are microscopic parasites that infect fish, amphibians, and now, birds and at least one terrestrial mammal. For over a hundred years they were classified at protozoa. More recent research reveals that classification to be wide of the mark:
on Metafilter continues
Myxozoa have a complex life cycle that involves infecting two different hosts at different times, an aquatic invertebrate (usually a worm) and a vertebrate (usually a fish).
In the early 1990s, it was confirmed that, tiny as they are, myxozoa are multicellular. So they’re not protists after all. Furthermore, they have special harpoon cells that look like the nematocysts, or stinging cells of cnidarians — the group that includes jellyfish, sea anemones, and corals.
Early on, the animal kingdom diverged into those with radial symmetry which includes the cnidarians; and those with bilateral symmetry, which includes all the mammals, birds, reptiles, amphibians, fish, arthropods -- and worms.
Enter Buddenbrockia plumatellae
, a myxozoa that appears
as an active, muscular, parasitic worm inside its host, the bryozoan
, or moss-animal
(which are pretty weird themselves).
Buddenbrockia has rarely been seen since its discovery in 1851, and as a ‘worm’ has always been classified as a bilaterian (at least). But in 2002
, genetic analysis placed it with the myxozoa.
Then in 2007
, further study revealed:
- The Buddenbrockia worm is not bilaterally symmetrical: 'It has no mouth, no gut, no brain and no nerve cord. It doesn't have a left or right side or a top or bottom. We can't even tell which is end is the front!' -- Peter Holland (Oxford University) It has two axes of symmetry and four longitudinal bands of muscle, so it's actually a tetra-radial worm.
- They have cnida (stingers) like all cnidaria.
- Further genetic analysis indicates they’re related to medusozoans — the jellyfish.
This means that the worm-like body shape evolved at least twice from two completely different kinds of animal. “It is one of the most striking examples of convergent evolution you will ever see. . . convergent evolution [not] of individual organs or body parts, such as eyes or wings, but of the whole body shape.”-- Peter Holland
Why we should care
Myxozoa are parasitic animals that live in marine and freshwater aquatic habitats. They are found in virtually all fish. Currently the most economically damaging species Myxobolus cerebralis is the cause of the “whirling disease
” in salmon and trout, with up to 90 percent mortality in farm-reared fish.
Mxyozoa are responsible for the decline of native endemic Australian frogs
. The introduced Cane Toads did not seem to have brought the parasite with them, but do act as a “spillback” carrier.
Mxyozoa were previously believed to infect only invertebrates and cold-blooded vertebrates. However, researchers at Oregon State University have discovered a new species that infects birds
In the Czech Republic, a myxozoa parasite has been confirmed for the first time in mammals
(the common shrew).
“It’s pretty clear it has now moved into warm-blooded animals, which some other research has suggested may include moles, shrews and even humans with compromised immune systems.
So there you have it
“The Cnidaria have undergone a niche expansion that allows the exploitation of warm-blooded terrestrial vertebrates by essentially aquatic animals”. -- Jerri Bartholomew (Oregon State University)
- Long ago, an ancestor of the jellyfish took an evolutionary detour into a parasitic lifestyle.
- A radially-symmetrical cnidarian somehow evolved into a wriggling elongated worm.
- Jellyfish are invading dry land as microscopic parasites in frogs, ducks, shrews and. . .