A new paper is calling for more attention to be paid to poorly-known micro-organisms capable of killing off amphibians and fishes.

Dead frog

Because frogs decompose so quickly, it's rather rare to see a dead frog. When there's more than one, it's generally a sign that something's really wrong- such as a disease outbreak or pollution.

Image: Jodi Rowley
© Australian Museum

Although I’m an amphibian biologist, I spend a lot of time thinking about amphibian diseases. In fact, my PhD research investigated the fungal disease chytridiomycosis- or, more specifically, how amphibian behaviour might affect the outcome of disease.

Chytridiomycosis, caused by the amphibian chytrid fungus, affects the skin of amphibians, is capable of killing rapidly, and is responsible for population declines and extinctions in amphibians globally. Not surprisingly, it is the most well-studied disease in amphibians. But it’s not the only fungal or ‘fungal-like’ disease out there.

One particular group of fungal-like parasites called mesomycetozoeans (what is it with fungi and difficult to pronounce names!?) are very poorly-studied parasites capable of causing high mortality rates in fish and amphibian populations. Mesomycetozoeans are microscopic, but can cause visible lesions on skin, muscle or internal organs in amphibians and freshwater fishes, and these infections can kill.

Although we don’t know much about them, several aspects of the biology of mesomycetozoeans are particularly worrying. They are highly virulent (deadly!) under certain conditions, aren’t at all fussy about which species they infect (even jumping from fish to frog, for example), and have a free-living infectious stage. Together, these characteristics make them possible candidates for causing the extinction of their host species, just like the better-known amphbian chytrid fungus.

Climbing Galaxias, Galaxias brevipinnis
A Climbing Galaxias caught (and released) in a tributary of Falkner Creek, Yambulla State Forest, west of Eden, New South Wales 24 Jan 2008. Image: Robert McCormack
© Robert McCormack

Like many parasites, mesomycetozoeans are also currently being introduced into new environments and new species via the global wildlife trade. Internationally-travelling fish are generally not rigorously checked for parasites- particularly such small and poorly-known ones- and are often released into the wild. That cool new species of freshwater fish imported into the country for your aquarium may be carrying some killer hitch-hikers.

Mesomycetozoeans have already been linked to dramatic population declines in at least one European fish species, the sunbleak (Leucaspius delineatus), and there are indications that they are already impacting amphibian populations in America and Europe. Global climate change and habitat modification is likely to make things even worse for freshwater fish and amphibians.

There is an urgent need to survey for mesomycetozoean parasites in the field and in specimens transported in the aquatic wildlife trade. Tighter control of the aquatic wildlife trade is also needed to prevent or reduce the global spread of wildlife disease.

Given the state of amphibians globally (an incredible one-third of all species threatened with extinction), amphibians don’t need more diseases to deal with!

More information:
Rowley, J.J.L., Gleason, F.H., Andreou, D., Marshall, W., Lilje, O. & Goslan, R. (2013). Impacts of mesomycetozoean parasites on amphibian and freshwater fish populations. Fungal Biology Reviews 27: 100-111. http://dx.doi.org/10.1016/j.fbr.2013.09.002

This research was a collaboration between the Australian Museum Research Institute, University of Sydney, James Cook University, Bournemouth University, the Centre for Aquatic Health Sciences (Campbell River) and the Museum National d’Histoire Naturelle.