The Atlantic Horseshoe Crab (Limulus polyphemus) is an ancient invertebrate that has been on the planet for over 450 million years, meaning it pre-dates the dinosaurs. It is an arthropod belonging to the subphylum Chelicerata, also containing spiders, scorpions, mites, ticks, and the extinct trilobites (thus horseshoe crabs are not, in fact, crabs.)
Horseshoe crabs possess nine eyes: the two obvious compound eyes, two smaller median eyes (they superficially look like they might be nostrils at first glance due to their placement), and five microscopic simple eyes spread across the body that are only capable of detecting sunlight. Additionally, their mouths are located between their legs, and the hair-like gnathobases that they use to break down their food are attached to the bases of their legs, meaning horseshoe crabs actually have to walk in order to chew!
Horseshoe crabs are a vital human resource, as their blood contains amebocytes that clot upon contact with bacterial endotoxins. Thus, the medical industry almost exclusively uses horseshoe crab blood to test drugs and vaccines for the presence of harmful bacterial contamination before releasing them for human use. Before the use of horseshoe crab blood, rabbits were used to test drugs for contamination: the rabbit was injected with the drug, and if it got sick or died, the drug was contaminated.
Some other interesting horseshoe crab facts:
- Their blood is blue, because it contains a lot of copper, unlike our blood, which is red because it contains a lot of iron.
- Females are on average 30% larger than males
- The tail of a horseshoe crab is used solely for flipping the crab over if it ends up upside down: the tail is not used at all for defense and cannot sting you (people often associate the tails of horseshoe crabs with those of stingrays).
- Horseshoe crabs should never be picked up by their tails, as the muscle connecting the tail to the body is surprisingly weak and can easily tear under pressure. A horseshoe crab without a tail in the wild is almost guaranteed to die, as it has no way of flipping itself over should it end up upside down.