Ask a Scientist: How do horseshoe crabs save lives?

Julie Davis is a living marine resources extension specialist with the S.C. Sea Grant Consortium.
Julie Davis is a living marine resources extension specialist with the S.C. Sea Grant Consortium. S.C. Sea Grant Consortium

How could horseshoe crabs save your life?

Julie Davis is a living marine resources extension specialist with the S.C. Sea Grant Consortium. Here she explains how the ocean’s primordial blue-bloods are helping biomedical research. Questions and answers have been edited.

Q. Beachgoers can’t help but take notice when they come across a horseshoe crab, with their helmetlike shells and spiked tails. What else is special about these sea creatures?

A. Horseshoe crabs’ ancestors date back 440 million years. They are actually more closely related to spiders and scorpions than to crustaceans – the group that includes crabs, lobsters and shrimp. Something you might not know about horseshoe crabs is that they have bright blue blood. Interestingly, we rely on their blood to detect bacterial contamination in critical medical treatments, so if you’ve ever received IV treatment in a hospital you’ve likely benefited from horseshoe crab blood.

Q. Why do horseshoe crabs have blue blood? Why is that so important?

A. Unlike humans, horseshoe crabs do not have hemoglobin – an iron-based molecule that gives our blood its red color. Instead, horseshoe crabs use hemocyanin, which is copper-based, to carry oxygen through the bloodstream. It is the copper that makes their blood blue.

The blood of horseshoe crabs contains a special compound called Limulus Ameobocyte Lysate (LAL) that clots whenever bacterial contamination is present. Decades ago, researchers showed this protein could detect as little as a picogram – a trillionth of a gram – per milliliter in medical treatments. That made it the most sensitive test available for bacterial contamination. Today, the LAL test is critical in detecting contamination in medical treatments and equipment, ranging from vaccines to pacemakers.

Q. How is the blood of horseshoe crabs harvested?

A. Thousands of horseshoe crabs are collected by hand from the shoreline of the East Coast each year. The live crabs are delivered to one of five laboratories, where they are bled in a sterile facility by expert technicians. Approximately 25 percent of their blood is removed before the animals are returned to the water. A quart of the horseshoe crab blood extract is worth around $15,000.

Q. The crabs are tagged before they’re returned to the water. What has been learned from tagging?

A. The U.S. Fish and Wildlife Service works with several states along the East Coast to tag horseshoe crabs. Tagging studies allow scientists and resource managers to learn about the movements of horseshoe crabs. A study funded by the S.C. Sea Grant Consortium examined post-bleed survival and found that the majority of animals – 80 to 90 percent – survive the procedure. Unlike in other states where horseshoe crabs are harvested for use as bait in other fisheries, in South Carolina our only use of our horseshoe crab resource is for biomedical purposes. In North Carolina, horseshoe crabs are harvested both for biomedical purposes and for bait, but the harvest is limited.