The chytrid skin fungus is killing frogs around the world, and researchers worry that it is already driving some species to extinction–but until now, no one understood how the fungus killed. Now, new research shows that the fungus disrupts the flow of nutrients through the frogs’ skin, ultimately leading to cardiac arrest.
In the study, which will be published tomorrow in Science, the scientists found that the fungus interferes with the frogs’ ability to absorb electrolytes, the electrically conducive molecules that are vital for muscle and nerve function. Diseased green tree frogs had dramatically lower levels of potassium and sodium in their blood and urine. Says study coautor Wyatt Voyles: “It’s a failure of the electrical system, leading to mechanical failure. If you don’t have a normal electrical system pacing the heart, it won’t pump blood” [Wired.com].
Further experiments confirmed that the electrolyte imbalance led to a heart shutdown. The scientists took electrocardiogram recordings of the frogs’ hearts in the hours before death; and found changes to the rhythm culminating in arrest. Drugs that restore electrolyte balance brought the animals a few hours or days of better health, some showing enough vigour to climb out of their bowls of water; but all died in the end [BBC News]. Researchers’ next task will be to determine exactly how the fungus interferes with the electrolyte absorption: it could be a result of cell damage in the skin, or a toxin produced by the fungus.
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Image: Jamie Voyles, Alex Hyatt, and Frank Fillipi
The question of how salamanders regenerate their legs when amputated is an ancient one that dates back to the days of Aristotle. Now scientists have come one step closer to solving the mystery. Contrary to what researchers previously believed, when a salamander’s legs are removed the cells near the amputation site revert to adult stem cells, but do not become pluripotent, or capable of developing into any body part. That explains why a salamander who loses a tail doesn’t regrow a leg in its place.
In the study, published in Nature, scientists explain that when a salamander’s limb is amputated, the muscle, bone, and skin cells at the amputation site change into a clump of adult stem cells called a blastema. Before this experiment, researchers had hypothesized that these undifferentiated blastema cells — which all look identical — are pluripotent and thus able to form many different cells types. But it was not clear how the original cells from adult tissue were reprogrammed, or how the blastema cells went on to form the correct tissue types [Nature News].
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The union between the native California tiger salamander and the non-native barred tiger salamander, which was brought in huge numbers from Texas beginning 60 years ago by California bait dealers [The New York Times], has produced an alarming hybrid offspring. A new study of the hybrid’s behavior in artificial ponds serves as a reminder that invasive species can alter ecosystems in unexpected ways: in this case, by getting too cozy with the natives of central California.
The new hybrid “superpredator” grows larger than either of its parent species, and its bigger mouth enables it to suck up a wide variety of amphibian prey, said lead study author Maureen Ryan…. Mostly on the menu are smaller pond species, such as the Pacific chorus frog and the California newt—both of which were “dramatically reduced” in population by the hybrid in the experiments [National Geographic News].
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Over the past 30 years, the salamanders that used to thrive in the tropical cloud forests of Mexico and Guatemala have been disappearing, and until now no one had even noticed that the stealthy amphibians may be spiraling downward towards extinction. Back in the 1970s, biologist David Wake studied salamanders in the San Marcos region of western Guatemala, and he recently returned to the region to survey the current salamander population and compare it to his previous data. What he found shocked him, Wake says. “Cold facts written on a piece of paper don’t convey the impact on my psyche when I went there,” he said. Species that could be seen 10 to 15 times an hour in the 1970s were “completely gone” [National Geographic News]. Studies in Mexico that compared current salamander populations to historical data produced similar results.
Since the 1980s biologists have raised alarms about worldwide declines in amphibians attributed to habitat destruction, disease and climate change, among other menaces [Science News]. Scientists studying dwindling frog populations have focused largely on chytrid, a fast-killing fungus, as the possible culprit, but Wake says that only a few of the salamanders he found on his recent trips showed signs of chytrid fungus. Instead, he blames global warming. Wake says that warming temperatures on the steep, forested slopes of Guatemala’s volcanoes are forcing the salamanders up to higher to less hospitable elevations.
