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Amphibians like White's Tree Frogs (right) and Monteverde Golden Toads (below) are at risk.![]() ![]() |
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POSTED 2 JULY 1998.
Could a newly-discovered member of an obscure group of fungi be causing the worldwide extermination of frogs? ![]() The victims include Costa Rica's colorful golden toad, and Australia's gastric brooding frog (the fertilized eggs of this strange creature mature inside mom's stomach!). Since the decline was first noticed almost two decades ago, many species have gone mysteriously extinct or become extremely rare. There are lots of possible explanations for the die-off, including increased ultraviolet light caused by the reduction in stratospheric ozone, and pollution by chemicals that mimic natural hormones. But until now, despite the investigation of 1,200 scientists in the Declining Amphibian Populations Task Force, there was no good clue why frogs, toads and other amphibians were disappearing around the globe.
Sleuthing
Round organisms in the skin seemed to be causing the disease, but what were they? Nichols showed them to an expert in protozoans, who said they must be fungi or algae. He showed them to an algae expert, who said they must be fungi or protozoans. |
Three thalli (the fungal equivalent of a body) of the still-unnamed frog fungus. A thallus absorbs nutrients with thread-like rhizoids. When mature, the contents of the thallus will divide to form zoospores, which are released through a discharge papilla.
Each zoospore swims through water using a single whip-like flagellum. |
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![]() Then, in 1996, a White's tree frog at the National Zoo was killed by a similar-looking pathogen, and three other species at the Zoo later succumbed as well. Armed with better samples, Nichols resumed the investigation. Searching the Web, his associate, Allan Pessier located a site maintained by Mel Fuller and Joyce Longcore at the University of Maine.
Meet the chytrids
She based the identification on the detailed structure of the zoospores -- the self-propelled spores that distinguish chytrids. "Things that grow in similar environments start looking like each other, and there's not much morphology [structural detail] to go on," she says.
Longcore has now grown the new organism in culture and, with Pessier and Nichols, has written a paper describing and naming it.
But is the little round fungus really the cause of the decline in amphibians? There's no way of knowing, but, according to the June 27, 1998 edition of New Scientist, 24 species of frogs and toads have now died from it, indicating that it has spread worldwide and has great lethality.
Making sense of it all
Nichols thinks it's likely that the apparent epidemic is due to a change in the environment. "In my opinion, these chytrids have probably been out there for thousands or millions of years;
Although the discovery may be a vital first step toward the goal of reversing the appalling decline of amphibians, it's only a first step since nobody yet knows how to treat the disease in captive colonies, let alone in the wetlands and jungles that are home to so many amphibians. "Can we go out and cure it in the environment?" asks Longcore. "No. But there are questions that can be asked, things that people really want to know. Where did it come from, and why is it causing a problem now?" |
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![]() -- David Tenenbaum Frog courtesy of Declining Amphibian Populations Task Force at the Open University, UK. Toad courtesy of US Fish and Wildlife Service. Fungi Courtesy of Joyce Longcore, University of Maine. Poison Dart Frog by Marcos A. Guerra. © 1991 Smithsonian Institution Chytrid research will be published in an upcoming issue of the Proceeding of the National Academy of Sciences. The Why Files also covered frogs -- deformed and flying. |
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