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Feb 28, 2019 · Scientists have long known that horseshoe crabs, scientifically known as the genus Xiphosura, were related to arachnids in some way. While most arachnids live on land, a number of them live in...
Mar 9, 2019 · A study he led with Integrative Biology Professor Prashant Sharma used robust genetic analysis to demonstrate that horseshoe crabs are arachnids like spiders, scorpions and ticks. Credit:...
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- A tale of two sisters
- See Photos of Amazing Underwater Wildlife
- Arachnocrabs
- By land or by sea?
The primordial ocean dwellers are squarely situated in the arachnid family tree, scientists claim in a new study.
Despite their name, horseshoe crabs—bizarre, ancient aquatic critters—don’t look like horseshoes and they’re definitely not crabs.
They’re arachnids. Just like spiders, scorpions, amblypygids, mites and a bunch of other leggy, land-dwelling animals.
That’s what scientists recently concluded after sequencing an enormous amount of genetic material from horseshoe crabs and arachnids, and then using those sequences to draw the most plausible family tree. (Read more about horseshoe crab evolution.)
“This particular part of the tree of life has always been quite challenging to solve,” says study leader Jesús Ballesteros of the University of Wisconsin-Madison, who reported the results recently in the journal Systematic Biology.
“But one of the things that was surprising in this analysis is that regardless of how we treated the data, we consistently found the same results … the horseshoe crabs are always nested inside the arachnids [on the family tree].”
For decades, scientists have known that horseshoe crabs and arachnids, which both belong in the subphylum Chelicerata, are closely related, but it’s been tricky to sort out just how closely related.
Horseshoe crabs, the blue-blooded burrowers in the genus Xiphosura, first appeared in the fossil record about 450 million years ago. Around that same time, arachnids crawled into existence.
As the old version of the story went, arachnids and horseshoe crabs descended from a common ancestor, an aquatic chelicerate of some sort, and then split into sister branches. One lineage quickly moved onto land and diversified into as many as 100,000 species, becoming today’s arachnids. (See pictures of fossil spiders found with “glowing” eyes.)
Another, the horseshoe crabs, stayed put and survived major mass extinction events essentially as-is; today, there are only four surviving species of horseshoe crabs found throughout the world’s oceans, some of which can be more than a foot long.
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Each spring, snowmelt raises Grüner See (“green lake”) by around 30 feet, flooding its surroundings and creating an underwater park. When diving over benches and trees, writes Henauer, “you cross the border of a magic world.”
Austria
Each spring, snowmelt raises Grüner See (“green lake”) by around 30 feet, flooding its surroundings and creating an underwater park. When diving over benches and trees, writes Henauer, “you cross the border of a magic world.”
Photograph by MARC HENAUER, National Geographic Your Shot
But in more recent years, genomic sequencing work by Ballesteros and others has challenged this story, suggesting that instead of existing as a sister group to arachnids, horseshoe crabs are arachnids.
This most recent study reached that conclusion after assessing the family trees that can most easily explain how genetic sequences are distributed among 53 species of arachnids, horseshoe crabs and sea spiders (which, despite the name, are not actual spiders, but another chelicerate lineage), as well as various crustaceans and insects.
The majority of those gene trees—roughly two-thirds—place Xiphosura well within the arachnid lineage, appearing as a sister group to hooded tick spiders.
But Rosa Fernández, who also studies arachnid evolution at the Center for Genomic Regulation in Barcelona, notes that there are lingering uncertainties. For instance, one-third of the sequenced genes keep horseshoe crabs outside arachnids.
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Horseshoe Crabs Mate in Massive Beach "Orgy"
If that result holds, the story of arachnid evolution needs a bit of tweaking—in essence, scientists need to explain how one small branch on the family tree ended up in the sea while everyone else is hanging out on land. Did horseshoe crabs split from terrestrial arachnids and return to sea? Or did arachnids move onto the land more than once? (See 10 beautiful photos that will make you love spiders.)
“Until now, we—or at least myself—had the idea that Xiphosurans evolved from a marine trilobite-like ancestor of all arthropods,” Obst says, referring to the group of organisms that includes insects and crustaceans, in addition to arachnids and others. “Now it looks like horseshoe crab ancestors lived on land and were rather small.”
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Horseshoe crabs are marine and brackish water arthropods of the family Limulidae and the only living members of the order Xiphosura. Despite their name, they are not true crabs or crustaceans: they are chelicerates, most closely related to arachnids such as spiders, ticks, and scorpions.
Feb 18, 2022 · If that were the case, then the horseshoe crabs, which were thought to be mere neighbors of the arachnids, were actually members of the clan. Dr. Sharma and his colleagues’ 2014 study was...
Fact Sheet. Conservation. Physical Description. Though it’s called a crab, horseshoe crabs are more closely related to arachnids, such as spiders and scorpions. Like those animals, horseshoe crabs have hard exoskeletons called carapaces. The hoof shape of their bodies is where they get their name.
Horseshoe crabs also have 10 eyes —a pair of compound eyes on the front shell and more photo receptors along their tail. Horseshoe crabs’ bodies are divided into three segments. The head, or ...
