Questioning the chytrid hypothesis

I had an interesting discussion with someone this week about chytrid, to the effect that the story is much more complicated that it's often made out to be. Part of this was stimulated by three scientific papers published recently.

The first suggests that the disease may have been around unrecognized for much longer than most people think:

Batrachochytrium dendrobatidis, a novel pathogen approaching endemism in central California. Dis Aquat Organ. 2009 83: 1-9
The recent emergence of amphibian chytridiomycosis has precipitated competing hypotheses regarding the endemic versus novel nature of the causative agent, Batrachochytrium dendrobatidis (Bd). We conducted a retrospective survey of the California Academy of Sciences' (San Francisco, California, USA) amphibian collection, testing for presence of Bd in 4 amphibian species collected from central California between 1897 and 2005. The earliest detection of Bd was found in 2 Rana catesbeiana in 1961, and the data support the hypothesis that Bd was a novel pathogen introduced into central California prior to 1961 that spread out geographically and taxonomically from at least one central location and is now endemic throughout most of central California. The taxonomic pattern of infection prevalence and the ecological constraints of the 4 species we tested suggest that, although Bd was initially detected in R. catesbeiana, the more efficient and most likely local vector for Bd in central California is actually Pseudacris regilla.


The next paper points at one possible reason why that might be:

Addition of antifungal skin bacteria to salamanders ameliorates the effects of chytridiomycosis. Dis Aquat Organ. 2009 83: 11-6
Chytridiomycosis, caused by the skin fungus Batrachochytrium dendrobatidis (Bd), has caused population declines of many amphibians in remote protected habitats. Progress has been made in understanding the pathogen's life cycle, documenting its devastating effects on individual amphibians and on populations, and understanding how and why disease outbreaks occur. No research has directly addressed the critical question of how to prevent declines and extinctions caused by outbreaks of the disease. We have identified a number of bacterial species of amphibian skin that inhibit Bd in vitro. Here, we demonstrate that a species of anti-Bd skin bacteria can be successfully added to skins of salamanders Plethodon cinereus, and that addition of this bacterium reduced the severity of a disease symptom in experimentally infected individuals. This is the first demonstration that manipulating the natural skin microbiota of an amphibian species can alter the pathogen's negative effects on infected amphibians and appears to be the first demonstration that an epibiotic manipulation of any wildlife species can lessen the effects of an emerging infectious disease. It suggests that probiotic or bio-augmentation manipulations of cutaneous microbiota could have the potential to reduce susceptibility of amphibians to the disease in nature. This is the first approach suggested that could slow or halt epidemic outbreaks and allow successful reintroductions of amphibian species that have become locally or globally extinct in the wild. Our results also suggest a mechanism for the association of climate change and the likelihood of chytridiomycosis outbreaks via the effects of the former on antifungal bacterial communities.

The third paper also supports the idea of the interaction between the pathogen and the environment:

Skin microbes on frogs prevent morbidity and mortality caused by a lethal skin fungus. ISME J. Mar 26 2009
Emerging infectious diseases threaten human and wildlife populations. Altered ecological interactions between mutualistic microbes and hosts can result in disease, but an understanding of interactions between host, microbes and disease-causing organisms may lead to management strategies to affect disease outcomes. Many amphibian species in relatively pristine habitats are experiencing dramatic population declines and extinctions due to the skin disease chytridiomycosis, which is caused by the chytrid fungus Batrachochytrium dendrobatidis. Using a randomized, replicated experiment, we show that adding an antifungal bacterial species, Janthinobacterium lividum, found on several species of amphibians to the skins of the frog Rana muscosa prevented morbidity and mortality caused by the pathogen. The bacterial species produces the anti-chytrid metabolite violacein, which was found in much higher concentrations on frog skins in the treatments where J. lividum was added. Our results show that cutaneous microbes are a part of amphibians' innate immune system, the microbial community structure on frog skins is a determinant of disease outcome and altering microbial interactions on frog skins can prevent a lethal disease outcome. A bioaugmentation strategy may be an effective management tool to control chytridiomycosis in amphibian survival assurance colonies and in nature.


There's a long way to go before there is any hope of practical intervention in the chytrid pandemic, but it is certain that we will need some new thinking if there is any hope of averting complete disaster.

Cane toads in Australia

A tiny frog from Peru

At 3,000 meters elevation in the Andes herpetologists were surprised to discover a frog so small it could sit on a coin with room to spare. Further study showed that this new species, named Noble's pygmy frog, is the smallest frog in the Andean mountain range:

Smallest Andean frog discovered in cloud forests of Peru via kwout

A new species of minute Noblella (Anura: Strabomantidae) from southern Peru: the smallest frog of the Andes. Copeia 2009(1): 148-156

Red eyed treefrogs (Agalychnis callidryas) mating

Chytrid fungus devastates 'chicken' frog

Fungus devastates 'chicken' frog via kwout

Call for help: Breeding African dwarf frogs

Mike asked me: Have you you bred African dwarf frogs? Mine have just laid eggs. I've just looked up breeding on the internet and found an article that says that they are almost impossible to raise from eggs? Did you find this to be true?

