Spawning Horseshoe Crabs, Adam’s Point, May 2023 (Photo: Stephen Halsey)

Of the many fascinating species found around Great Bay, the horseshoe crab may be the most unique. Despite their name, horseshoe crabs are not actually crabs. They belong to an ancient group of arthropods, more closely related to arachnids (like spiders and scorpions) than the crabs you see today. Known as “living fossils,” this species first appeared in the fossil record ~450 million years ago, which is 200 million years before the dinosaurs!

There are four species of horseshoe crab around the world, with only one (Limulus polyphemus) in North America. This species can be found along the entire Atlantic coast, from Maine to Florida. It has even been observed as far as the Yucatan peninsula! They have a smooth, hard shell shaped like a horseshoe (hence the name) and a pointed tail that they use as a rudder to steer while they swim or to right themselves if they are flipped over. This tail, and horseshoe crabs in general, are completely harmless. They rely solely on their hard shell for defense against predators.

Horseshoe crabs are vitally important to human and ecosystem health. Their eggs provide much needed fuel for many species of migratory birds, sea turtles, and fish, and their blood is an integral part of the biomedical industry. Their blue, copper-based blood quickly clots in the presence of bacterial toxins. This allows medical researchers to use it for testing intravenous drugs, vaccines, and medical devices to ensure they are free of bacterial contamination. To collect the blood, horseshoe crabs are caught in the wild, brought to facilities where their blood is drawn, then returned to the ocean after their blood has been collected. If you have ever gotten a vaccine, taken medication, or had surgery, thank a horseshoe crab!

With its many benefits to humans and ecosystem health, not to mention its ability to survive for millions of years, one would think that the horseshoe crab is here to stay. Unfortunately that might not be the case. Horseshoe crab populations worldwide are decreasing at alarming rates due to overharvesting from commercial fishing operations, loss of spawning habitat, and high demand for their blood from the biomedical industry. Through dedicated stewardship and research, horseshoe crab scientists and advocates around the world, including here at Great Bay, are doing everything they can to preserve this species. Click this link for more information on horseshoe crab conservation at Great Bay and what you can do to help.


Horseshoe Crabs – NH Fish and Game Department (

Atlantic Horseshoe Crab (Limulus polyphemus) | U.S. Fish & Wildlife Service (

UNH Research Reveals Water Temperature Influences Horseshoe Crab Spawning | UNH Today

The Horseshoe Crab: Natural History, Anatomy, Conservation and Current Research

Tessa Corsetti, Great Bay NERR Biologist