Most people know jellyfish and their ilk -- the cnidarians, of sea pen, anemone, coral, and man'o'war fame -- live in water and (happily for us) stay pretty well confined to it. But as it turns out, that's not entirely the case.
In 1935, in a fit of profound navet, the government of Queensland introduced the cane toad to Australia to control the insects devouring its sugar cane fields. The result was a biblical-scale plague of the noxious amphibians, who proceeded to grow fat off the land but not off of sugar cane beetles. Apparently, the cane fields did not provide enough daytime shelter for the otherwise indefatigable toads.
Nowadays Australians are still dealing with the fallout, which includes precipitous declines in predatory reptile and marsupial species who might try to eat the toxic toads, and the recent discovery that even dried-out, flattened ex-toads can still pollute waterways with their poisonous carcasses (and can also still be sent conveniently through the Australia Post to a friend or favored enemy with the appropriate postage affixed).
Given their quick rise to numerical dominance in northeastern Australia, one might reasonably be concerned they have contributed to the amphibian declines seen in recent decades in Australia as elsewhere by bringing their parasites with them. Though chytridiomycosis, the tongue-twisting disease brought on by an aquatic fungus from southern Mexico, has wrought havoc there as elsewhere in the world, some populations of native, endangered Green and Golden Bell frogs and Southern bell frogs have disappeared even in chytrid-free zones. So scientists began searching for what else might be causing these deaths, which sometimes involved strange liver abnormalities.
These symptoms were consistent with diseases caused by a group of protist-like creatures called myxozoans. These creatures -- which tend to form shapeless bags of cells called plasmodia in their hosts and use spores to disperse -- are widespread aquatic parasites of fish and amphibians, perhaps most famously as the culprits of whirling disease of salmon, caused by Myxobolus cerebralis. They are generally two-host parasites, which require passage through both host species alternately to complete their life cycle. The alternate host for M. cerebralis is none other than the annelid sludge worm, Tubifex tubifex, which caused a stir a few years ago when odd mystery blobs were found in a South Carolina sewer (I blogged about -- and you can view -- the repulsive video here).
So scientists -- who reported their findings in an April 2011 edition of PLoS ONE -- dutifully tested symptomatic toads for myxozoans, and lo, myxozoans were found -- in their livers, bile ducts, brains and nerve cords. Indeed, they were found in preserved specimens as far back as 1966, 31 years after cane toads were introduced, but no earlier. It was not looking good for the cane toad's already abysmal reputation.
But here was the odd thing: cane toads came to Australia by way of Hawaii. And the populations of cane toads in Hawaii showed no sign of myxozoans. Further, the cane toads of South America, their homeland, did have myxozoans. But the South American myxozoans were genetically distinct from those in Australia. What could explain this?
Enter the "parasite spill-back" theory, in which, to no one's surprise, cane toads still feature as villains. Somehow the cane toads imported from South America to Hawaii and Australia were miraculously myxozoan-free. But the native frogs were not. They had their own myxozoan parasites, but because their populations were isolated or at least not super-abundant (like cane toads), they were likely uncommon.
Cane toads changed all that. They were also susceptible to the pathogen, and each infected toad became a mobile myxozoan incubator. As the toads swarmed the land, they also amplified the parasite, increasing its prevalence in native toads and frogs in the process and spreading it to populations that may have never encountered it before.
But that is not what is most interesting about the cane toad parasite. What is most interesting is that until recently, no one knew what a myxozoan really was.
Early guesses included that it was a protist of some sort, but then it turned out that they have multicellular spores, cell-to-cell junctions found only in invertebrate animals, and special harpoon cells that look for all the world like the cnida, or nematocysts, the stinging cells of cnidarians -- cnidarians like jellyfish, sea pens, anemone, and coral. But that idea seemed pretty far-fetched. The complex, multicellular spores of myxozoans contain the polar capsules, which the spores fire to attach themselves to their hosts, not to capture prey. And the polar capsules, unlike those of known cnidarians, lack any sort of chemical or mechanical triggers or neurons in charge of firing.
