Why Is a Horseshoe Crab’s Blood Blue?

Hi, my name is Nevbahar. I am a young wildlife ambassador, and I studied the horseshoe crab. When I first saw one, I thought it looked like a creature from another time. I was right! Horseshoe crabs belong to a very ancient group of animals. Fossils of their relatives go back about 450 million years, long before dinosaurs appeared on Earth.

Even though we call them “crabs,” horseshoe crabs are not true crabs. They have a hard shell, many legs underneath, and a long tail called a telson. The tail may look dangerous, but it is not a weapon. Horseshoe crabs use it for balance and to help turn themselves over if they get flipped upside down.

But one of the most amazing things about horseshoe crabs is something we normally cannot see.

Their blood is blue.

Our blood is red because the protein that carries oxygen in our bodies contains iron. Horseshoe crab blood uses a different protein called hemocyanin, which contains copper. When oxygen attaches to the copper, their blood turns blue.

Their blue blood has also become very important to people. Horseshoe crab blood contains special cells that react strongly when they meet dangerous toxins made by certain bacteria. Scientists discovered that they could use this reaction to create a test called LAL, which has been used to check medicines and medical equipment for bacterial contamination. In a strange way, an animal that has been living in the ocean for hundreds of millions of years has helped make modern medicine safer.

But there is a difficult side to this story. Horseshoe crabs are collected for biomedical use and some of their blood is removed before many are returned to the ocean. In 2024, more than one million horseshoe crabs were collected along the Atlantic Coast solely for biomedical purposes, and scientists estimated that about 185,000 died from the collection and bleeding process or related causes. Scientists are now working with laboratory-made alternatives that can test for the same dangerous bacterial toxins without needing horseshoe crab blood. In 2026, the FDA reported progress toward allowing greater use of these alternatives.

Horseshoe crabs are also very important to other animals. Every spring, they crawl onto sandy beaches during high tides, especially around the new and full moons, and females bury thousands of tiny eggs in the sand. Those eggs become an important meal for migrating shorebirds, including sandpipers, Red Knots, Ruddy Turnstones, and other birds that need lots of energy for their long journeys.

This connection happens right here in Brooklyn. At Plumb Beach, nearly 3,000 horseshoe crabs may gather to spawn. Their eggs help feed migrating shorebirds stopping around Jamaica Bay. This was one of my favorite things to learn because Izabella studied sandpipers. Her birds and my horseshoe crabs are part of the same story. If there are fewer horseshoe crab eggs, a hungry bird arriving after a very long flight may find less food.

Horseshoe crabs still need our help. Scientists do not have one simple population number for the entire Atlantic Coast because different areas are doing differently. The latest major assessment found that the horseshoe crab population in the New York region is in poor condition, even though some other regions are doing better and some surveys have shown improvement in recent years. That is why protecting spawning beaches and carefully managing how many horseshoe crabs are taken is so important.

There is some good news. The National Park Service already prohibits horseshoe crab harvesting in and around Jamaica Bay. New York has also passed a new law that gradually reduces horseshoe crab harvest and will prohibit taking them from state waters for commercial or biomedical purposes beginning January 1, 2029. These changes show that people are beginning to understand that horseshoe crabs are worth more than what we can take from them.

Kids can help too. If we see horseshoe crabs spawning, we can watch them without disturbing them. We should never pull a horseshoe crab by its tail. If we find one stuck upside down, we can gently turn it over by holding the sides of its shell, with an adult helping if needed. Scientists also tag horseshoe crabs, and people who find a tagged crab can report it and help researchers learn where these animals travel. We can also keep beaches clean, protect spawning areas, learn about shorebirds, and teach other people why horseshoe crabs matter.

Before studying horseshoe crabs, I saw an animal with a strange shell and a long tail. Now I see something much bigger. I see an animal whose history reaches back hundreds of millions of years. I see blue blood that has helped modern medicine. I see thousands of eggs hidden underneath the sand. I see sandpipers arriving hungry after a long migration.

And I see a Brooklyn beach where all of these stories meet.

A horseshoe crab may look like something from the past.

But whether it has a future depends partly on what we choose to do today.