I have bred African dwarf frogs (Hymenochirus species) on several occasions and found them fairly easy. I used a 45x30cm planted heated tank with an air-driven sponge filter to provide good water quality and a very gentle current.
When the eggs hatch, start the larvae on infusoria or pond water then onto sifted Daphnia and/or brine shrimp. Once they get onto grindalworms you're away!
Good luck!

American bullfrogs eat everything

Amphibians as classroom pets

When I was at school a million years ago, a dedicated biology teacher kept a range of animals in their class, including several amphibians. Although times have changed in schools, a few dedicated teachers still like to do this - but is it a good idea?

The Herpetologists Education Committee of the Society for the Study of Amphibians and Reptiles (SSAR) has produced a document which summarizes the situation and suggests practical steps that teachers should take:

Considerations and Recommendations for Raising Live Amphibians in Classrooms

Frogs jumped from Andes to Amazon

The amazing diversity of colorful, poisonous frogs that live in the Amazon rainforest today have evolved from ancestors that spread into the region from the Andes Mountains and the surrounding areas, according to a study published in this week’s PLoS Biology. This is the first study to show that the Andes have been a major source of diversity for the Amazon basin, which is now one of the largest reservoirs of amphibian diversity on Earth. The finding runs counter to the idea that Amazonian diversity is the result of evolution only within the tropical forest itself.

“Basically, the Amazon basin is a ‘melting pot’ for South American frogs,” says graduate student Juan Santos, lead author of the study, who is based at The University of Texas, Austin. “Poison frogs there have come from multiple places of origin, notably the Andes Mountains, over many millions of years. We have shown that you cannot understand Amazonian biodiversity by looking only in the basin. Adjacent regions have played a major role.” Santos and collaborators peered about 45 million years into the past using novel biogeographical techniques to study the deep evolutionary history of poison frogs in space and time. Because of the lack of an extensive fossil record for the tropical forest, their work used DNA sequences to investigate the frogs’ evolutionary history.

The scientists created an evolutionary tree, or phylogeny, using 223 of the 353 species of poison frogs known from throughout this region. By analyzing the evolutionary relationships among the poison frogs, they discovered that Amazonian diversity is the result of at least 14 dispersals of ancestral frogs into the region beginning about 23 million years ago. All living Amazonian poison frogs evolved from these ancestors, most of which (11 dispersals) came from the Andes Mountains. The scientists also discovered that frogs have historically immigrated out of the Amazon basin to adjacent areas, and to and from other regions within the Neotropics.

“The Amazon rainforest is not just gradually accumulating diversity over time,” says Prof. David Cannatella, who is also an author on the study. “Ancestral frog species moved into and out of the area, and we can predict that other organisms restricted to these wet tropical forests may show a similar pattern of dispersal, evolution and diversification.”

Amazonian amphibian diversity is primarily derived from late Miocene Andean lineages. 2009 PLoS Biol 7(3): e1000056

Dendrobates pumilio breeding video

Outdoor enclosures for amphibians

I've often felt it is a great shame that we don't have populations of fire salamanders in the UK. You only need to pop across the Channel to northern France and there they are. Although they range across Europe from Scandinavia to Morocco, their distribution is very patchy and represents remnant populations left behind after the last ice age, hence the reason why they do not occur naturally in the UK.

A friend of mine recently sent me this picture of an outdoor enclosure they've built for fire salamanders. If done right, outdoor enclosures work very well for this species. Unlike some species, e.g. smaller newts and highly agile frogs, they are less likely to escape by climbing out, and if they enclosure is well situated (not too much sun and some shelter), they are ideally suited to the British climate. This enclosure provides much more space than a plastic box and this species breeds well in this type of setup.

This enclosure is built from blockwork but I know of a very successful breeder in the south of England who used to keep Salamandra in outdoor enclosures made from outdoor-grade plywood. Such enclosures need a layer of packed gravel for drainage and to prevent the animals digging out, then a substrate of soli, composted bark chips, leafmould, dead leaves etc. Rotting wood, stones and piles of bark provide shelters. There needs to be a water area, which for Salamandra needs to be shallow and easy to climb out of to prevent drowning. Most important if the animals are to be left out all year is a frost-free hibernaculum ("hibernation" shelter) consisting of a deep sheltered area. Polystrene boxes sunk below ground levels or rockeries work well for this but must not be in danger of flooding. It can also be a good idea to add some protection such as bubblewrap or horticultural fleece across the top of the enclosure in very cold weather.

Once the enclosure has matured the animals will find some of their own food but will also need supplementary feeding with worms, slugs, woodlice, etc added to the enclosure. It's relatively easy to train Salamandra by spraying the enclosure lightly at dusk or just after dark, a time when they are naturally active, and then adding food. They will soon figure this out and can then be observed without disturbing them, which overcomes one of the problems with outdoor enclosures, that it is more difficult to monitor the animals than in indoor setups.

The mesh across the top is not to prevent the animals escaping but to protect them from dangers such as rodents, dragonflies, birds (especially magpies) and cats. Sadly, it won't protect them from any two-legged vermin which might find their way into your garden, so an outdoor security light and a shotgun might also be necessary to ensure their safety.