Why Is Horseshoe Crab Blood Blue? Blue Blood Explained

How hemocyanin turns blood blue

Hemocyanin means “blue blood protein”. Each place on the molecule that grabs oxygen holds a pair of copper atoms, which together bind one oxygen molecule. When oxygen attaches, the copper changes its chemical state and the protein absorbs orange and red light, so the blood looks blue. Without oxygen it is almost colorless.

In the Atlantic horseshoe crab, hemocyanin floats freely in the blood fluid at about 50 g per liter rather than being packed inside cells, as hemoglobin is in our red blood cells. It carries less oxygen than our blood, but it works well enough for a slow-moving animal living in cool coastal water.

Horseshoe crabs are not unusual in this. Octopuses, squid, many snails, many crabs and lobsters, and spiders and scorpions all use hemocyanin, as explained in Why is octopus blood blue?

The blood cells that detect bacteria

What makes horseshoe crab blood valuable is not its color but its cells. The blood contains one main type of cell, the amebocyte, which works as the crab’s immune defense. Amebocytes are packed with granules that release clotting proteins when they meet endotoxins, molecules from the outer walls of a large group of bacteria known as gram-negative bacteria.

The result is a gel-like clot that seals off invading bacteria. This matters for an animal with an open circulatory system, in which blood washes through body spaces instead of running in closed vessels, so an infection could spread quickly.

In 1956 the American researcher Frederik Bang reported that bacteria made horseshoe crab blood clot. In the 1960s he and Jack Levin showed that the reaction was triggered by endotoxin and came from the amebocytes, which led to a test that uses extracts of those cells.

What is the LAL test?

The extract is called Limulus amebocyte lysate, or LAL. To make it, the blood cells are separated, burst open in water, purified and freeze-dried. When a sample containing endotoxin is mixed with LAL, it clots, turns cloudy or changes color, depending on the version of the test.

Endotoxins can cause fever and serious reactions if they get into the bloodstream, even when the bacteria themselves are dead. Since the US Food and Drug Administration approved LAL in 1977, it has been the standard way to check injectable drugs, vaccines, intravenous fluids and medical devices such as implants for contamination. It replaced a slower and costlier test that used rabbits. In Asia, a similar test is made from the blood of Asian horseshoe crabs.

How horseshoe crabs are bled

Biomedical companies collect Atlantic horseshoe crabs along the US East Coast, mostly during the spawning season, when crabs gather on beaches and in shallow water. At the laboratory, a needle is inserted through the soft hinge behind the front shell, and part of the blood is drained. The industry says up to about 30% of a crab’s blood is taken, although some reports suggest more.

The crabs are then returned to the sea, usually one to three days after they were collected. Their blood volume recovers within about a week, but blood cell numbers can take two to three months to return to normal. Hundreds of thousands of crabs are collected for this purpose each year.

Does bleeding harm the crabs?

Yes, to a degree. Estimates of how many crabs die after bleeding range from about 3–15% in early industry studies to 10–30% in later research. A 2010 Massachusetts study found that 22% of bled females died when they were returned to the water at once, and about 30% when they were held overnight, as often happens commercially. US fishery managers use an estimated death rate of about 15% when they plan harvests.

Survivors can also be affected. Bled crabs tend to be sluggish for days or weeks, and females may spawn less. Because the Atlantic horseshoe crab is listed as Vulnerable and its eggs feed migrating shorebirds such as red knots, any extra losses matter. Even so, far more crabs die each year in the harvest for whelk and eel bait, in which every crab taken is killed.

Synthetic alternatives

Scientists at the National University of Singapore developed recombinant factor C, or rFC, a lab-made copy of the protein in the crab’s blood that first detects endotoxin. It went on sale in 2003, made by genetically modified cells rather than taken from crabs. It is not triggered by fungal sugars that can cause false positives in LAL, and studies have found it at least as reliable.

Adoption was slow at first, partly because only one company sold it. That has changed. The European Pharmacopoeia, which sets official drug-testing methods in Europe, now includes rFC, and in 2025 a dedicated chapter for recombinant endotoxin tests became official in the United States Pharmacopeia. Several large drug companies have switched some testing to rFC, a shift that could greatly reduce the number of crabs bled each year.

Blue blood beyond horseshoe crabs

Blue blood is just one of the features that set horseshoe crabs apart. They breathe with book gills, chew with their legs and are more closely related to spiders than to the crabs they are named after, as Is a horseshoe crab a crab? explains. Their color is simply chemistry: copper in their blood instead of iron.