September 2, 2011 | 1
It’s no secret that frogs and toads in the Americas, Australia, and Europe have suffered extinctions and massive declines at the hands of a chytrid fungus on a global genocidal rampage. What is much more of a mystery is exactly what a chytrid fungus is. Or, for that matter, what to do about it. I am here to help you on the first count. And a paper this month in Biodiversity and Conservation suggested a new tack on the second: a naturally-occurring chytrid-eater called a water flea.
Last week I wrote about a story I did for High Country News looking at a few surviving boreal toad populations that have remained chytrid-free. I didn’t have room in that article to describe what they are, which I will do now. Because chytrids are very, very interesting creatures, and the majority of them are not on a global genocidal rampage.
Chytrids (KIT-rids) are fungi. Their chitin cell walls, absorptive feeding modus operandi (secrete enzymes to environment, absorb resutling goo), and DNA tell us this. And they are remarkable fungi, for unlike any other fungi, they can swim. The defining trait of chytrid-hood is being a fungus, and possessing, for at least part of your life, a flagellum, or tail. Chytrids are also almost always small, and lack cell walls, or septa. A typical chytrid may be nothing more than a spherical sporangium containing spores, sometimes sporting a few small fibers at its base called rhizoids. Sometimes the rhizoids have nuceli and are called rhizomycelia, and sometimes they support more than one sporangium. These rhizoids (root-like objects), can be inserted into whatever bit of detritus they are digesting to increase the surface area and thus food haul. Mature sporangia look like this, or this or even this.
But, there comes a time in every chytrid’s life when it’s time to shove off for wetter, more nutritious pastures. When that happens, the sporangium (the round or flask-shaped thing), releases a little swarm of swimming cells called zoospores through a hole, a neck, or by popping a lid called an operculum. They look like sperm, and sometimes they can act that way too. During sex, two zoospores simply fuse, may continue swimming confusedly for a while, and then settle down to form a thick-walled resting spore in which meiosis (reductive cell division) occurs to restore the proper number of chromosomes to the next generation of zoospores.
Otherwise, a zoospore simply swims until it bumps into a promising bit of new food. It then settles down to transform itself into a new sporangium, possibly made fabulous by a shrubby bunch of rhizoids. Chytrids may resemble the first, simplest fungi to evolved from protists.
Chytrids aren’t rare organisms either. It used to be thought that, by virtue of their tails, they were primarily aquatic. But now we know they surround us in forest, farmland, and even desert. They simply wait for wet conditions that favor them. They specialize in eating bits of plant and animal detritus and are thus “saprotrophs”. A few parasitize algae or small aquatic animals. A very few are plant parasites of corn, potatoes, cabbage, or alfalfa. A single chytrid may make its life meal out of a single pine pollen grain. In fact, pine pollen (with its characteristic Mickey Mouse ear “wings”) is actually used as bait to lure chytrids into labs. You can also, apparently, use insect wings or cellophane (which is simply a thin film of reconstituted cellulose) to lure in chytrids, who as a group seem not to be picky about their detritus.
So it came as a big surprise when, in 1997, Joyce Longcore isolated a chytrid from a captive blue poison dart frog that had died at the National Zoological Park in Washington. Scientists concluded in 1998 that this organism was chiefly responsible for the sudden die-offs they’d observed in Australia and Central America, where, for instance, the golden toad and harlequin frog had suddenly gone extinct in otherwise relatively pristine nature preserve in the Monteverde Cloud Forest. This was fairly shocking, as, though chytrids parasitize plants and algae, they had never been known to parasitize a vertebrate before.
But these chytrids certainly were. Specifically, they were feeding on a fibrous protein called keratin in frog skin, which also happens to be a component of things like your skin, hair, and nails, and other vertebrate bits like scales, claws, feathers, quills, baleen, shells, beaks, horn and hooves. As you may gather, vertebrates shed a lot of keratin, so perhaps it is not surprising that at least one chytrid made the leap from keratin decayer to amphibian attacker — especially in a habitat where chytrids excel: water.
Here is what the frog chytrid embedded in its host looks like:
Now came some critical questions: how was this chytrid killing frogs and toads? And where had it come from? There are still two hypotheses to answer the first question: the chytrids clog up the frogs’ skin to the point where they have trouble breathing (frogs do a lot of breathing through their skin), or, as seems to be becoming the more accepted answer, the chytrid messes up frogs’ ability to exchange electrolytes and ions through their skin, leading to, essentially, heart attack after about two weeks.
