In the early summer, watch for the translucent molts of growing horseshoe crabs. Here a "shell" is seen among hundreds of eggs that have also washed ashore on the bay beach.

Despite its common name, the horseshoe crab (Limulus polyphemus), is more closely related to spiders and scorpions than crabs. All are invertebrates from the phylum Arthropoda, or arthropods. This group of animals includes insects (Insecta); spiders, scorpions, ticks and mites (Arachnida, or arachnids); crabs, lobsters, shrimp and barnacles (Crustacea, or crustaceans); and the 4 world-wide species of horseshoe crabs (Merostomata). (Find out more!)

Horseshoe crabs are one of the world’s oldest animals. For over 300 million years (at least 100 million years before there were dinosaurs on earth) horseshoe crabs have survived and remained fairly unchanged.

Horseshoe Crab Biology

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. These 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 point of a sharpened pencil is about 1 mm.) The young crabs will spend the next 3 weeks in the water. When they come back to shore they will be about ¼ of an inch wide and have a tail and functioning eyes and digestive system. These juvenile crabs will stay close to shore for about 2 years before they move into deeper water. As larvae and hatchlings, juvenile and subadults, they will shed their shells (molt) as they grow. Males are sexually mature after about 8 or 9 years and 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.

A female horseshoe crab will lay 90,000 eggs or more during a spawning cycle. From so many eggs it is estimated that only about 10 horseshoe crabs will make it to adulthood.

Importance of Horseshoe Crabs to Humans

Horseshoe crabs play an important role in making sure that many of the medicines that used by people are safe and free of bacteria. 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 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.

Study of the horseshoe crab eye has taught scientists and doctors much of what we know today about the human eye.

Native Americans used and taught early settlers to use the horseshoe crabs to fertilize 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.

Horseshoe crabs are used as bait for catching eels and whelk.

Within the boundaries of Fire Island National Seashore, the harvest of horseshoe crabs is not permitted.

Ecological Importance of Horseshoe Crabs

Many spring migratory shorebirds, including the red knot, ruddy turnstone and sandpiper, rely upon the energy-rich horseshoe crab eggs for food.

Horseshoe crab eggs and larvae are an important food for many fish (America eel, killifish, silversides, summer flounder, winter flounder).

Learn More!

For More Information

• Horseshoe Crab: History & Biology/Shorebird Connection/Human Use/Research/Fisheries Management
  Mid-Atlantic Sea Grant Program and National Oceanic and Atmospheric Administration
• Long Island Horseshoe Crab NetworkDowling College
• Horseshoe Crab: New York State Fishery and Regulations  
  New York State Department of Environmental Conservation
  (Fire Island National Seashore is one of the areas now closed to the harvest of horseshoe crabs in New York)
   

Fun Horseshoe Crab Facts

Did you know that horseshoe crabs have blue blood? The protein (hemocyanin) that carries oxygen in horseshoe crab blood contains copper. When oxygen binds to the copper, the blood turns blue. As the oxygen is removed from the copper (as the blood circulates and delivers oxygen to the tissues of the horseshoe crab) the blood becomes colorless. The protein (hemoglobin) that carries oxygen in human blood contains iron. When oxygen is bound to the iron, human blood is red. As the oxygen is removed from the iron, the blood becomes blue.

The name horseshoe crab refers to the shape of the animal. The front half of a horseshoe crab looks like a horse's hoof or a horseshoe.