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Horseshoe Crabs
(Limulus polyphemus)

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Female horseshoe crabs with males (satellites) attached.

Horseshoe crabs are often referred to as "living fossils" because they haven't changed in the over 450 million years they've roamed the ocean floor. These ancient creatures aren't crabs at all, in fact they are more closely related to marine scorpions and spiders. Horseshoe crabs are complex creatures with a unique anatomy and even more unique blood.

Horseshoe Crab Blood

If you've ever had a flu shot, know someone with a pace maker or joint replacement, or have given your pet a rabies vaccination, you owe a debt of gratitude to the horseshoe crab. Vaccines, injectable drugs, intravenous solutions, and implantable medical devices, both for humans and animals, are quality checked for safety using a test that comes from the blood of horseshoe crabs. This, of course, includes the COVID-19 vaccine, which would not be considered safe without first being tested using horseshoe crab blood.


Every year, over 500,000 horseshoe crabs are captured and “bled” in laboratories for their blue blood. Commercial fishermen, contracted by biomedical companies withdrawing the blood, capture the crabs during their spring spawning season, then transport them to labs where up to 30% of their blood is removed from their bodies. The crabs are then released far from the site of capture, to ensure they are not bled a second time.


Why would anyone do such a thing to this ancient, harmless creature? If you’re alive, you’ve probably benefitted from this practice more than once. The blue blood of horseshoe crabs, is used to detect harmful bacteria in vaccinations, prosthetics and other medical devices like pacemakers.


To manufacture Limulus amebocyte lysate, commonly referred to as LAL, biotech companies collect adult horseshoe crabs, remove their blood, then release them alive. Approximately 15 percent of the animals do not survive the bleeding procedure. As of 2016 the bleeding mortality associated with LAL production was estimated to be 70,600 animals per year.


Even as alternatives are being developed that will retire or reduce the use of horseshoe crab blood, we will always be indebted to the horseshoe crab's contribution to our health.

Horseshoe Crab Anatomy

Horseshoe crabs have three parts to their anatomy; the head, abdomen and tail


1) Head: Also called the prosoma. Includes and protects the brain, eyes, heart, mouth and nervous system. This U shaped "helmet" is how the horseshoe crab got its name, since it is in the shape of a horseshoe.


Eyes: Horseshoe crabs have 10 eyes, 8 on the top of the shell (dorsal) and two on the bottom (ventral) near the front of the carapace above the legs and mouth. The largest pair are compound eyes on the top of the carapace. These are the most obvious eyes and are used mainly for finding a mate.


Mouth: Located between their legs and surrounded by spines. Their chelicerae crush worms, snails, and dead fish that are fed into the mouth.


Legs: Horseshoe crabs have ten walking legs and two chelicerae used for crushing food. Males have modified front legs in order to hook onto the female during spawning. These are often referred to as “boxing gloves.”


2) Abdomen: Also called the opisthosoma. There is also a noticeable rib down the center.  


Book Gills: Located near the back of the abdomen. Page-like structures used for swimming and extracting oxygen from the water. The first pair of the 6 book gills is called the operculum, and it serves as a cover for the other five pairs, which are the respiratory organs. In addition, the operculum houses the opening of the genital pores through which eggs and sperm are released from the body. They are also used to propel juvenile horseshoe crabs through the water. The hemolymph (a loose equivalent to blood) flows through the multiple pages (or lamellae) of the ten books (see ERDG's website for more about book gills and horseshoe crab anatomy)


3) Tail: Also called a telson. Is long, rigid and pointed. Is used as a rudder in the water and to right horseshoe crabs if they are flipped. The tail is not used to pierce or sting prey. It is completely harmless.

Size: Females are about 20-30% larger than males. The average width of the female's prosoma (the "helmet") is approximately 8.5 inches (21.5 cm).

Life Cycle

Eggs are laid in May and June during full and new moons. Once laid, they hatch in approximately 5 weeks. Larvae may remain in the sand for a few more weeks depending on weather. Once hatched, larvae will molt in a week or so and resemble the adults but very tiny. It takes females between 10-12 years to reach sexual maturity, and males between 9-11 years.

Spawning and Migration

Spawning: Pairs of crabs migrate to shallow water of beaches in May and June generally during new and full moons and the highest tide. The female has approximately 90,0000 eggs stored near the front of her shell at the beginning of the spawning season. She digs a shallow nest and lays thousands of eggs at a time. Male crabs attach themselves to the back of a female's shell with modified front legs (boxing gloves) and are ready to fertilize the eggs once they are laid. Other unattached males called “satellites” hang around while the female is laying eggs and also contribute to fertilization of the eggs.

Migration: Migration is for adult crabs, since young crabs stick more to the shallows for many years. Adult crabs live in deeper water during the year and only come to shore to spawn in the spring.

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