Scientists discover horseshoe crabs are spider-related

The horseshoe crab need not do much to haunt your nightmares. Their hard shell hides their slippery paws and claws, they use their long, thorny tail to turn and bleed milky blue blood. Think less "Sebastien of the Little Mermaid" and more "Facehugger from Alien".

But it turns out that the horseshoe crab is not adorable kind of crustacean or alien – in fact, scientists have proven that the horseshoe crab is not a crab. It's an arachnid.

The horseshoe crab (Xiphosura) is already one of the strangest creatures in the animal kingdom. While only four species survived the modern era, some half a meter in length, these species survived relatively unchanged for 450 million years and by multiple events of mass extinction, giving them the nickname "living fossils."

As Xiphosura's blood is so sensitive to toxins, scientists also collect blood (which is a beautiful shade of blue-sky) to use in contamination tests on things like medical equipment. (Blood is so valuable, researchers from Florida recently asked the public to report sightings of horseshoe crabs mating on the next full moon.) Sorry, kids, it looks like Disney World will have to wait!)

Now a new research paper, published by the University of Wisconsin-Madison and reported for the first time by National Geographic, has launched the horseshoe crab in a new light. Studying a large amount of genetic material as well as data from genome sequencing projects, the researchers traced the pedigree lineage of the horseshoe crab.

Discovery?

Horseshoe crabs did not evolve separately for ground arachnids like spiders and scorpions. They are actually classified as aquatic arachnids.

"This particular part of the tree of life has always been quite challenging to resolve," said lead researcher Jesús Ballesteros to National Geographic.

"But one of the things that was surprising in this analysis is that regardless of how we treat the data, we consistently find the same results … the horseshoe crabs are always nested within the arachnids [on the family tree]"

So if you're arachnophobic, you can comfortably add the horseshoe crab to your list of fears.

And if you're a human with eyes, you probably can too.