The horseshoe crab does not have to do much to track your nightmares. Their hard shell hides jerking legs and tongs, they turn around with their long, prickly tail and bleed milky blue blood. Think less about "Sebastien from The Little Mermaid" and more about "Facehugger from Alien." However, it turns out that horseshoe crab is neither a delightful caricature of crustaceans nor an alien species ̵
The horseshoe crab (Xiphosura) is already one of the stranger creatures in the animal kingdom. While only four species of the animal have survived to the modern era, some of which are about half a meter long, these species have survived 450 million years and relatively unaffected by several mass extinctions, giving them the nickname "living fossils".  Because Xiphosura blood is so sensitive to toxins, scientists also harvest their blood (a beautiful color of sky blue), for example, to test for contamination in medical devices. (The blood is so precious, researchers in Florida recently called on the public to observe sightings of horseshoe crabs mating under the next full moon, I'm sorry, children, Disney World will have to wait!)
Now, a new research paper published by the University of Wisconsin-Madison and first reported by National Geographic has shed new light on horseshoe crabs. Researchers studied a range of genetic material as well as data from genome sequencing projects and traced the lineage of the horseshoe crab family tree.
The horseshoe crabs did not develop separately to arachnids on land such as spiders and scorpions. They are actually classified as aquatic arachnids.
"This particular part of the tree of life has always been a great challenge," said lead researcher Jesús Ballesteros to National Geographic.
"Surprising in this analysis, however, was that regardless of the way we treated the data, we always found the same results: The horseshoe crabs are always nested in the arachnids [on the family tree].
If you If you're an arachnophobist, you can now easily add the horseshoe crab to your list of fears.
And if you're a human with eyes, you probably can.