Horseshoe crabs are living fossils - they go back over 300 million years, outlived the dinosaurs and are the oldest life on earth. And you can thank these ancient creatures for your health and your life.
Horseshoes are not crabs, they are arachnids like spiders and scorpions. The family resemblance is hidden under the domed shell, where the crab's mouth is surrounded by 10 articulating legs that scuttle the horseshoe across the seabed like a spider in search of prey.
There are four species of horseshoe crabs, which are found only on the eastern shores of the United States and in Southeast Asia. They range throughout the water column from estuaries to the continental shelf, but are most-often seen in spring when they swarm onto beaches to mate in large nesting groups.
Each female lays up to 100,000 eggs that are fertilized by smaller males and are a major food source for migratory shore birds, fish and crustaceans. In summer, horseshoe crab shells litter the beaches as each individual molts 17 times before reaching maturity at ten years old.
The blood of horseshoe crabs is blue and is incredibly sensitive to toxins in bacteria. It is harvested to test for disease-bearing pathogens on everything that goes inside the human body: shots, IV drips, medical devices and intravenous drugs. It played a key role in the manufacture of vaccines used to combat COVID-19. You could say we owe our lives to these ancient blue bloods.
Science is also interested in the complex vision system of horseshoe crabs. They have ten eyes on the top and underside of the shell as well as an array of light-sensitive photoreceptors along the body and tail. The compound eyes have up to 1,000 facets and the photoreceptors are remarkably light sensitive.
The next time you see a horseshoe crab on the beach, show a little respect and gratitude for this ancient creature of the sea.