, it is estimated that between 10% and 30% die as a result of the process.

Because of this high demand for horseshoe crabs, populations along the East Coast, and particularly in Delaware Bay, are in serious decline. 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.

Complicating the issue is the fact that 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. These birds are migrating to their breeding grounds in the Arctic, some from as far away as the southern tip of South America, and the availability of nutrient-rich crab eggs during this time is critical. Recent declines in the horseshoe crab population have triggered similar and more drastic declines in shorebird populations. Even though immediate action may not be needed to save the Atlantic horseshoe crab from extinction, immediate protection of horseshoe crabs is necessary to protect the species that rely on them.