They may look like a trinket you might find in a zoo giftshop, but glass frogs are a diverse group of amphibians in the family Centrolenidae which contains 2 sub-families, 12 genera and 160 species, spread throughout the tropics. From studies of mitochondrial and nuclear genes researchers determined that glass frogs originated in South America and dispersed multiple times into central America with the species evolving so isolated from each other that they became unable to interbreed, a phenomenon called allopatric speciation. They occur throughout tropical lowland forests to mid elevation mountain forests. Adults rarely grow larger than 30mm.
Glass frogs are aptly named as they have translucent skin on their bellies and legs, showcasing the intricate workings of their internal organs. Observers can see their hearts beating and food moving through their gut! How this evolved was a mystery for a long time; clear skin as camouflage made sense, but why did the frogs not develop this on the top of their bodies too? Typically, the skin on their backs is a vivid mottled green. By creating opaque and translucent gelatine frogs and placing them in the rainforest in Ecuador, researchers identified that with their translucent legs and bright green backs, the frogs blended into the green foliage, but their legs softened their outline, making it incredibly difficult for aerial predators to detect them. This is a phenomenon called edge diffusion. Many predators have been determined to identify prey from their outline, so this is a particularly clever camouflage method the glass frogs have evolved.
Fungus and frogs: a tragic tale
Like so many tropical species, frogs are threatened by habitat loss and fragmentation. In addition, thanks to their very porous skin, frogs are particularly susceptible to environmental disruption meaning they are often used as an indicator species. This is defined as a species whose presence or absence or changes in abundance can indicate some fluctuation in an ecosystem and are often used to diagnose the health of that habitat.
One big issue unique to amphibians is chytridomycosis, an infectious fungal disease caused by Batrachochytrium dendrobatidis that is behind dramatic population declines of tropical species. No one is exactly sure where it arose, but studies of museum specimens found evidence of the disease all the way back in 1938 in an African clawed frog, although the prevalence was very low, and no chytrid positive specimen was found outside of Africa until 23 years later. This suggests it was a stable endemic disease that posed very little threat to amphibians in general, but became an issue when the international trade in the African clawed frog began in the 1930’s. African clawed frogs generally resistant Today the fungus has spread across the world into the tropical regions of South America, eastern Australia, the Western United States, Africa and Europe.
The pathogens infect the skin of the amphibians, leading to ulceration. Amphibians need to osmoregulate and respire through their water permeable skin, and when this is disrupted by the infection, they can’t perform essential homeostatic functions and disease in the wild. eventually they die. The disease doesn’t affect tadpoles, as they only have keratinised tissues in their mouthparts, so they simply carry the fungus with them into adulthood, where it goes on to infect their entire body. The infectious spore of the fungus can swim in the water, where it can embed itself in the skin of any unfortunate amphibian that happens to be around. The spores can also persist in damp soil and can be transferred by other animals, like birds or livestock, or by people, on their equipment or boots.
Chytridomycosis can be treated successfully in captivity by bathing the amphibians in an antifungal solution, but impossible to treat in the wild. The only way to tackle the issue currently is to reduce the spread of the disease by disinfecting the footwear and equipment of anyone working in a known chytrid positive area.
Save the frogs is an international organisation of scientists, educators and policy makers working to protect amphibians across the world. They have created, restored and protected key habitat, pushed through legislation and improved awareness through education. You can listen to Episode 264 for an interview with Dr Kerry Kriger, the founder of the organisation, for more information about what his team are doing to save the frogs!
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Guayasamin, J.M. Castroviejo-Fisher, S. Ayarzagüena, J. Trueb, L. and Vilá, C. (2008). ‘Phylogenetic relationships of glassfrogs (Centrolinidae) based on mitochondrial and nuclear genes.’ Molecular Phylogenetics and Evolution.
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