Anisakis spp. (et al.)

In the ocean, squids occupy a sort of middle position in the food web. While they are voracious predators that attack anything that moves, they themselves are prey for a variety of animals like larger predatory fish and marine mammals. Even the mighty giant squid is food to whales and fish at various stages of their lives. This makes them the perfect vehicle for parasites looking to climb their way up the food chain, starting from infecting small plankton to end up in the bowels of a large fish or a whale.

Left: An Antarctic flying squid (Todarodes filippovae). Top right: Anisakis larvae encysted on squid stomach (indicated by red arrows), Bottom right: Anisakis physeteris larva from a squid.
Photo of flying squid and infected squid stomach provided by Dr Chenhua Li, photo of larval Anisakis from this paper

To investigate the role played by squid in parasite transmission, scientists from the University of Otago dissected a whole bunch of squids and what they found was a scientific treasure trove of parasites. They had on their hands a mixed platter of large squids,  including the likes of arrow squid (Notodarus sloanii), flying squids (Todarodes spp.), glass squid (Teuthowenia pellucida), cock-eyed squid (Histoteuthis sp.), and many more - include a giant squid (Architeuthis dux)! In total, they were able to obtain 11 different species of squid, fit for a giant cephalopod banquet. But this would have been a banquet with parasite fillings, because they also found six types of roundworms, three types of tapeworms, and a mystery fluke among those cephalopods.

Of the six different roundworm species, three of them were in the Anisakis genus - a notorious public health hazard. Anisakis worms complete their life cycles in whales, but they can cause problems when they end up in humans and their occurrence in people is highly dependent on eating habits. For example in Japan, due to the popularity of raw seafood, Anisakis causes over 19000 cases of illness per year, whereas it is far less common in other regions with different seafood consumption preferences.

The scientists also found the larvae of Crassicauda, a genus of very long (>3 m) whale parasite that lives in all kinds of awkward nooks and crannies in the whale's body, such as under their skin, in their kidneys, in their genitalia, or even occupying multiple parts of the body simultaneously. The complete life cycles of those worms are unknown, though evidently squids are involved for at least a part of it. Similarly mysterious is Lappetascaris, a nematode which has previously been featured on this blog. The full life cycle of these worms are also shrouded in mystery, but based on what little we know, they would have been on their way to a swordfish or other large predatory bony fishes. 

Meanwhile, the tapeworms are ridesharing these squid in order to reach a different destination - sharks. Two of the tapeworms found were trypanorhynchans, which also use various fish as vehicles to reach their final hosts. They are armed with four gnarly hook-lined tentacles that anchor them to their hosts' intestinal wall once they finally reach a shark's bowel. The other tapeworm species was a genetic match with a tapeworm which has previously been reported to dwell in the belly of porbeagle sharks, which is perhaps to be expected as these sharks have been documented to frequently dine on squid.

Amidst all the worms riding these squid to various different destinations was a fluke lurking in the mysterious googly-eyed glass squid. It was not readily identifiable to a known species but based on its DNA, it seems to be genetically similar to a species of fluke which lives in the throat of ocean sunfish, from the family Accacoeliidae. Very little is known about the life cycle of this family of flukes, and what little we know indicates they use various oceanic invertebrates such as pelagic snails and man o'war as intermediate hosts. Given the habits and diet of the glass squid, it might have inadvertently picked up these flukes while chowing down its usual prey.

So simply by looking under the hood of some squids have given us some remarkable insights into the ecological interactions in the ocean, along with parasitic worms are able to zip between hosts via such interactions. For those parasites, cephalopods are like well-connected travel hubs through which they can reach a myriad array of the ocean's inhabitants, all linked via a network of transmissions.

Reference:

Li, C. H., Bennett, J., Braid, H., Howard, R., Stevens, D., McBride, L. E., Clough, S., Fritzgerald, E., & Bolstad, K. (2026). The Battle of Helminths Deep: Molecular Insights Into Oegopsid Squid Parasites in Aotearoa New Zealand. New Zealand Journal of Zoology 53: e70035.

