Horseshoe crabs have never quite fit in with the rest of the ocean's animals. Considered living fossils, their circular bodies and sharp tails are often presented as frightening. But horseshoe crabs aren't scary, they're just misunderstood. A new scientific study has created a definitive family tree for horseshoe crabs, showing that they're best classified as arachnids.
Scientists have long known that horseshoe crabs, scientifically known as the genus Xiphosura, were related to arachnids in some way. While most arachnids live on land, a number of them live in the water. The diving bell spider, for example, to trap oxygen.
An old line of thinking assumed there was split in the family. On one level this makes sense—arachnids have become one of the most diverse classes of anthropods on Earth, with over 100,000 species. Meanwhile, the horseshoe crab genus Xiphosura has barely evolved over thousands of years, with a mere four species to its name. That also makes them harder to classify.
“This particular part of the tree of life has always been quite challenging to solve,” says of the University of Wisconsin-Madison, lead author of the , speaking to . “But one of the things that was surprising in this analysis is that regardless of how we treated the data, we consistently found the same results … the horseshoe crabs are always nested inside the arachnids [on the family tree].”
The study took a look at whats known as genomic sequencing, analyzing a animal's DNA sequence one painstaking genome at a time. Ballesteros and others have been sequencing horseshoe crabs for some time, allowing for the creation of what's known as a phylogenetic dataset, a fancy way of saying a family tree. Looking at the genomic sequencing of 53 species of arachnids, horseshoe crabs and sea spiders, Ballesteros' team found overwhelming data putting Xiphosura within the anarchid family. Approximately two-thirds of these genetic trees came to that conclusion.
The horseshoe crab's new family is just further emphasizes how much left there is to discover about these creatures. Biomedical companies known their worth—horseshoe crab blood is amazingly potent at detecting E. coli, among other things, making their blood worth as much as $14,000 per quart. Out of time, the arachnids are likely still full of mysteries.