New research proves that the horseshoe crab is a relative to the spiders after all, and does not belong to the family of crabs.
Evolutionary biologists first thought that the blue-blooded horseshoe crab with 10 long legs belongs to the family of crabs and lobsters. In 1881, E. Ray Lankester theorized and categorized them in a group among spiders and scorpions. However, the molecular evolutionary patterns of the tiny creature could not confirm Lankester's theory.
Family Ties Between Horseshoe Crabs And Spiders
In a recent genetic study published in the journal Systematic Biology, evolutionary biologists from the University of Wisconsin-Madison identified the creature that has been crawling the earth for nearly 500 million years belong to the Ricinulei group, the arachnids that are also known as hooded tick spiders.
"We simulated coalescent gene trees to explore the effects of increasing levels of ILS on the placement of horseshoe crabs," notes the abstract of the study authored by Jesús Ballesteros and Prashant Sharma.
The earliest-known spider fossil dates back to 400 million years. However, the scientists could not believe the evidence set before their eyes and they thought there was an error that affected the data. As a result, the classification of the horseshoe crab was delayed for 140 years.
"Scientists assumed it was an error, that there was a problem with the data," said Ballesteros.
The study concludes that the horseshoe crabs belong to a group of aquatic arachnids, similar to aquatic mites. The biologists' doubts may also stem from the horseshoe crabs' 10 legs and 10 eyes, unlike common arachnids that have eight legs and eight eyes.
Those who have seen a horseshoe crab, particularly in an upside-down position, will not be surprised to hear that it belongs to the arachnid group. They look more of spiders with a shell and curious tail. Also, they have chelicerae, those arms of a spider that move food into its mouth.
They don't have the crab's antennae or its jaw. Zoologists also believe that the name horseshoe crab is a misnomer since Lankester classified them to a group that belongs to spiders and scorpions almost 140 years ago.
Today, the number of horseshoe crabs are diminishing due to the acidification of the oceans, which is bad for arthropods. Pharmaceuticals are also harvesting them for their blue blood for medical purposes, and makeup companies are using their shells.