Scientists analyzed genetic data and various number of plausible ways to conclude that horseshoe crabs are the relatives of spiders, scorpions. Before the study, they were considered to be closely related to crabs, lobsters and other crustaceans. In 1881, E. Ray Lankester, an evolutionary biologist placed them in arachnids. But there has never been enough data to prove that.
Now, biologists Jesus Ballesteros and Prashant Sharma from University of Wisconsin-Madison have published a study in Systematic Biology which says with high-degree of confidence that ancient horseshoe crabs do belong with the arachnid family tree.
This theory has further pressed the need to revise on all the previous hypotheses on the evolution of arachnids. This has created a major shift in the understanding of arthropod evolution.
Arthropods are considered one of the most successful species on Earth because of their ability to occupy land, water and sky. Horseshoe crabs have always been doubtful to be classified under arthropods because analysis has again and again shown their relation with arachnids like mites, scorpions, spiders etc. Scientist always assumed that there was an error like problem with the data or something.
Additionally, horseshoe crabs show some physical characteristics similar to arthropods. The group also appeared in fossil record around 450 million years ago, surprisingly, with a lineage of sea scorpions. Age has created some problems with trace back as finding a common ancestor becomes difficult. Also, evolution happened faster in these groups creating further complications.
Scientists in the study compared complete genomes of 3 out of 4 living horseshoe crab species against genome sequences of 50 other arthropod species. The data was in contrast with the researchers who generally cherry-pick data or rule out data that doesn't fit. This method helped in understanding the 'complicated evolution'.
It is still unclear on how the common ancestor evolved. It's possible that it first evolved in water then made it to land or first, evolved in land and then, recolonised the sea. The researchers are now working towards finding out the history of terrestrialization.
By: Neha Maheshwari