fungus amungus
Amphibians like White's Tree Frogs (right) and Monteverde Golden Toads (below) are at risk.


golden toad

POSTED 2 JULY 1998. Could a newly-discovered member of an obscure group of fungi be causing the worldwide extermination of frogs? White's tree frogThe intriguing discovery could explain why it's such a rotten time to be a frog - or amphibian in general. These creatures -- born in water, living on land, and able to breathe through both lungs and skin -- have been disappearing -- even from pristine, protected habitat.

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
We still don't know for sure, but at least there's a strong candidate "amphibian exterminator." The story begins in 1991, when Don Nichols, who's now a pathologist at the National Zoological Park in Washington, D.C., started investigating deaths in a colony of arroyo toads in California and saw "some weird skin disease caused by some organism I'd never seen before."

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.

thallic symbol? By a process of elimination, Nichols figured they might be fungi, even though they "did not match any classic fungi that are pathogens in animals." Frustrated, he left the project on idle.

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
Longcore is one of world's few experts in chytrids [pronounced ki-trids], fungi that reproduce asexually and make spores that can move around under their own power. fungus slideShe quickly recognized electron micrographs of the pathogen. "As soon as I saw them, I was quite sure they were chytrids," she says, of a species that had never been identified.

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
Nobody knows for sure whether the chytrid is new, or whether it's the sole cause of death, or just the final blow that kills an amphibian that's been weakened by something else.

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; don't mess with methey look fairly well adapted to growing in amphibian skin. Some imbalance in the environment is allowing this to bloom or is making the frogs more susceptible to it."

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?"



-- 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.


The Why Files

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