Myxozoans are a group of single-celled parasites which had evolved from jellyfish-like ancestors, thus making them a type of single-celled animal. There are about 2400 known species and they mostly infect fish, with a handful of them infecting other kinds of vertebrate animals including amphibians, turtles, ducks, and even shrews. The species being featured in this post, Myxobolus medusae, infects fish like most other myxozoans, but not just any fish, it's one with a notorious, but overblown, reputation - the red piranha (Pygocentrus nattereri). Despite its fearsome reputation, the red piranha are commonly caught and regarded as a regular food fish, so researchers in this study were able to obtain the piranha from local fishermen around Lake Sacaizal, and describe a previously undocumented species of myxozoan.
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| Left: Myxobolus medusae cyst (indicated by arrowhead) in the eye of a piranha, Right: Illustration of M. medusa spore. From Fig. 1 and 2 of the paper. |
While myxozoan infections are often visible as white cysts in the host's tissue, the spores themselves are actually microscopic and come in various different shapes. Some myxozoans produce spores that have a pair of long wispy tails, but the spores of M. medusae are far more unique and extravagant, with multiple branching tendrils, like the medusa of Greek mythology, in unicellular form (hence its species name). But why have such an elaborate structure in the first place? The researchers suggested those appendages might help the spores disperse in water where they act like a web that catches the current and carry the spores far and wide.
But this parasite also has another connection to its medusa namesake, namely where M. medusae lives in its host. The medusa in Greek mythology can turn someone into stone with a stare from her eyes - and that's where M. medusae lives in the piranha. Myxozoans can occur in various different parts of the host's body, and the genus Myxobolus is an exemplar of that. With almost a thousand known species, they inhabit just about every part of a fish's anatomy including the gills, kidneys, liver, ovaries, muscles, and even the cartilages of the skull and spine, where they constricts and compress the fish's spinal cord and brainstem, resulting in symptoms called "whirling disease". In the case of M. medusae, they appear as a white cyst lodged in the eye's interior.
Myxobolus medusae is not the only parasite to inhabit fish eyes, they are also the favoured infection site for other species of Myxobolus, and a number of trematode flukes. But why the eye though? For the aforementioned fluke, hanging out in the eye would hinder a fish's ability to see, which makes it more vulnerable to birds - the next host in the flukes' life cycle. But it wouldn't do any good for M. medusae if its host gets eaten by a predator, because its spores need to make their way to worms, not the belly of a hungry bird. However, the eyes are still considered prime real estate for any would-be parasites because along with the rest of the central nervous system, the eyes are "immune privilege sites" which are mostly off-limits to the immune system, thus they can act as potential parasite shelters.
This also applies to those eye flukes too, scientists have found that flukes which infect the fish's eyes are able to infect wider range of fish species than those infecting other parts of the host's body.
Since each species of fish have a slightly different immune system, for the body-dwelling flukes, they are more limited in their host choice because their tricks for overcoming one fish species' immune system might not work for another. But since eye flukes don't have to deal with the immune system, they are free to infect a wider range of fish. So M. medusae might also be hiding in the eye for the same reason.
So while beauty might be in the eye of the beholder, in this case, a medusa is found in the eyes of a piranha.
Reference:
de Sena, N. M., Eduard, J., Pereira, C. M. B., Neto, J. L. S., & Velasco, M. (2025). Myxobolus medusae n. sp., a new species of Myxozoa with dendritic appendages. Parasitology International 109:103106.
