Horseshoe Crab Monitoring

Background / Life History

The American horseshoe crab (Limulus polyphemus) is a marine arthropod that is more closely related to spiders, ticks, and scorpions than true crabs. Recent fossil evidence suggests that horseshoe crabs, very similar in form to our present crabs, have been around for at least 445 million years, and haven't changed much over time. American horseshoe crabs are found along the Atlantic coast from northern Maine to the Yucatan Peninsula.

HB 6484 Signed Into Law in CT 

This is an Act regarding the Hand-harvesting of Horseshoe Crabs. The law takes effect on Oct. 1, 2023. Anyone violating the law will face a $25 fine per harvested Horseshoe Crab.

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Article on Horseshoe Crabs 2023

Horseshoe crabs are long-lived and can be as old as 20 years. It takes at least 9 to 12 years for a horseshoe crab to reach sexual maturity. Like other arthropods, horseshoe crabs must molt in order to grow. A crab will molt at least 6 times in their first year of life and about 18 times before they reach sexual maturity. Once reaching sexual maturity, it is believed that the crabs no longer molt so that they can invest more energy into reproduction.


In the late spring and early summer (predominately mid-May to the end of June), horseshoe crabs arrive at beaches to spawn. Spawning generally coincides with high tides during the full and new moons; breeding activity is usually highest during the evening hours of the full moon. Breeding adults prefer sandy beach areas that are protected from the heavy surf. Additionally, females may spawn over several evenings, with as many as five or more mating partners during each spawning period, and deposit as many as 100-000-eggs total.

The eggs, which will hatch in two to four weeks, provide an important food source for migratory shorebirds and finfish. Each year an estimated 425,000 to one million shorebirds amass in the Delaware Bay Estuary to feed on eggs that they rely on to buildup energy reserves during their northward migration. Furthermore, horseshoe crabs provide an important food source for Atlantic loggerhead turtles and sharks.

Ecological Role

In addition to playing an important ecological role, horseshoe crabs are harvested for medical and physiological research, and as bait in the commercial fishery for eels and conch. Horseshoe crab blood, which turns blue when exposed to oxygen because it is copper-based, contains a clotting agent called Limulus Amoebocyte Lysate (LAL). A unique property of LAL is that it clots when it comes in contact with bacterial toxins. Because of this, pharmaceutical companies test vaccines, drugs, prosthetics, and other medical devices with LAL to ensure that they are sterile. Currently, there is no FDA approved synthetic substitute for the LAL test.

Stock Status

Recent data analyzed by the Atlantic Sates Marine Fisheries Commission (ASMFC) suggests that the horseshoe crab population in the mid-Atlantic region is stable or declining. However, the Delaware trawl survey shows a significant decrease in abundance. Additionally, decreasing numbers of migratory shorebirds, such as the red knot, raises concern about the horseshoe crab population.

Spawning Surveys

The Town of Greenwich Conservation Commission currently conducts horseshoe crab surveys each year under the direction of Sacred Heart University's Project Limulus.

Project Limulus is:

  • A Study Examining the Ecology of the Long Island Sound Horseshoe Crab (Limulus polyphemus) population.
  • A Community-Based Research Program Providing Opportunities for All People to Become Active Contributors to On-Going Scientific Research.
  • A Data-Gathering Network to Potentially Direct Conservation Programs for the Horseshoe Crab
  • An Educational Tool to Increase Public Awareness of Limulus and its connection to the Long Island Sound Ecosystem and Human Health
  • Part of the International Union for the Conservation of Nature (IUCN) Horseshoe Crab Species Specialist Group

Surveys in the Town consist of dividing public beaches (under TOG supervision) into several 5m X 3m quadrats. All horseshoe crabs within and outside of the quadrats are counted and identified by sex. Numbers of mating pairs are recorded as well. In addition to conducting horseshoe crab surveys, the Town also tags crabs with a button tag so that population estimates and migration patterns can be determined. 

More Information

For more information, visit the Project Limulus website.