Microscopic parasitic jellyfish astonished the biological community when they were first identified, as evolutionary degeneration hadn’t been thought to reach such extremes. By now it has become clear that they swarm throughout water everywhere and have even been found on dry land. Now yet another previously unknown species of microscopic parasitic jellyfish called Streblospio benedicti has been identified in marine worms living in Charleston Harbor, South Carolina, scientists from the Oregon State University, and College of Charleston reported in the Journal of Parasitology this month.
This is quite the breakthrough. Myxozoa researchers had focused on fish because people eat fish, explains Stephen Atkinson of Oregon State University. Asian universities in particular have been pouring money into this field because when you eat certain fish raw, you eat live parasitic jellyfish, which can make you sick.
“Nobody looked at worms,” says Atkinson. “That is what I am doing as a career goal – worms are the story.”
It turns out that the horrifying little endoparasites, called myxozoans, are anything but rare. In fact a paper from 2018 estimates that myxozoa represent at least 20 percent of the phylum Cnidaria, which includes jellyfish, sea anemones and coral.
This discovery transformed the understanding of cnidaria. Once thought to be basically free-living, now the cnidaria are split into two categories: independent, such as jellyfish and anemones, and parasitic – chiefly, the myxozoa.
“There are 2,500 known species of freshwater and marine myxozoa, but every time we look in fish and invertebrates we find novel species. My guess is that this suggests that the true diversity of these organisms is at least ten times higher than formally described,” Atkinson tells Haaretz. “Probably nobody has ever looked for this type of parasite in this type of polychaete. ‘Look and ye shall find’ is the rule here. The parasites have probably always been there; and there are most likely many more there that we have not yet seen.”
If anything, after selecting the worm for study, researchers were shocked to find “ONLY ONE” species of myxozoan after checking more than 2,000 individual worms, Atkinson shares. The capitals are his.
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Who, by the way, is the terrestrial victim of jellyfish? Hungarian shrews, that’s who. The spores grow in their livers, but happily, the rodents seem to evince little suffering from the infection.
Sex and sick trout
Much remains unknown about these microscopic creatures. When first discovered, myxozoa were thought to be unicellular because they’re so tiny, but further study elucidated that although they are simple animals, that they are multicellular.
Not that they have that many cells. At first, the spore stage is indeed sort of a single cell. “The only part that enters the worm to start the infection is like an amoeba - the ‘infectious sporoplasm’ is a single cell but has more than one nucleus,” Atkinson explains. “And sometimes the sporoplasm contains complete cells within it (“secondary cells”) instead of multiple nuclei.”
Weird stuff. Now, from the worm, what comes out to infect fish is called actinospores, which have multiple cells in tri-radial symmetry: three “shell” cells, three stinging cells and a sporoplasm with two to hundreds of infectious cells within them.
What emerges eventually from the fish host is usually bilaterally symmetrical with six cells.
Like many parasites, myxozoa seem to need two hosts: a worm or other invertebrate – and a fish (or other higher host).
And they reproduce sexually. Inside the worm, the myxozoan spore splits into a male half and female half, which combine with halves of other spores. From the worm, the newly mixed and matched spores pass to the fish (or other host), where they proliferate.
In rainbow trout, a single spore infecting the unfortunate fish turns into a billion spores. Since the myxozoans are parasites, they by definition take nutrients from the host, which is why they can make the fish so sick.
These things aren’t large enough or complex enough to have a mouth. They help themselves to the host’s chemicals. “Like amoebas, they could be excreting digestive enzymes into their immediate environment inside the host and absorbing broken-down nutrients,” Atkinson speculates.
Which came first, the myxozoan or the jellyfish?
Myxozoans have been known for about 200 years but their kinship with cnidarians is a recent discovery. And it isn’t clear who begat whom. Myxozoa have very few genes. If they began as jellyfish, it hadn’t even been thought possible to lose that many genes.
Cnidarians began about 500 million to 550 million years ago, in the great Cambrian explosion: There have even been discoveries of fossilized jellyfish. We will quite certainly never find a myxozoan fossil because they’re too small. So we cannot actually ascertain if myxozoa are jellyfish that experienced reverse evolution, or if the simple myxozoa were first, and jellyfish evolved from them.
Atkinson suspects that jellyfish came first and myxozoans were a later development. First of all, fossil cnidarians found in China from around half a billion years ago look exactly like modern ones.
“Clearly cnidarian diversity was there 500 million years ago. That sets a very early date for lots of jellyfish,” Atkinson says. “So, could myxozoa have existed beforehand, and evolved into those? Maybe, but it’s putting a bit of a crunch on the timing because complex life wasn’t around much before that.”
(Some scientists believe multicellular life existed as far back as 2.1 billion years ago, but others think the evidence for that is debatable, and for sure if anything was moving around the ocean floors back then, it wasn’t worms.)
Assuming that myxzoans began their career as parasites, they had to have some complex being to latch onto, Atkinson points out. Consequently, complex beings had to exist before the myxozoans, and worms are a good candidate.
“It is also possible that before the worms, myxozoa feasted on primitive bryozoan animals (typically a few millimeters long, but they can make large colonies). Bryozoa were recently renamed “ectoprocta” because their anus lies outside of their crown of tentacles. That is good to know.
So we don't know about myxozoans, but it looks like free-living jellyfish came first, then worms evolved (no relation), then a branch of cnidarians split off and degenerated into the Myxozoa that parasitized bryozoans and worms, then fish later on.
Particularly exotic food poisoning
Which brings us to why the Japanese and South Koreans are investing heavily into myxozoan research.
To recap, myxozoa are everywhere, and Atkinson for one suspects that every species of fish on Earth has at least one myxozoan parasite, probably more. The little beasts are tissue-specific: One species could infest the unhappy fish’s liver, another could live in the fish’s muscles, another in its skin and so on.
Yes, when you eat a fish, you are probably eating its population of microscopic parasitic jellyfish. And if you eat that fish raw (sushi, sashimi or slothfulness), you are getting them alive and kicking.
Usually that doesn’t matter: You digest them. But there is at least one exception. When you eat uncooked olive flounder, the myxozoa don’t infect you, per se. You are not their natural host and they will not proliferate in you. But they will fire their stinging cells in your gut and you may get swelling and gastroenteritis. It is a particularly exotic form of food poisoning.
“I would expect raw tuna could get tuna myxozoa in us, but we would just digest them. They wouldn’t be triggered,” Atkinson says. “They’re just more protein.”
Where else are these charming creatures to be found? In aquatic vertebrates – frogs, salamanders and water fowl, specifically ducks. They are also in some turtles, and probably, water-dwelling ones like crocodilians are likely enough to get infected one day, if they haven’t been so far. “If you look at enough crocodiles, you’ll probably find them,” Atkinson speculates. “They are living in a soup of spores from all different myxozoans – it will happen somewhere, somehow. Life will find a way, I’m sure.”
At least they aren’t known to have reached plants yet.
Does Atkinson think more worm-infesting myxozoa will be found? Why yes, he does think so. “We know already that almost every species of fish has one or more myxozoan parasites,” he continues to enlighten. “And all of those parasites have a life cycle that could involve a worm. So there should be many to find in worms (both freshwater and marine). We plan to look at more marine worms to find more myxozoa.”
One last point. These monsters are the bane of fish farms. The only real way to avoid massive myxozoan infestation is to breed fish in constantly-flowing creek or spring water. You take fish that are already stressed from overcrowding, add myxozoans and you get sick fish. The end.