Copiatestes thyrsitae

Copiatestes thyrsitae is fluke that lives on the gills of fish, and while there's nothing out of the ordinary here as far as parasites go, it sometimes also wraps itself around the ankles of seabirds and dragging them to their deaths. That sounds more like the work of a horrific sea monster than a ordinary fish-dwelling fluke, so in order to understand how this otherwise ordinary fluke ends up committing acts of avicide, we need to look at its life cycle. 

Top left: A krill infected with Copiatestes thyrsitae, Top right: A mass of Copiatestes removed from a dead bird's leg,
Bottom left: A Copiastestes fluke attached to debris, Bottom right: A mass of Copiastestes flukes viewed under a microscope.
Photos from Figure 2 of the paper. 

The life cycle of this parasite involves a very convoluted journey. Like other flukes, Copiatestes clones itself in a first intermediate host, where they do that though is currently unknown, though it's most likely a marine snail. The larvae produced by all this asexual activities go on to infect krill, where they hang out for a few weeks before emerging more fabulous than ever before, festooned with air bladders and a mop of long sticky filaments.

These larvae aren't just dressed to impress. Their new drip allows them to ride the waves and maybe snag onto a passing fish where it can settle down and live out the rest of its life, tucked in the fish's blood-rich gills. But the surface of the sea is also frequented by another type of animal - seabirds. And for many Copiatestes, instead of a rendezvous with a fish, they end up getting wrapped around a bird's leg. But this is an accident with a tragic ending for both fluke and bird. The fluke will never reach a fish that way, and its fate will be to get dried out on a bird's leg.

The numerous filaments on Copiatestes stay sticky even after the fluke dies, so they end up collecting sand and other debris, and go from a sticky nuisance to being deadly shackles, dragging the birds down like heavy chains. Cases of these flukey anklets have been widely documented over the decades, but not particularly well-studied. This is a major concern because they have been implicated in mass mortalities of seabirds.

A group of scientists in New Zealand decided to figure out what drives the abundance of these ankle-grabbers. They collected thousands of krill and deployed plankton nets in the Otago Harbour, and found both infected krill and the free-swimming stage of the fluke. While the prevalence is relatively modest (5.9%), this really adds up when you realise just how abundant krill can get. The scientists found that the infected krill tends to be smaller, though it is not clear if this is because of the fluke stunting their growth, or if smaller krill are more susceptible to infection

While most infected krill only harboured a single fluke, the scientists did found one unlucky krill which was stuffed with four Copiatestes. Furthermore, this fluke is relatively common among the plankton samples in their flamboyant, free-swimming form.  Based on their samples, it seems that Copiatestes prevalence seems to increase with temperature, so the warmer it gets, the more Copiatestes there are in the krill.

So why would ocean temperature affect Copiatestes abundance? Well digenean trematodes have that asexual stage in their snail first intermediate host, and as temperature increases, so does the rate at which they produce and release their free-swimming larvae. This probably also applies to Copiatestes, which means finding the missing link in the fluke's life cycle - the mysterious first intermediate host - would be a key to managing this parasite.

Global warming can have an impact on all aspects of life on this planet, and sometimes the outcome is not always so straightforward. Who would have thought that some heated snails would lead to dead birds? The authors of this paper have also produced a video (with animation!) about Copiatestes, as well as how you can get involved in monitoring this parasite if you are living in Aotearoa New Zealand.

Reference:

Bennett, J., Little, M., Kitson, J., Lewis, R., Carson, S., Edwards-Ingle, J., & Poulin, R. (2026). Temperature driving a mass killer: assessing the risk of trematode outbreaks for New Zealand seabirds. Parasitology, 153: 599-607.

