A horseshoe crab photographed in Money Island, New Jersey. There are four living species of the ancient creatures.
Photograph by Joel Sartore, National Geographic Photo Ark
Bizarre horseshoe crabs are actually spider relatives
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].”
A tale of two sisters
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.
“The problem with Xiphosurans is that there are only four extant species,” says Matthias Obst of Sweden’s University of Gothenburg, who also studies the evolutionary relationships within chelicerates.
So there’s “very little internal diversity, which usually helps to stabilize certain groups in the tree of life.”
Arachnocrabs
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.
Horseshoe crabs, found throughout the world's oceans, are a familiar sight on beaches of the eastern U.S.
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.
By land or by sea?
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.”
He finds that a bit tough
to explain. But no matter how that story shakes out, it’s true that
horseshoe crabs will remain as weird as ever.
A tale of two sisters
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.
“The problem with Xiphosurans is that there are only four extant species,” says Matthias Obst of Sweden’s University of Gothenburg, who also studies the evolutionary relationships within chelicerates.
So there’s “very little internal diversity, which usually helps to stabilize certain groups in the tree of life.”
Arachnocrabs
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.
Horseshoe crabs, found throughout the world's oceans, are a familiar sight on beaches of the eastern U.S.
Horseshoe crabs, found throughout the world's oceans, are a familiar sight on beaches of the eastern U.S.
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.
By land or by sea?
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.”
He finds that a bit tough
to explain. But no matter how that story shakes out, it’s true that
horseshoe crabs will remain as weird as ever.
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