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

2020-2-2 14:21| 发布者: hujian| 查看: 73| 评论: 0

摘要:   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. ...

 


 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.

  “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.”

[video]https://www.youtube.com/embed/2XHH7CCvbTI[/video]

  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.

  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.

  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. 

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

  PHOTOGRAPH BY CONSERVATION INTERNATIONAL, MARK V. ERDMANN

  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.

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>

  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.

  Lead Image: 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



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