Random Facts About … How Horseshoe Crab Blood Keeps You Safe


By David Martin – 

We live in horseshoe crab territory. These flat, rounded sea creatures have a hard shell and five pairs of legs for crawling along the ocean floor — also for crawling out of the ocean and onto beaches where they breed and lay eggs. The male horseshoe crab is about 15 inches from head to tail with the female about a third larger. They are actually in the arachnid family, related to spiders. The species of horseshoe crab that lives in the Atlantic Ocean along the Eastern seaboard has large numbers in our area with the largest population of horseshoe crabs in the world being north of us, in Delaware Bay. 

Horseshoe crabs have been around from way before dinosaurs. When a species survives for 445 million years, something is going very right. For the horseshoe crab, it might be that shell or the breeding practices that has a single female laying 100,000 eggs over several nights — or it might be a chemical in the horseshoe crab’s blood that detects even the slightest presence of toxins and reacts by coagulating around the toxin to render it isolated and harmless. 

When scientists discovered this anti-toxin property of horseshoe crab blood, they figured out how to use the property to detect endotoxins in medicines and medical devices. Limulus amebocyte lysate (known as LAL), isolated from the horseshoe crab’s blood, can detect endotoxins even in concentrations of one in a trillion. The milky blue blood of the horseshoe crab (made blue from the blood’s concentration of copper just as the iron in our blood makes it red) is the only natural source of LAL, which is used to check the safety of vaccines such as those developed to protect us from COVID-19. LAL is also used to ensure the purity of other injected medicines along with surgically implanted artificial knees and hips. 

Companies that collect horseshoe crab blood don’t kill the animals. Instead, they draw about a third of a horseshoe crab’s blood and then return the animal to the wild in a place far enough from the collection site that it’s unlikely the same animal will be targeted again for a blood draw. The LAL created from the blue blood is worth more than $60,000 a gallon. 

Although almost all the crabs survive the blood letting, research has shown that, following a blood draw, females take longer to participate in breeding and don’t return to breeding sites as often as they did before blood was taken from them. The effect of bleeding horseshoe crabs might be contributing to their lower numbers. According to National Geographic, “In 1990, biologists estimated 1.24 million crabs spawned in Delaware Bay. … By 2002, that number had dropped to 333,500. In recent years, numbers of Delaware Bay spawning crabs have hovered around the same amount, with the 2019 survey estimating about 335,211.” 

The good news for horseshoe crabs is the development of a lab-produced LAL, which will render it no longer necessary to bleed half a million horseshoe crabs each year. One version of the lab-produced alternative has been approved for use in Europe.

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