Horseshoe Crab (Limulus polyphemus)


Class: Merostomata
Order: Xiphosura
Family:    Limulidae
Size:    Length: Up to 24 inches (60 cm)
Weight: 3 to 4 pounds (1.3 to 1.8 kg)
Diet: Worms, small molluscs and algae
Distribution: Along the Atlantic coast from Nova Scotia to Yucatan
Young:  Up to 25,000 eggs per season
Animal Predators:  Loggerhead turtles, pufferfish, leopard sharks and sea gulls 
IUCN Status: Lower Risk, Near Threatened
Terms: No special terms
Lifespan: Up to 15 years in captivity



·       Fossil horseshoe crabs date from the Ordovician period, 500 million years ago.

·       Horseshoe crabs have two pairs of eyes that allow them to see in all directions.

·       Horseshoe crab blood contains a clotting factor that can be extracted and used to detect bacteria in human blood, intravenous drugs and prothetics.



Horseshoe crabs are actually not crabs; they are more closely related to spiders and scorpions. They are dark brown animals that crawl backwards along the ocean bottom. Flap-like gills are used as paddles when swimming. Horseshoe crabs resemble miniature army tanks, with a hard, thick, horseshoe-shaped shell, and a long tail (also known as a telson) extending from one end. Their tail supports their body when moving and also helps them turn over if a current flips them over accidentally. The mouthparts are called chelicerae and resemble those of a spider. Until they reach maximum size, horseshoe crabs outgrow their shell and moult. While the crab is still inside the old shell, a new shell begins to grow. When the old shell splits, the crab crawls out. The new shell, which is about one-quarter larger than the old one, is soft at first, but hardens quickly. Female horseshoe crabs are larger than males. 



This particular species of horseshoe crab is found along the Atlantic coast of North America. There are three other species—Tachypleus gigas, Tachypleus tridentatus and Carcinoscorpius rotundicaudafound in the eastern Pacific Ocean, along the coast of Asia from Japan to Vietnam, and they are very similar in both habits and appearance to the Atlantic coast horseshoe crab. 


Feeding Habits

Horseshoe crabs feed on worms, small molluscs and algae. Horseshoe crabs crush and grind their food with the spiny bases of their legs and then push the small food particles into their mouth.



When spring arrives, the crabs begin their annual migration to shore. Females attract males by releasing a pheromone—a chemical—into the water. Once a male has hooked onto her abdomen, the female leads him to the water’s edge, where she prepares several nests just below the high-water mark in the sand and deposits 2,000 to 25,000 tiny eggs in them. She then drags the male over the eggs, and he fertilizes them. Not many make it to adulthood—birds flying north after wintering in the south stop on the beach to gorge on the eggs. The eggs hatch several weeks later as larvae. Maturity is reached at the age of 10, after numerous moults. 



Horseshoe crabs prefer shallow water with sandy or muddy bottoms, and will sometimes come up out of the water onto a beach. They look fierce with their long pointy tail and pincers, but they are actually gentle, and harmless to humans. 



Due to their declining numbers, quotas are being put into place in the U.S. to limit the amount of horseshoe crabs that can be caught. 



Horseshoe Crab Wildlife Fact File, IM Pub, US