With their blue blood and armor of ten spindly legs, horseshoe crabs have long puzzled scientists about their place in the evolutionary tree. Initially thought to be closely related to crabs and lobsters, in 1881, evolutionary biologist E. Ray Lankester placed them in a group more akin to spiders and scorpions. While considered ancestors of arachnids, sparse molecular sequence data cast doubt.
Evolutionary biologists Jesús Ballesteros and Prashant Sharma from the University of Wisconsin-Madison aimed to clarify this evolutionary puzzle. Their recent study, published in Systematic Biology, delves into extensive genetic data, bolstering the case for firmly situating horseshoe crabs within the arachnid family tree.
The researchers analyzed copious genetic data through various approaches, providing a high degree of confidence that horseshoe crabs are indeed part of the arachnid lineage.
“By showing that horseshoe crabs are part of the arachnid radiation, instead of a lineage closely related to but independent of arachnids, all previous hypotheses on the evolution of arachnids need to be revised,” explains Ballesteros, a postdoctoral researcher in Sharma's lab. This discovery marks a significant shift in our understanding of arthropod evolution.
Arthropods, considered among the most successful animals on Earth, encompass a diverse array of species occupying land, water, and air. This group includes insects, crustaceans, and arachnids.
The classification of horseshoe crabs within arthropods has posed challenges due to genetic analyses consistently linking them to arachnids like spiders and scorpions. Ballesteros notes that scientists previously perceived this connection as an error, highlighting the significance of this research in rectifying our understanding of horseshoe crab evolution within the broader arthropod family.
Horseshoe crabs present a unique amalgamation of physical traits observed in various arthropods. While sharing a hard shell with crabs, they stand out as the sole marine animals known to respire through book gills, reminiscent of the book lungs employed by spiders and scorpions for survival on land.
Despite only four extant horseshoe crab species, their origins trace back about 450 million years, coexisting with extinct lineages like sea scorpions. These living fossils have withstood major mass extinctions, and today, their blood plays a crucial role in biomedical testing for bacterial contamination.
Tracing their evolutionary history poses challenges due to the complexity of unravelling relationships among ancient bursts of speciation. Professor Sharma and postdoctoral researcher Ballesteros emphasize the difficulty of finding a common ancestor, especially considering rapid evolution among these arthropod groups, complicating their interconnections.
Ballesteros addressed intricate gene relationships by comparing the complete genomes of three living horseshoe crab species with 50 other arthropod species. Regardless of the approach, horseshoe crabs consistently nestled within the arachnid family tree, cautioning against cherry-picking data and emphasizing the need for rigorous analysis.
The mystery of why horseshoe crabs remained aquatic while other arachnids ventured onto land remains unanswered. Belonging to the Chelicerata group, which includes sea spiders, these water-dwelling arthropods prompt questions about the intricate history of terrestrialization.
The study suggests that the conquest of land by arachnids is a complex process with potential scenarios: the common ancestor of arachnids evolving in water, with only spiders and scorpions transitioning to land, or a common ancestor evolving on land, subsequently leading to horseshoe crabs recolonizing the sea.
Professor Sharma emphasizes the overarching question about the history of terrestrialization, probing into the multifaceted paths arachnids took in their evolution.
Ballesteros, currently studying the evolution of blindness in spiders residing deep within Israeli caves, echoes the innate human curiosity driving evolutionary inquiries. “How did all this diversity come to be?” he reflects, appreciating the privilege of delving into the incredible intricacies of existence.
Source: University of Wisconsin-Madison