Fungal and fungal-like parasites and the diseases which they cause have been increasingly associated with amphibians and fish population declines at a global scale. This review summarises our current knowledge of the Mesomycetozoea, a poorly-studied group of fungal-like parasites that includes emerging virulent parasites capable of causing high mortality rates in fish and amphibian populations. This review considers the potential impact of emerging parasites in this class on global aquatic biodiversity. Several aspects of the biology of mesomycetozoean parasites are associated with the potential to drive hosts to extinction, including their high virulence under certain conditions, low host specificity, and a free-living infectious stage. In addition, mesomycetozoeans are currently being introduced into novel environments and hosts via the global freshwater wildlife trade. These parasites have been linked to dramatic population declines in at least one native fish throughout Europe, and there are indications that they are already impacting amphibian populations in America and Europe. The impact of mesomycetozoeans on freshwater biodiversity is likely to increase in response to stressors such as global climate change and habitat modification. Surveillance for mesomycetozoean parasites in the field and in specimens transported in the aquatic wildlife trade needs to be increased, and a tighter control of the aquatic wildlife trade is urgently needed.