Soon sequencing of highly conserved ribosomal DNA -- commonly used to judge the relatedness of different groups of distantly-related organisms -- indicated myxozoans were indeed some sort of animals. But what sort? Just looking at them doesn't help much. Most of them are tiny, aquatic parasites that are sac-shaped or plasmodial -- bags of cytoplasm filled with the nuclei of many amoeba-like cells that fused. Simply put, as a group, they really don't look like any other animal.
Almost all those we know about infect fishes, but some infect amphibians or turtles that spend at least part of their lives out of water, and one -- Soricimyxum fegati -- infects a mammal: the common shrew. But vertebrates are actually the secondary host. The definitive host of most myxozoans is usually an annelid worm (earthworms are annelids, and so are sludge worms and polychaete worms), or, in the case of a sub-group of myxozoans called the malacosporans, a tiny but fascinating aquatic animal called a bryozoan, or moss-animal, which I covered here at Sci Am back in December.
The parasite produces a different spore type in each host. The spore produced in vertebrates generally contains 1-2 amoeba-like infective germ cells and 2-7 polar capsules. The spore shell is composed of segments or "valves" joined by sutures. They may be smooth, flared into wing-like projections, or ridged, as you can see in the photograph at the top of this post.
On the other hand, the spores released by invertebrate hosts tend to be star-shaped. The corresponding spore pair of single species look so different that until recently they were actually thought to belong to two different taxa within the myxozoa.
These spores appear as three or four hooks joined at the base. The genius of this design for hitching a ride on a passing vertebrate should be obvious.
Once a polar capsule from a viable spore fires and draws the spore close to the host, the amoeba-like germ cell(s) crawl out and penetrate the host, where they go on to form their sacs or plasmodia (in a manner very similar, if much more parasitic, to the completely unrelated plasmodial slime molds).
This all seems weird enough, but, straining credulity, it gets weirder. The Myxozoans have managed to produce a species that is not just a shapeless bag of cells or nuclei, but is, in fact, a bona fide, muscle-bound worm. A cnidarian worm.
In a paper with the refreshingly straightforward title "Buddenbrockia is a Cnidarian Worm", researchers reported in Science on a myxozoan that is not simply a passive lump of parasitic cells. In the words of the authors of the Science paper, Buddenbrockia worms are "highly active, with continuous and vigorous sinuous writhing within the body cavity of bryozoan hosts." Once they bail out on their moss-animal hosts (they are malacosporans), they continue to undergo "repeated coiling and straightening." Under the microscope, they look uncannily like a nematode worm -- a ubiquitous bilaterally symmetrical soil denizen that is unquestionably not a cnidarian.
But there were problems with Buddenbrockia's status as a cnidarian. To start with, they seem distinctly bilateral and their worm-like behavior even more so. Some "elongate" cnidarians do exist, but none move as Buddenbrockia does. Though they lack anything resembling a nervous system, gut, or external sense organs, there's also the matter of those muscles. Buddenbrockia has four distinct blocks of muscles that run the length of its body and are comparable to nematode musculature. Though some cnidarians have four longitudinal muscles that extend the length of the organism, they are found in bona fide jellyfish -- not in worms.
Moreover, most cnidaria have only two primordial tissue types: ectoderm and endoderm. In between is mesogloea -- the jelly of jelly fish. In "higher" animals, a third layer occurs between endoderm and ectoderm -- the mesoderm. Without going into gory detail, that is whence most all of your internal squishy bits come, including most of your muscles. So we have a problem here. We have a muscular worm with a mesoderm-like layer and the appearance of bilateral symmetry.
What truly defines cnidarians? They tend to be radial. They have, typically, just the two tissue layers. And they tend to alternate body types: the swimming medusa generates and the sessile polyp, and vice versa. The fusion of their egg and sperm can make swimming larvae called planulae.
But above all else, it is the cnida, or nematocyst, that is the defining cnidarian trait. All cnidarians have them. These distinctive harpoon-cells contain a long filament that, when fired, turns inside out to attach or deliver venom to a host, object of prey, or predator. And Buddenbrockia plumatellae, just like every other myxozoan, has these in spades. They're not only found in its spores, but also in the skin of the worm.
