A new study in Molecular Biology and Evolution led by Professor Gonzalo Giribet's lab disproves the monophyly (a group composed of only one ancestor) of Arachnida, suggesting horseshoe crabs colonized the sea from a land ancestor.
Chelicerata (Arachnids, horseshoe crabs, and their kindred taxa) are one of the two major branches of arthropods. They have long been considered a diverse group with many Paleozoic marine fossils. For instance, the horseshoe crabs is considered a primitive taxon and the sister group of the terrestrial Arachnida (scorpions, spiders, mites, daddy-long-legs, etc.). It's always been difficult to decipher the evolutionary relationships of Chelicerata because of their ancient rapid radiation and high evolutionary rates in several lineages. Scientists accept the monophyly of Arachnida despite a lack of support in molecular data and conflict between morphological and molecular analyses.
To address this conflict the study's researchers assembled a data set of 506 high-quality genomes and transcriptomes sampling all living orders of Chelicerata, and generated a 514-taxon morphological data set of living and fossil Chelicerata. Analyses of the two data sets failed to recover arachnid monophyly, instead placing horseshoe crabs within Arachnida.
The findings have far-reaching implications, as it may indicate that horseshoe crabs have colonized the marine environment secondarily, from a terrestrial arachnid ancestor. "This may be difficult to swallow for some researchers, but we cannot forget that there are also many mites that returned to the aquatic environment," Giribet said. "Also, horseshoe crabs return to lay their eggs on land, something unknown for any other marine group, and instead follow the pattern of secondarily adapted aquatic animals, like sea turtles or crocodilians, which return to land to lay their eggs."
Image: Traditional (left) versus new (right) hypothesis on arachnid evolution. Arachnids (scorpions, spiders, mites, daddy-long-legs, etc.) are depicted in green while the non-arachnid chelicerates are in blue. The new hypothesis places some of the marine chelicerates within the terrestrial arachnids. Credit P. Sharma’s Lab.