The chytrid’s origins are still debated too. One popular hypothesis posits the fungus was native to Africa and spread by humans; South African scientists looking at preserved African clawed toads from 1879 to 1999 found it present in a specimen from 1938 and believe it was a stable endemic infection. Then these frogs were widely exported for laboratory, aquarium, and, oddly, pregnancy test use (in the days before the confusing blue lines, you had to pee on (or, if you were boring, pour your urine on) a toad, and if it subsequently ovulated, you were pregnant). As the exported frogs were released by well-meaning people or escaped on their own into the America, Australia, and elsewhere, spreading an otherwise relatively innocuous chytrid to naive populations, up to 30% of worldwide amphibian species have felt the painful results. It has been described as “the most spectacular loss of vertebrate biodiversity due to disease in recorded history”*
So is it possible we could right what we have wronged? People have tried control strategies before, like painting frogs with protective bacteria, peptides, fungicides, or eradicating the fungus from water (unfortunately, B. dendrobatidis has an annoying ability to both survive on keratinaceous detritus or other animals and even when eliminated, to return to ponds via vectors). None have worked well in practice.
Because some amphibian species seem able to tolerate chytrids, and some can tolerate chytrids as long as they don’t exceed certain zoospore concentration thresholds, someone suggested it might be possible to encourage the chytrids’ natural enemies, if such could be found. Now, one has been found, and it is an incredibly voracious — not to mention cute — chytrid killer. It’s a minute crustacean called a water flea.
These tiny animals are common in freshwater but rare in the ocean. They’re related to brine shrimp (aka sea monkeys!) and fairy shrimp, and fit into the crustaceans like this (look for Branchiopoda). They have a carapace that covers an unsegmented thorax and abdomen. Like a cyclops, they have a single, compound, centrally positioned eye. They bear two pairs of antennae on their head: one sensory, and one for paddling. Most of the time, they reproduce parthenogenetically (without males, by cloning themselves), and they are really good at it.. Daphnia magna, the species studied for chytrid control, can rear their first brood within a week of hatching and a new brood every 2-3 days for up to two months.
Here’s a video of a different species of the little animal, with impressive slow-mo pooping action:
Normally, water fleas eat bacteria and the ever-popular “organic detritus”. But chytrid zoospores are just about the size of bacteria, and exist in the same habitats that water fleas do. Hmmm.
Julia Buck, Lisa Truong, and Andrew Blaustein at Oregon State University decided to feed fluorescently-labeled chytrid zoospores to captive Daphnia magna. And lo, the water fleas guts lit up like christmas trees, while controls’ did not (paper is open access).
If it were possible to “augment” (dump more of them into frog ponds?) populations of already-native water fleas, that could help tip the chytrid balance in favor of amphbians’ ability to fight them off. It does sound like a great idea. But as the authors note,
Furthermore, it may be possible to augment the numbers of Batrachochytrium-eating zooplankton in natural systems for effective biological control, although previous species introductions for biological control have met with varied success (Cory and Myers 2000).
Um, understatement alert? Anyone remember the cane toads post? It’s a great idea worthy of investigation, but as with all attempts to alter the environment for the better, we must remember the iron-clad Law of Unintended Consequences. It’s likely what landed amphibians in this mess in the first place.
*It’s a repeat of the story we see played out again and again on Earth: our carelessness about moving things around (shipping palates, dirt, firewood, ship ballast, non-native trees, etc.) around has led to catastrophes for other species: American Elm (Dutch Elm Disease), American Chestnut (chestnut blight), Sugar maples (Asian Longhorn beetles), five-needle pines (White Pine blister rust), amphibians (chytridiomycosis), and most recently, bats (White Nose Syndrome). Those are just examples from North America; I know Asia and Europe have their own troubles with pests and pathogens carelessly imported from the Americas as a byproduct of international trade. I just saw this press release about the accidental export of cypress canker from California to the rest of the world this morning.
Buck, J., Truong, L., & Blaustein, A. (2011). Predation by zooplankton on Batrachochytrium dendrobatidis: biological control of the deadly amphibian chytrid fungus? Biodiversity and Conservation DOI: 10.1007/s10531-011-0147-4
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