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Tech & Science These sharks have evolved to walk on land—and they did it quickly

18:00  24 january  2020
18:00  24 january  2020 Source:   nationalgeographic.com

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Sharks have roamed the world’s oceans for hundreds of millions of years. In that time, many species have barely changed. But some strange sharks are still evolving —and have even learned to walk . Meet the walking sharks .

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a cat lying on a dirt road: Walking sharks can briefly leave the water as they © Photograph by Conservation International, Mark V. Erdmann

Walking sharks can briefly leave the water as they "walk" between tide pools, making them great predators.

Sharks have roamed the world’s oceans for hundreds of millions of years. In that time, many species have barely changed. But some strange sharks are still evolving—and have even learned to walk. Meet the walking sharks.

These three-foot-long creatures live near Australia, and, as their name implies, move their pectoral fins in the front and pelvic fins in the back to plod along the seafloor—or even atop coral reefs, outside the water, at low tide. Such mobility allows the sharks to wriggle between tide pools and different areas of the reef to prey upon crabs, shrimp, small fish—just about anything they can find.

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Some sharks have learned to walk — but don't panic just yet. Just as Katy Perry and SNL This brings the number of species of walking sharks up to nine, and researchers believe there are more Researchers say their ability to withstand low oxygen environments and walk on their fins gives them

How they walk and where they do it . Walking sharks , also called "epaulette" sharks for their spots that resemble the military decor, " walk " on their muscular fins to forage for small fish along shallow reefs and sea grass. It 's a behavior partially driven by changing sea levels and shifting landscapes

“During low tides, they became the top predator on the reef,” says Christine Dudgeon, a researcher at the University of Queensland in Brisbane, Australia.

Now, a long-term study by an international group of collaborators has turned up four new species of walking sharks since 2008, bringing the total to nine walking shark species. In a paper published this week in the journal Marine and Freshwater Research, the researchers have also shown that these species all evolved in the last nine million years.

That’s highly unusual, says Gavin Naylor, director of the Florida Program for Shark Research at the University of Florida, since most sharks evolve slowly. Sixgill sharks, for example, denizens of the deep sea, “seem stuck back in time,” Naylor says. “We see animals from 180 million years ago with exactly the same teeth.”

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How did these sharks even evolve to waddle on land in the first place? The answer to that was lurking in their DNA. Dudgeon and her team closely studied the sharks ’ mitochondrial DNA or mDNA, which is passed down exclusively through the maternal side and carries the code for mitochondria

The idea of a walking shark might sound like an apocalyptic Sharknado sequel brought to life, but at a "But their ability to withstand low oxygen environments and walk on their fins gives them a The adorable-and-not-at-all-threatening creatures have even been known to clamber across land during

But the walking sharks are likely still evolving in their native tropical waters surrounding Australia, Papua New Guinea, and eastern Indonesia.

“This may be the one place in the world where speciation is still going on for sharks,” Naylor says. Studying these animals will help researchers better understand the animals and why “some change, and others stay the same,” he adds.

Back in time

About 400 million years ago, the ancestors of sharks and all other jawed vertebrates diverged. Since then, only about 1,200 species of sharks and rays have come into existence. The animals are, mostly, very slow to evolve, slow to reproduce, and long-lived, Naylor explains.

a close up of an animal: The leopard epaulette shark (Hemiscyllium michaeli) is a species of walking shark found in coral reefs in the Milne Bay region of eastern Papua New Guinea.© Photograph by Conservation International, Mark V. Erdmann

The leopard epaulette shark (Hemiscyllium michaeli) is a species of walking shark found in coral reefs in the Milne Bay region of eastern Papua New Guinea.

In other situations, this combination of traits would make an animal less adaptable and more vulnerable to extinction, since, in many cases, continued evolution is necessary to survive changing conditions.

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Having evolved the ability slow their heart rate and breathing, they can gradually limit blood flow to certain parts of the brain, so they can stay during the more extreme tides and exploit the “We found the sharks , which use their fins to ‘ walk ’ around shallow reefs, only split off evolutionarily from their

These animals evolved nostrils positioned further and further back along the snout. For example, the pelvis had evolved to be much reduced in size and separate from the backbone. This is because whales evolved from walking land mammals whose backbones did not naturally bend side to side

You could say, for example, that “these things should be extinct,” Naylor says. “How do you exist for so long with a slow rate of evolution?”

But sharks, obviously, are not extinct—and have flourished, outlasting many other aquatic creatures that have come and gone during their reign in the sea. Apparently, they’ve figured out a formula that works, despite an ever-changing ocean. (Related: Six amazing sharks you may not have heard of.)

Naylor says that the rich coral reefs in the walking sharks’ range are dynamic, shifting continually over recent epochs as sea levels rise and fall, currents shift, reefs flourish or wither, and temperatures change. This dynamism is likely what’s driven their speedy evolution and diversity.

“It’s the shark equivalent of the Galápagos, where you can see shark evolution in action.”

In addition, these animals are “homebodies,” Dudgeon explains, laying eggs in the reefs and not traveling far from where they were born. This doesn’t allow a lot of gene flow—and seemingly surpassable obstacles, like small stretches of deep water—have provided enough separation for the animals to uniquely evolve in different places.

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Little-known walkers

Until 2008, scientists believed there were only five species of walking sharks, which are also known as epaulette sharks. Though most of these animals have similar anatomy, they have different coloration and marking patterns. The more detailed genetic analysis in the new study reveals that there are actually nine species, and exactly when they diverged from each other in the recent past.

Dudgeon worked with Gerry Allen, of the Western Australian Museum, and Conservation International’s Mark Erdmann to gather DNA samples from sharks around the region, taking tiny pieces of the fins without hurting the animals. Samples from museum specimens were also used. These were then sequenced and analyzed in Naylor’s lab, and compared to create a phylogenetic tree, a genetic map of the walking shark genus, Hemiscyllium.

Like most sharks, they face threats such as overfishing and harvesting for the aquarium trade. Some of the species, restricted to relatively small areas, are thus vulnerable, Dudgeon says.

Since several of them are only recently well described, and little studied, there is a lack of data; only three of the nine known species are included on the International Union for Conservation of Nature’s Red List of Threatened Species. Luckily, none of them are thought to be threatened or endangered yet, but in some cases, “we really just don’t know how they’re doing,” Dudgeon says.

“These species have been pretty much overlooked,” she adds.

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