A-Z of Weird Animals: Chytrid Fungus and the Fate of Amphibians

I had always thought the rainforest was supposed to be hot, but as I walked up the jungle stream I wondered if anyone else had ever caught tropical-hypothermia. I couldn’t feel my feet anymore, the thigh high rubber boots enlisted to keep my toes warm and dry had failed miserably at their job the first time I had stepped in a deep hole in the stream, plunging waist-deep into chilly water. During the day it would’ve been welcome, and on a nicer night it could’ve been quite pleasant. But it was past two in the morning and had been raining solidly for hours: great heavy sheets of rain sluicing from low-hanging clouds. The downpour limited the ability of my headlamp to penetrate the blackness, and I could only barely make out the bobbing lights of my colleagues ahead of me in the river, floating up and down in the dark like disembodied spirits as they negotiated holes and riffles and boulders. I was high in the cloud-forest of Honduras, walking up the middle of a river in pitch-blackness, being deafened by a tropical storm, but for a very good reason.

I was looking for frogs.

Cloud forest. It’s beautiful, but also cold. Credit: University of Colorado.

Not your typical backyard pond frogs, but tree frogs. Small arboreal frogs that mostly live 30m up in the air, making their homes in pools of water collecting in tree holes or orchid flowers. At night they venture down to the forest floor, and congregate on riverbanks in search of – what else – a mate. The males find a perch and sing, hoping to woo a female. The female, once fertilized (without even being bought dinner), climbs back up into the tree, finds a pool of water, and lays her eggs. Unfortunately for the would-be lotharios, scientists were out in force to ruin the mood.

Hello little tree frog. Sorry for interrupting your seduction attempts.

Wading through the river, I reached up and overturned a fern frond that dangled over the bank. I scanned the bottom of it: no frog. Just a hairy caterpillar, peeved at being exposed to the rain. I let the leaf down gently, and trudged on. Then ahead, I heard a commotion, a distant shout over the thunder and the rain. The ghostly lights bobbed more vigorously. I speed-waded up to the congregation of lights. In the middle of the group, the leader of the research team was holding a frog. It was dead. Its normally green skin had a reddish tinge, and when rubbed, the skin sloughed off in swatches. Its legs were paralyzed, held awkwardly away from its body, as if frozen mid-jump. Its murderer was new to these parts: Batrachochytrium dendrobatidis, or Bd, the chytrid fungus.

A frog, dead from chytrid fungus. Credit: frogblog

The chytrid fungus (pronounced with a “k”, like “kiss”, but I wouldn’t if I were you) is a contemporary scourge of amphibians around the world. In some populations, it causes almost no damage, but in other areas of the world it kills 100% of amphibians that it infects. It’s widely blamed for the massive, on-going decline in amphibian populations around the world – potentially threatening the very existence of amphibians – and has no known control mechanism: once it is found in a population, the only real option scientists have is to pray (not a skill native to many scientists).

But what is chytrid fungus? And where did it come from?

Chytrid fungus (Batrachochytrium dendrobatidis) is not an animal (but I make the rules of this list), but it is weird and interesting. Like all fungi, it belongs to the Kingdom Fungi (personally, I think the Fungus Kingdom would be a more amusing taxonomical label) – a group of eukaryotic organisms which likely has around five million members, including yeasts, moulds, and mushrooms. Originally assumed to be plants, fungi were eventually recognized as being entirely distinct from both plants and animals, and actually more related to animals than to plants.

A fungus from the West Coast, colloquially known as ‘Fairy Puke’.

Chytrids are a specific division of fungus (“Fungus Kingdom, Chytrid Division,” he said, flashing his badge), consisting of ~1000 species of ancient, unicellular, aquatic fungi. Most of them are harmless, just reproducing and getting on with life. A few are parasitic, mostly attacking small aquatic arthropods. But one, the Batrachochytrium dendrobatidis, has evolved a very unhealthy attachment to vertebrates, much to the detriment of its hosts. The infection begins with a zoospore, a unicellular oval-shaped body propelled by a single flagella. The zoospore is asexual, and its only job is to find a host. It travels through a body of water – a river, a pond, the stagnant pool in a tree hole – detecting microscopic chemical scents emitted by its prey, the way a shark follows a blood trail. But, unlike a shark, the zoospore isn’t in any hurry. It can live for months, or years, without finding a host as long as the environment stays moist.

A chytrid zoospore. Yes I know what it looks like, no you’re not that clever. Credit: UBC

Once the zoospore finds a host, it infects the keratin cells on surface of an amphibian’s skin. Keratin is a structural protein, and a major component of skin, hair, nails, and feathers. It’s a tough and resilient protein, used by the body as a first line of defence against the outside environment. When zoospores infect the keratin cells of amphibians, the body senses that the line of defence has been breached, and mounts an immune response. This immune response induces “hyperkeratosis” – the keratin cells become tougher and thicker, reinforcing themselves against the attack. This prevents anything from getting into the body, but it also prevents anything getting out. Skin is fundamentally an area of exchange, it’s a thin barrier that both absorbs and exudes moisture and ions (when you sweat and your skin tastes salty your body is exuding salt through the skin to maintain its internal ion balance). In most species, a chytrid infection would cause dry skin. But amphibians operate differently.

A chytrid sporangium growing on a species of algae. The sporangium is full of zoospores. Credit: stccmop.org

Amphibian skin is very thin, only one or two cell layers thick, making it a critical site of exchange. Frogs and other amphibians essentially ‘breath’ through their skin: most of their gas exchange, the intake of oxygen and removal of carbon dioxide, is done through the skin. They drink through their skin, and absorb electrolytes like salt and potassium. When a chytrid infection causes hyperkeratosis, the frogs skin becomes too thick for gas exchange: it’s like when Wall Street shuts down, all the other systems dependant on that exchange get shot to hell.  Unlike Wall Street, I’m sure we’ll one day find a healthy use for chytrid fungus.

Electrolytes are needed for muscle reflex, and without them the frogs become paralyzed, unable to turn themselves over in the water. Without gas exchange they slow down, becoming lethargic and weak. Their skin toughens and sloughs off. Eventually, they die.

While this occurs, the chytrid zoospores are growing, developing into zoosporangia – factories for the creation of new zoospores. Using the amphibian’s body for fuel, the zoosporangia grow a new batch of zoospores, which are released into the environment as the frog dies. Once present in an area, the chytrid fungus is almost impossible to remove: the amphibian population, if vulnerable, will die or evolve resistance. Darwin’s survival of the fittest in brutal reality.

But not all species of amphibian are vulnerable – some can carry the zoosporangia on their skin without adverse effects. These amphibian “Typhoid Mary’s” may have been responsible for the spread of chytrid fungus around the world. But they might also hold the key to a cure.

That, and more, on Thursday.

That was depressing, so here’s a cute frog.

Neil Griffin

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