There's more evidence. The worms have a unique cell junction in their body walls and use freshwater bryozoans as hosts, both characteristics of the malacosporeans, the myxozoan subtaxon. The ribosomal DNA sequences of B. plumatella is quite similar to a particular malacosporean, Tetracapsula bryozoides. And if you look carefully, you will note that the muscles of B. plumatellae are actually tetraradial, and that the worm has two axes of symmetry in cross section, rather than one. Finally, the Science team looked at 50 protein-coding genes from Buddenbrockia, and found when they compared sequences with other animals (more similar DNA tends to indicate closer relatedness), Buddenbrockia fit comfortably in the Cnidaria, most likely on the medusozoan lineage -- the same one that has given us the jellyfish. (That ancestral myxozoan may or may not have actually looked like a jellyfish, and they certainly do not look like them now, but I hope you will forgive my liberty in using them to lure you here. : ) )
The authors of the Science paper hypothesize that, given the lack of other worms in Cnidaria, wormhood was not the ancestral state for the group, and Buddenbrockia evolved on its own within the cnidarians by the loss of the opening to the ancestral "bell" of its medusa/polyp, followed by the evolution of its own hydrostatic skeleton. However, the development of its muscles may have used the same, conserved genetic developmental pathways for producing a mesoderm layer that bilaterally symmetrical animals use to produce theirs. In other words, though mesoderm and muscles wasn't present in the first Cnidarians or shared with bilaterally symmetric animals (bilaterians), the developmental genes for the capacity to make a mesodermal muscle layer may have been.
There's one more piece to the myxozoan story. Lingering skepticism that myxozoans were truly cnidarians (the above results had a measure of statistical certainty called a "bootstrap value" that was uncomfortably low for some) led another team to look for genes inextricably linked to cnidarians in myxozoans. These genes, the "minicollagens", are specific to the phylum cnidaria and code for certain proteins of the nematocyst. A group from Aberdeen University in Scotland and the Natural History Museum in London looked for the genes in Tetracapsuloides bryosalmonae, the parasite responsible for salmonid proliferative kidney disease. They found them there, as published in the Proceedings of the Royal Society B in 2010, and once again, the genes indicated the myxozoans are the sister group to the jellyfish-spawning Medusazoa.
It goes to show that when seemingly "advanced" organisms go parasitic (and in the process, evolve like crazy and become somewhat "degenerate" compared to previous incarnations), all hell can break loose taxonomically. Fungal zygomycete parasites seem to have done the same thing. The Microsporidia were misclassified as protists near the base of the eukaryotic tree for years before outed as true fungi, and even have their own form of coiled-up harpoon-like apparatus called a "polar filament".
So to sum up, all this means: 1) close relatives of coral and jellyfish have colonized the land via their terrestrial hosts, and are even now drifting hither and yon via a nearby toad or shrew, and 2) somehow, a radially-symmetric ancestral cnidarian managed to evolve its way to robust, motile wormship. A worm with no front, back, top, bottom, gut, mouth, or anus, mind you, but a muscular wriggling worm nonetheless.
Sometime in deep time, long ago, one of the earliest cnidarians took a parasitic detour that has ended with the only known cnidarian invasion of land, a feat Holland et al. describe as a heretofore unknown "major event" in the early evolution of animals. As Australia has discovered, in some places, the invasion isn't over.
Hartigan A, Fiala I, Dykov I, Jirků M, Okimoto B, Rose K, Phalen DN, & lapeta J (2011). A suspected parasite spill-back of two novel Myxidium spp. (Myxosporea) causing disease in Australian endemic frogs found in the invasive Cane toad. PloS one, 6 (4) PMID: 21541340
Jimenez-Guri, E., Philippe, H., Okamura, B., & Holland, P. (2007). Buddenbrockia Is a Cnidarian Worm Science, 317 (5834), 116-118 DOI: 10.1126/science.1142024
Holland JW, Okamura B, Hartikainen H, & Secombes CJ (2011). A novel minicollagen gene links cnidarians and myxozoans. Proceedings. Biological sciences / The Royal Society, 278 (1705), 546-53 PMID: 20810433