Blue Blood and Horseshoes

Updated: Jun 27, 2021

By Celine Teh

Prehistoric creature, horseshoe crab. (

What a huge crab!

Is this a crab, spider or a scorpion? How long has its species lived? What is its correlation with vaccines? What does this creature do with my pet?

The horseshoe crab is a wondrous prehistoric species that is one of the greatest living fossils ever discovered on Earth, as its oldest fossil dates back to 445 million years ago (the Paleozoic Era), making it older than the dinosaurs. Surprisingly, their genus, Xiphosura describes marine arthropods related to arachnids, which means that they are more related to spiders and scorpions than crabs. This popular misnomer came from its exoskeleton, those crab-looking legs, and the horseshoe-shaped body when viewed on its back.

There are four species recorded today, one found in the Atlantic Ocean along the North American coastline, and the rest found in the Indian Ocean and the Pacific Ocean along the coast of Asia. They feed on worms, clams, and algae. A horseshoe crab typically lives for more than 20 years, and follows a life cycle similar to sea turtles; in late spring and summer, eggs are laid along the beach at night and hatch, then juvenile horseshoe crabs crawl offshore and deep into the ocean bed until they return to spawn as adults. In the Delaware Bay (United States) ecosystem, horseshoe crabs are crucial, because they and their eggs are an important diet for fish, birds, and reptiles in and around the region.

Migrating birds feed on horseshoe crabs. (National Geographic)

Alien blood

Unlike a mammal’s red blood, a horseshoe crab’s blood is blue due to the presence of copper in hemocyanin (proteins that transports oxygen, similar to hemoglobin in humans). Normally, the blood is transparent, but it turns blue when oxygenated..

The blue blood consists of special cells, named granulocytes, that release a substance that contains blood-clotting cells when intruders are detected. Limulus amebocyte lysate, in particular, is extremely sensitive to endotoxins, a fever-causing agent on the exterior cell walls of bacteria, and causes blood to coagulate around the pathogen or toxin, trapping it in a gel to prevent it from spreading to and affecting other parts of the body.

When sufficient endotoxin enters a man’s blood, it will cause fever, shock, organ failure, and in extreme cases, death. This is why it is vital to keep intravenous substances and objects endotoxin- and bacteria-free. Here, horseshoe crabs’ blood comes onto the stage. Limulus amebocyte lysate has been developed into a test for the level of endotoxin and sterility (the absence of living bacteria), called the LAL test. In Asia, a similar test, short-formed the TAL test, has been developed using another related species. LAL and TAL replace the pyrogen test (rabbit fever test for endotoxin) in international pharmacopeia tests and make injectable drugs, vaccines, and implantable medical devices safer for humans and animals. Thus saving countless lives.

The blood of horseshoe crabs being collected. Unfortunately, some cannot survive the bleeding while others cannot reproduce. (National Geographic)


In 1990, it was estimated that there were 1.24 million crabs in Delaware bay; within a short span of 29 years, the population has fallen drastically to 335 211 by 2019. The American horseshoe crab (Limulus polyphemus) has been classified as “vulnerable” to extinction. According to the International Union for Conservation of Nature (IUCN), The coastal horseshoe crab (Tachypleus gigas) and the mangrove horseshoe crab (Carcinoscorpius rotundicauda) are about to be listed on the IUCN Red List of Threatened Species, and the Chinese horseshoe crab (Tachylaus tridentatus) is already listed “endangered”. There are multiple causes for this.

First, coastal developments and habitat loss greatly decrease its population. Secondly, horseshoe crabs are easily captured to be used as bait in the American eel and conch fisheries or made into dishes in Southeast Asian countries, especially in Thailand, Malaysia and Vietnam. Both consumers prefer females, thus lowering reproduction rates.

Thirdly, horseshoe crabs have been exploited for their blood for biomedical use. To manufacture endotoxin LAL/TAL tests in America, the companies collect these creatures, bleed them for one-third of their blood, then release them alive back to the ocean. The bleeding mortality rate was circa 15 percent. In Asia, crabs for TAL are bled to death and their remains are sold off to gain maximum profit. In addition, the time to reach sexual maturity increases and the reproduction rate decreases when 30% of the animal’s blood is removed, so the American method is not without negative side effects.

Thank you crabs!

So, next time when you receive any injectable medicines or get your pets vaccinated for rabies, keep in mind that these are made safe by the contribution, and even sacrifice, of the horseshoe crabs and thank them sincerely. You can also give back by fighting littering along beaches or flipping over a horseshoe crab by turning it over by the edge of its shell if you see one strained on the beach. They are absolutely harmless and non-poisonous, but just be careful not to drag its fragile tail - it is meant for overturning the creature, but it doesn’t work every time. For a demonstration on flipping a horseshoe crab, click here.

Save the crabs!


Arnold, C. (2021, May 3). Horseshoe crab blood is key to making a COVID-19 vaccine-but the ecosystem may suffer. Animals.

The horseshoe crab and public health. The Horseshoe Crab: Natural History, Anatomy, Conservation and Current Research. (n.d.).

Horseshoe Crab. National Wildlife Federation. (n.d.).

Mehmood, Y. (2019, January 8). What Is Limulus Amebocyte Lysate (LAL) and Its Applicability in Endotoxin Quantification of Pharma Products. IntechOpen.

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