Which Of The Following Animals Is Most Closely Related To Spiders?

Study confirms horseshoe crabs are really relatives of spiders, scorpions

University of Wisconsin–Madison postdoctoral researcher Jesús Ballesteros holds a small horseshoe crab. A study he led with integrative biology Professor Prashant Sharma used robust genetic assay to demonstrate that horseshoe crabs are arachnids like spiders, scorpions and ticks. Courtesy of Jesús Ballesteros

Blue-blooded and armored with 10 spindly legs, horseshoe crabs have mayhap always seemed a bit out of identify.

First thought to be closely related to venereal, lobsters and other crustaceans, in 1881 evolutionary biologist E. Ray Lankester placed them solidly in a group more similar to spiders and scorpions. Horseshoe crabs have since been thought to be ancestors of the arachnids, but molecular sequence data have ever been sparse enough to bandage doubt.

University of Wisconsin–Madison evolutionary biologists Jesús Ballesteros and Prashant Sharma promise, so, that their recent report published in the journal Systematic Biology helps firmly establish ancient horseshoe crabs inside the arachnid family tree.

Jesús Ballesteros holds a small horseshoe crab. Courtesy of Jesús Ballesteros

By analyzing troves of genetic data and considering a vast number of possible ways to examine it, the scientists now take a loftier degree of confidence that horseshoe crabs practice indeed vest within the arachnids.

"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," says Ballesteros, a postdoctoral researcher in Sharma's lab. "It'south a major shift in our understanding of arthropod evolution."

Arthropods are oft considered the nigh successful animals on the planet since they occupy land, water and sky and include more than a one thousand thousand species. This grouping includes insects, crustaceans and arachnids.

Horseshoe crabs have been challenging to classify inside the arthropods considering assay of the animals' genome has repeatedly shown them to be related to arachnids like spiders, scorpions, mites, ticks and lesser-known creatures such equally vinegaroons. Yet, "scientists assumed information technology was an fault, that there was a trouble with the information," says Ballesteros.

"By showing that horseshoe crabs are function of the arachnid radiation, … all previous hypotheses on the development of arachnids need to be revised. It's a major shift in our understanding of arthropod evolution."

Jesús Ballesteros

Moreover, horseshoe crabs possess a mix of physical characteristics observed amongst a diversity of arthropods. They are difficult-shelled like crabs but are the merely marine animals known to breathe with book gills, which resemble the book lungs spiders and scorpions use to survive on state.

Simply four species of horseshoe crabs are alive today, but the group offset appeared in the fossil record well-nigh 450 million years ago, together with mysterious, extinct lineages like sea scorpions. These living fossils have survived major mass extinction events and today their blood is used by the biomedical industry to test for bacterial contagion.

Prashant Sharma

Age is just 1 of the problems inherent in tracing their evolution, say Ballesteros and Sharma, since searching dorsum through fourth dimension to find a common antecedent is not easy to accomplish. And evidence from the fossil tape and genetics indicates evolution happened quickly amid these groups of animals, convoluting their relationships to one another.

"I of the most challenging aspects of building the tree of life is differentiating old radiations, these aboriginal bursts of speciation," says Sharma, a professor of integrative biological science. "Information technology is difficult to resolve without big amounts of genetic data."

Even then, genetic comparisons become catchy when looking at the histories of genes that can either unite or separate species. Some genetic changes tin can exist misleading, suggesting relationships where none exist or dismissing connections that do. This is owed to phenomena such as incomplete lineage sorting or lateral gene transfer, by which assortments of genes aren't cleanly made across the evolution of species.

Ballesteros tested the complicated relationships between the trickiest genes past comparing the complete genomes of three out of the four living horseshoe crab species against the genome sequences of 50 other arthropod species, including water fleas, centipedes and harvestmen.

A scorpion hides in an egg carton in a cage in a campus animal facility. Photograph by Kelly April Tyrrell

Using a complex set of matrices, taking intendance not to introduce biases in his analysis, he painstakingly teased the data apart. Still, no matter which manner Ballesteros conducted his analysis, he establish horseshoe crabs nested inside the arachnid family tree.

He says his approach serves every bit a cautionary tale to other evolutionary biologists who may be inclined to red-pick the data that seem most reliable, or to toss out information that don't seem to fit. Researchers could, for case, "force" their data to identify horseshoe crabs amongst crustaceans, says Sharma, merely information technology wouldn't exist accurate. The research squad tried this and found hundreds of genes supporting incorrect trees.

Ballesteros encourages others to subject their evolutionary information to this kind of rigorous methodology, because "evolution is complicated."

Prashant Sharma shows off a preserved sea spider recovered from the deep sea in Antarctica. Photo by Kelly April Tyrrell

Why horseshoe venereal are h2o dwellers while other arachnids colonized country remains an open question. These animals belong to a grouping called Chelicerata, which also includes sea spiders. Sea spiders are marine arthropods similar horseshoe venereal, just they are not arachnids.

"What the study concludes is that the conquest of the land by arachnids is more complex than a single tradition event," says Ballesteros.

It'south possible the common ancestor of arachnids evolved in water and but groups like spiders and scorpions made information technology to state. Or, a common ancestor may accept evolved on land and then horseshoe crabs recolonized the sea.

"The big question we are after is the history of terrestrialization," says Sharma.

For Ballesteros, who is now studying the development of blindness in spiders living deep inside caves in Israel, his motivations get to the heart of human nature itself.

"I get to look with childish curiosity and enquire: 'How did all this diverseness come to be?'" he says. "It'due south incredible what exists, and I never thought I would have the privilege to be able to exercise this."

The study was funded past the Thousand. Guyer postdoctoral fellowship and supported by National Science Foundation grant IOS-1552610.

Source: https://news.wisc.edu/study-confirms-horseshoe-crabs-are-really-relatives-of-spiders-scorpions/

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