Myxozoa

Image: Ivan Fiala

Some jellyfish just don't know their place. They're meant to be out there, free in the open ocean. Not nestled in other people's flesh and skeleton, waiting for the emancipatory event of someone else's death.

We're looking at a group of parasitic creatures so strange it was a century before anyone had a clue what on earth they were. They're called Myxozoans, which means Slime Animal. Aaahh! With a name like that, you can be anything you want, baby!

Image: Ivan Fiala
Spores

Cnidaria is what they've opted for. The phylum full of jellyfish and sea anemones, corals and siphonophores, tentacles and stinging cells. It just so happens that Myxozoans have lost a huge number of the body parts we expect of Cnidaria. Or indeed of animals in general. That's the thing with parasites: they're surrounded in other people's organs so they need almost none of their own.

Indeed, getting rid of unnecessary body parts shows admirable confidence in your parasitic abilities. Where would we be if we all walked around with our umbilical cords attached "just in case"? No. Either trust in your ability to feed yourself, or trust in your ability to get other people to feed you.

Image: Ivan Fiala
More spores

So let's start as all good stories should, with the rotting corpse of a dead fish. It can be freshwater or marine since there are well over 1,000 species of Myxozoa and between them they can parasitise pretty well any fish anywhere.

No rotting corpse can last long before the worms arrive to feast on soft, decaying flesh. Those inglorious bastards! What they don't realise, however, is that amid the tender slivers of meat lay, in great abundance, the spores of Myxozoa.

Image: Ivan Fiala
Spores, spores spores

These spores are tiny, for most species just 1 or 2 hundredths of a millimetre. Each spore is valved, like a tiny oyster, and contains within it a few polar capsules and one or two sporoblasts. The polar capsules fire a little thread into the worm's gut wall, just like the stinging cell of a jellyfish but without the venom. This anchors the spore to the gut wall and provides an opening for the sporoblasts to enter.

Image: Ivan Fiala
Just the one spore

Myxozoans were initially thought to be protists, micoorganisms which can be single-celled but are more complicated than bacteria. These spores are part of why they were placed in a class called Sporozoa, alongside other parasitic protists that produce spores. We hear a lot about the Animal Kingdom, not so much about the Plant Kingdom or the Fungus Kingdom. Myxozoa was thought to belong to the Protist Kingdom. You can't get much more wrong than that!

Once liberated from the spore the sporoblasts set to work. They move into the space between the cells of the worm's gut wall and divide over and over again. Eventually, after a whole lot of cellular division and cellular fusion, they finally form the next stage of their life cycle: the triactynomyxon.

Triactynomyxon

This thing has three hooks and one central style with a few polar capsules at the end. It looks a lot like a tiny grappling hook about a fifth of a millimetre long, ten times bigger than the spore. They're finished with the worm at this point, so they unceremoniously vacate via the back end if their erstwhile host is still alive, and seek out a fish. On the other hand the worm might get eaten by a suitable fish, which is just the kind of welcoming, proactive behaviour we like to see from our accommodation.

Image: John Owens

The hooks keep the triactynomyxon in place while the polar capsule fires a line into the skin or intestinal wall of the fish. It's a bit like Batman's grappling hook gun but in reverse. A giant grappling hook that fires a tiny Batman at the end of a rope. And Batman is a blob of germ cells called a sporoplasm, completely lacking a jawline let alone a strong one.

Image: Ivan Fiala
A young plasmodium

The sporoplasm soon divides into cells who seek out their favoured tissue. Some species like to munch on cartilage, others prefer certain organs. They then feed and grow into a plasmodium, a huge sack of cytoplasm full of nuclei. And when I say "huge" I mean it might actually be visible with the naked eye. Some can reach 1 cm (half an inch) across, which is enough to put you off your salmon.

It's in this ghastly nursery that new spores are created, ready to be released when the fish relinquishes its mortal coil and leaves it to the worms.

Image: Flying Penguin
Salmon full of plasmodia, each full of spores

Myxozoans don't always harm their host, though several are known to cause all sorts of diseases in commercial fish and who even knows what's going on among the non-commercial ones. A particularly nasty example is whirling disease, where fish end up swimming in circles due to damage to their spinal cord.

My favourite Myxozoan disease is a curious one caused by Kudoa thyrsites. This particular Myxozoan is incredibly widespread and attacks the muscle fibre of a huge variety of marine fish all over the world. It doesn't appear to cause any trouble in life but when the fish dies, a process called myoliquefaction sets in. The flesh becomes soft and unpalatable, which is bad news for a fisherman. For the Myxozoa, however, if they can get their act together they may one day turn us all into Slime Animals!