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Living fossils saving lives


Horseshoe crabs: living fossils

The half-moon shaped prosoma with two eyes is how you know its a horseshoe crab. Photo Source: UF/IFAS

The American horseshoe crab (Limulus polyphemus) has been shuffling its spidery jointed legs along beaches and estuaries for over 445 million years (1). We’re talking millions of years older than dinosaurs. They are more closely related to spiders, than true crabs. Most beachgoers recognize them from their half-moon shaped shell (prosoma) and long sharp tail (telson). Unknowing tourists sometimes mistake their long telson as a stinger. It does not sting at all but allows the horseshoe crab to right itself if turned upside down by waves or predators.

In the grand scheme of coastal ecology, horseshoe crabs are prey for sea turtles, crabs, fish and migrating shorebirds. Many migratory birds rely on their eggs to sustain arduous northern migration routes. One, in particular, the federally threatened rufa Red Knot (Calidris canutus rufa) migrates 9,000 miles from the tip of South America to the Canadian Arctic every spring. It is a route they time with peak horseshoe crab spawning activity (2). The rufa Red Knots competition comes in the form of a unique fishery on the Eastern United States. Fishermen collect live horseshoe crabs for pharmaceutical facilities to harvest one third to one half of their blood. After processing, they are re-released into the wild. Florida does not harvest horseshoe crabs for this purpose, but some are harvested as bait for an eel fishery (1).

Human health and blue blood

The horseshoe crab’s blue blood is highly sought after in the biomedical industry. If you’ve ever had a saline drip, flu shot, heart stent, epidural and you guessed it, the COVID-19 vaccine, a horseshoe crab was involved. Their blood is copper-based containing a compound that coagulates blood when exposed to bacterial contaminants or endotoxins. The compounds extracted are made into the “Limulus Amebocyte Lysate” or LAL test (1). Endotoxins can be deadly if minuscule amounts make their way into the bloodstream, which is why the LAL test is required by the U.S. Food and Drug Administration.

Horseshoe crabs are involved with every COVID-19 vaccine being administered.

The high demand for the COVID-19 vaccine has conservationists concerned it will impact wild horseshoe crab and migratory shorebird populations. Bird conservation organizations are working on a solution. In recent years, other countries have developed a synthetic alternative recombinant Factor C (or rFC for short). Unfortunately, United States regulations have kept the standard LAL test, but the conservation groups are trying to change the use of the LAL test and transition to the synthetic versions. Only time will tell the outcome.

For now, next time you’re forming footprints along a sandy beach and witness a horseshoe crab scuttling along the tide’s edge, take a moment to appreciate the evolutionary adaptations living millions of years can provide and the humility that we rely on these living fossils to save humanity.


  1. UF/IFAS Extension: The American Horseshoe Crab (Limulus polyphemus)
  2. Florida Fish and Wildlife Conservation Commission: Facts About Horseshoe Crabs and FAQ