Predator Found for Fungus Deadly to Frogs

CORVALLIS, Oregon, August 26, 2011 (ENS) – A freshwater species of zooplankton will eat a fungus that is killing amphibians around the world, zoologists at Oregon State University reported today.

The tiny zooplankton, Daphnia magna, could provide a tool for biological control of this deadly fungus, the scientists said, if field studies confirm its efficacy in a natural setting.

The fungus, Batrachochytrium dendrobatidis, known as chytrid fungus, can, at high levels of infestation, disrupt electrolyte balance and lead to death from cardiac arrest in its amphibian hosts.

Daphnia magna, a species of zooplankton that eats the chytrid fungus (Image courtesy OSU)

The OSU scientists have found that Daphnia inhabits amphibian breeding sites where chytrid transmission occurs and may be able to stem the unprecedented population declines and extinctions.

“About one third of the amphibians in the world are now threatened and many have gone extinct,” said Andrew Blaustein, a professor of zoology, co-author on this study and an international leader in the study of amphibian decline.

“It’s clear there are multiple threats to amphibians, but disease seems to be a dominant cause,” he said.

The chytrid fungus has defied all attempts to control it, even the use of fungicides on individual amphibians.

Until now, it has not been possible to eradicate the fungus from wild amphibian populations, nor is it possible to protect a natural wilderness area before the arrival of the chytrid fungus.

But Julia Buck, an OSU doctoral student in zoology and lead author on the study, and her colleagues saw evidence that zooplankton might make a meal of the troublesome fungus.

“There was evidence that zooplankton would eat some other types of fungi, so we wanted to find out if Daphnia would consume the chytrid fungus,” said Buck. “Our laboratory experiments and DNA analysis confirmed that it would eat the zoospore, the free-swimming stage of the fungus.”

“We feel that biological control offers the best chance to control this fungal disease, and now we have a good candidate for that,” Buck said. “Efforts to eradicate this disease have been unsuccessful, but so far no one has attempted biocontrol of the chytrid fungus. That may be the way to go.”

The chytrid fungus is not always deadly at low levels of infestation, Buck said. It may not be necessary to completely eliminate it, but instead just reduce its density in order to prevent mortality.

“Biological controls can work well in that type of situation,” she said.

The Costa Rican golden toad, Bufo periglenes, was last seen in 1989. (Photo by Charles H. Smith)

Although they have survived for hundreds of millions of years, amphibians may be especially vulnerable to rapid environmental changes and new challenges that are both natural and human-caused because they have a permeable skin, and exposure to both terrestrial and aquatic environments.

The first known record of chytrid infection in frogs wass in the African clawed frog, Xenopus laevis, in 1938. Because Xenopus are sold in pet stores and used in laboratories around the world, it is possible that the chytrid fungus may have been exported from Africa.

When large numbers of frogs began dying in Australia and Central America in the mid-1990s, scientists discovered the chytrid fungus was the cause of these deaths.

Chytrid is now reported in 43 countries and 36 U.S. states. The fungus has infected over 350 amphibian species by penetrating their skin.

The fungus can live at elevations up to 20,000 feet and is believed to have caused the extinctions of all known high elevation populations of seven frog species in Australia’s Wet Tropics between the late 1980s and early 1990s.

Chytrid infections also have caused the extinction of the Costa Rican golden toad, Bufo periglenes, which was last seen in 1989; the Wyoming toad, Bufo baxteri, extinct in the wild since 1991; and the Panamanian golden frog, Atelopus zeteki, extinct in the wild since 2007.

The fungus also wiped out the Australian gastric-brooding frog, of a genus, Rheobatrachus, unique because it contained the only two known frog species that incubated the prejuvenile stages of their offspring in the stomach of the mother.

Australian researcher Lee Berger wrote in a 2007 paper, “The impact of chytridiomycosis on frogs is the most spectacular loss of vertebrate biodiversity due to disease in recorded history.”

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