The blood of horseshoe crabs is blue. Limulus amebocyte lysate has unique medicinal value. The blood of limulus amebocyte lysate contains copper ions, and the color of copper oxidation is blue-green, so the extract of blue blood is limulus amebocyte lysate, which can accurately and quickly detect whether the internal tissues of human body are caused by bacterial infection. In pharmaceutical and food industries, it can be used to monitor toxin pollution. Scientists also use the blood of horseshoe crabs to study cancer.

Horseshoe crab is a national second-class protected animal, belonging to arthropods. It looks like a crab. Its body is blue-brown or dark brown. It has four eyes, two of which are compound eyes.

The ancestors of horseshoe crabs appeared in the Paleozoic Devonian in geological history, when dinosaurs had not yet appeared and primitive fish had just come out. With the passage of time, its contemporaries have either evolved or become extinct. Only horseshoe crabs, which came out more than 400 million years ago, still retain their primitive and ancient appearance, so horseshoe crabs are called "living fossils".

The earliest horseshoe crab fossils were found in Ordovician, and horseshoe crab fossils similar to modern horseshoe crabs appeared in Jurassic. Horseshoe crabs are as old as trilobites. Horseshoe crab is also called horseshoe crab, but it is not a crab, but related to scorpions, spiders and extinct trilobites.