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Pity the poor frogs: they’re one of the most endangered group of vertebrates on the planet, and new research shows that two of the factors in their plight are common, everyday farm chemicals. The study shows that atrazine, a weedkiller that’s widely used in agricultural areas, not only boosts the levels of parasitic flatworms in frog ponds, it also decreases tadpoles’ ability to fight off infections. If that wasn’t bad enough, previous research has found that runoff from phosphate fertilizers also boosts parasite levels. Taken together, researchers say, the weedkiller and the fertilizers are hitting frogs with a double whammy.
Amphibian populations around the world have been declining in recent decades, with many species on the brink of extinction. Infection with any of several species of tiny flatworms, known as trematodes, can trigger debilitating limb deformities in frogs. Severe infections can kill the amphibians. The question was why high rates of those deformities — and, presumably, trematode infections — began showing up across the nation in the mid-1990s [Science News]. The new findings suggest that the growing prevalence of the weedkiller atrazine in corn-growing regions since that time may be partly to blame for the woeful state of American amphibians.
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Two different studies separated by more than 1,700 miles hammer home the same point: evidence of global warming is everywhere. In Yellowstone National Park, researchers found that amphibian populations have declined dramatically over the past 15 years as some of their pond habitats have dried up and disappeared. Meanwhile, in Massachusetts’ Walden Pond, botanists discovered that more than a quarter of the plant species observed by Henry David Thoreau have disappeared since the author went to the woods to “live deliberately” in the 1850s.
The two studies, which both appear in the Proceedings of the National Academy of Sciences [subscription required], show that changes to the planet’s flora and fauna are already well underway. The Yellowstone study compared data from an amphibian survey done in 1992 and 1993 to data from a new survey conducted over the last three summers; researchers looked at the park’s “kettle” ponds, which are re-filled in spring by groundwater and snow melt running down from the hills [BBC News]. The researchers found that the number of permanently dry ponds had quadrupled, and even in the ponds that remained, amphibian populations had plummeted.
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A new study of a the fossilized remains of the Tiktaalik, the “walking fish” that illuminates how swimming fish evolved into land-dwelling amphibians, shows that there was more to the transition than the switch from fins to limbs. The study shows that the head and braincase were changing, a mobile neck was emerging and a bone associated with underwater feeding and gill respiration was diminishing in size, a beginning of the bone’s adaptation for an eventual role in hearing for land animals [The New York Times].
The creature, dubbed Tiktaalik roseae — or, to be less formal, Fishapod — lived 375 million years ago 600 miles north of the Arctic Circle in a subtropical floodplain that eventually became Ellesmere Island, where it was discovered in 2004 [Wired News]. The fishapod has already earned its reputation as a “missing link” in evolutionary history due to its sturdy, jointed fins and its dual breathing system, with both gills and lungs. But the new study suggests that changes to the animal’s head and the development of the first neck also played a critical role in its evolution.
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Cane toads, the green and warty invasive animals that are spreading ruin throughout ecosystems in northern Australia, may not continue their relentless march southward towards the sea. According to a new study, lower temperatures in the south may stop the toads in their tracks.
Researchers coaxed the toads into hopping through a 2-meter course in a laboratory to make a so-called “cane toad Olympics,” and measured the cold-blooded creatures’ hops at different temperatures. They found that in temperatures above 80 degrees Fahrenheit, the toads could hop at a speed of over one mile per hour. But below 60 degrees Fahrenheit, they slowed to a glacial .2 miles per hour. “This means that there’s no way the toads could invade southern parts like Melbourne, Adelaide … because they wouldn’t be able to move,” said researcher Dr Michael Kearney…. The winters of southern Australia would be too cold for the cane toads to forage or spawn, he said [The Age].
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