奧勒岡州立大學的動物學家在26日表示，他們發現一種淡水浮游動物，會吃掉一種在全球各地殺死兩生類的真菌。
科學家說，這種微小的浮游動物，學名是Daphnia magna，如果野外研究證實其自然狀態下的效果，就可以用來當做對這種致命真菌的生物控制工具。
這種真菌是一種壺菌，學名為Batrachochytrium dendrobatidis，在嚴重感染的時候，會破壞電解質的平衡，導致其兩生類宿主心臟驟停而死。
奧勒岡州立大學的科學家們發現，巨水蚤在有壺菌的兩生類繁殖地出現，可能可以阻止兩生類族群數量大量下降以及滅絕的危機。
國際兩生類滅絕研究的領導者，本研究的共同作者，動物學教授Andrew Blaustein說，「全球有三分之一的兩生類動物正受到威脅，其中有許多已經滅絕。」
他說，「很明顯有許多問題威脅著兩生類，但壺菌感染似乎是最主要的因素。」
所有企圖控制這種壺菌的嘗試都失敗了，即使是對個別的兩生動物使用殺菌劑也一樣。
直到現在，一直無法將壺菌由野生的兩生類族群中根除，也無法保護自然棲地免於壺菌的入侵。
不過本研究的主要作者奧勒岡州立大學的動物系博士生Julia Buck以及她的同事們發現，有證據顯示浮游動物會將惹麻煩的真菌當成晚餐。
「我們認為生物控制可以提供對付這種真菌的最佳機會，而現在我們有一個好的候選者，」Buck說道，「過去根除這項疾病的努力沒有成功，但沒有人嘗試過生物控制法來對抗壺菌，這或許是可行的一條路。」
低程度的壺菌感染並不總是會致命，Buck 表示。可能沒有必要完全消滅它，而只需要降低其密度以防止造成死亡。
她說，「生物控制在這類的情況下，可以達到很好的效果。」
首次已知的壺菌感染青蛙紀錄是1938年受感染的非洲爪蛙 (Xenopus laevis)。由於爪蛙在寵物店銷售，也在全球各地的實驗室中使用，所以壺菌很可能已經由非洲擴散出去了。
當1990年代中期澳洲與中美洲大批青蛙發生死亡時，科學家發現是壺菌造成的。
壺菌現在在43個國家以及美國的36個州有通報紀錄。這種真菌藉由穿透兩生類的皮膚，感染超過350種兩生類。
這種真菌能夠生活在海拔20000英尺的高度之上，被認為在80年代晚期至90年代早期，造成7種澳洲濕熱帶高海拔的青蛙全部絕種。
壺菌感染也造成了哥斯大黎加金蟾蜍(Bufo periglenes)的絕種，該物種最後一次出現紀錄是在1989年；懷俄明蟾蜍(Bufo baxteri)在1991年野外滅絕；巴拿馬金蛙(Atelopus zeteki)自2007年野外絕種。
這種真菌也使得Rheobatrachus屬的澳洲胃育蛙滅絕，這是種獨特的青蛙，因為它是已知唯一兩種會在媽媽胃裡面孵育子代青蛙的一種。
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.
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.
"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 first known record of chytrid infection in frogs was 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.
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