Natural History of Horseshoe Crabs

Tiny greenish horseshoe crab eggs lie on bay beach sand

Spring Spawning

Around the new and full moons of May and June, keep an eye on the bay shoreline for some of the coolest creatures on Fire Island. Horseshoe crabs are marine invertebrates with large, horseshoe-shaped shells and a long, pointed tail, or “telson.” Fossils of horseshoe crab ancestors predate dinosaurs so these fascinating creatures are sometimes referred to as, “living fossils.”

As adults, horseshoe crabs live in deeper water and come to shore to mate and lay eggs. Peak spawning occurs in New York in May and June, particularly during the evening high tides of new and full moons. After at least two weeks and up to several months later, the eggs hatch.

Tiny baby horseshoe crabs look just like an adult except that they do not have a tail and their eyes and digestive system are not yet fully developed. And they are small - only 3 mm across. The young crabs will spend the next 3 weeks in the water.

When juveniles return to shore they will be about ¼-inch wide and have a tail and functioning eyes and digestive system. These tiny crabs will stay close to shore for about 2 years before they move into deeper water. As larvae and hatchlings, juveniles and subadults, they will shed their shells, or molt, as they grow.

Male horseshoe crabs are sexually mature after about 8 or 9 years, or 16 molts. Females are not sexually mature until they are about 10 or 11 years old and have molted 17 times. Some horseshoe crabs continue to molt even after sexual maturity. No one knows how long horseshoe crabs live.


Horseshoe Crab Ecology

Many migratory shorebirds including the red knot, ruddy turnstone and sandpiper rely upon the energy-rich horseshoe crab eggs for food on spring migration. Horseshoe crab eggs and larvae are an important food for many fish (American eel, killifish, summer flounder, winter flounder).

Horseshoe crabs play an important role in the safety of many medicines used by people. Horseshoe crab blood cells (amoebocytes) attach to dangerous toxins produced by some types of bacteria (gram negative). When a crab is injured, the amoebocytes move to the wound and form a gel that surrounds and destroys the bacteria thus preventing an infection.

Horseshoe crab amoebocytes are used to test intravenous drugs and vaccinations to make sure that they are free from gram negative bacteria. Horseshoe crab amoebocytes are also used to diagnose spinal meningitis and other human diseases. Horseshoe crabs are collected, taken to a laboratory where the blood is collected, and then released back into the environment. Within the boundaries of Fire Island National Seashore however the harvest of horseshoe crabs is not permitted.
Study of the horseshoe crab eye has taught scientists and doctors much of what we know today about the human eye.

Learn more about horseshoe crab citizen science at Fire Island National Seashore and check the Calendar of Events for park ranger- and partner-led programs about horseshoe crabs.

Native Americans used and taught early settlers to use the horseshoe crab as a fertilizer for crops. Native Americans also ate horseshoe crab meat, used the shell to bail water, and used the tail as a spear tip. In the United States, horseshoe crabs continued to be used as fertilizer through the 1960s.

Notice to Visitors:The harvest of horseshoe crabs is prohibited within Fire Island National Seashore.


Last updated: May 31, 2018

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