Duplorbis koru

Student guest post time! One of the assessments that I set for students in my ZOOL329 Evolutionary Parasitology class is for them to write about a paper that they have read, in the manner of a blog post. The best blog posts from the class are selected for re-posting (with their permission) on here. So from the class of 2026, here's a post by Keegan McCleary (who also happens to be a keen photographer) titled "The Deep-Sea Nesting Doll: How a New Zealand Barnacle Hijacks the Hijacker"

 

How many layers of exploitation can one life support? In the crushing darkness of the Kermadec Ridge, north of New Zealand, a squat lobster carries a secret: a gill-dwelling parasitic isopod siphoning nutrients from its host. Look closer, and the story goes deeper. Inside that isopod’s marsupium resides a gang of T-shaped barnacles known as Duplorbis koru. It is a biological nesting doll where each organism is exploited by the one inside it.

 

Left: Spiral-shaped externae of Duplorbis kuro in the marsupium of its bopyrid host. Right: Duplorbis kuro cypris larva
Photos from Figure 2 and 4 of the paper

Researchers in 2025 provides the first detailed account of Duplorbis koru, a parasite that pushes the definition of "barnacle" beyond imagination and resolves a century-old evolutionary mystery. Unlike familiar barnacles clinging to boat hulls, this species has shed almost every typical animal feature: it has no shell, legs, eyes, gut, or excretory system. This extreme reduction is a hallmark of parasitic evolution, where a lineage abandons unnecessary traits in order to specialise entirely in its host.

 

By stripping away the "dead weight" such as eyes and legs, the highly modified barnacle becomes a master of internal architecture. Its body consists of two highly specialised organs: the interna, a root-like network siphoning nutrients, and the externa, pulsating fern-shaped sacs that house the next generation within the isopod’s brood pouch. This radical transformation is rare even among rhizocephalans, a group already renowned for extreme modifications. Many of these barnacles are parasitic castrators, diverting host reproductive energy toward parasite growth. In related species, male crabs are even feminised, their bodies transformed to resemble females. Duplorbis koru takes this further by exploiting the reproductive body of another parasite for its own benefit.

The species only leaves its host as a cypris larva, a tiny planktonic stage. At just 85 micrometres long, these larvae act as specialised infective agents, navigating the abyss for the scent of an isopod’s brood pouch. Once a target is found, they inject a motile, worm-like invader called a vermigon into the host. This stage is equipped with sensory organs finely tuned for this needle-in-a-haystack search, with only 5% of isopods carrying the hyperparasite, these larvae face incredible odds. DNA analysis finally cracked a century-old mystery surrounding this lineage of parasitic barnacles. Genetic data confirms D. koru is closely related to a group of modified parasitic barnacles called Duplorbidae and expands the family’s range from the Arctic to the South Pacific. This placement also highlights the evolution of hyperparasitism, a rare lifestyle in which it exploits a host that is already a parasite.

 

Beyond solving its evolutionary puzzle, D. koru carries a symbolic connection: its name references the Māori koru, a spiral representing new life and growth. This highlights how remote discoveries can resonate with local heritage. Modern surveys of the Kermadec Ridge have added dozens of new records, challenging assumptions about deep-ocean biodiversity. By hijacking the isopod, this hyperparasite likely redirects host energy to prioritise the barnacle’s reproductive success.

 

Even 1500 metres down, the smallest players can have outsized influence, with these regressive parasites playing highly specialised roles in deep-sea ecological systems. Understanding these interactions is essential, as parasites are not merely passengers but may actively be shaping biodiversity and energy flow in the ocean’s depths.

 

Reference:

Kokasko, G., Williams, J. D., Wong, K. J. H., & Chan, B. K. K. (2025). Description of a new species of the hyperparasitic barnacle Duplorbis Smith, 1906 and its cypris larva, with the molecular phylogenetic placement of the enigmatic family Duplorbidae Høeg & Rybakov, 1992 (Cirripedia: Rhizocephala). Journal of Crustacean Biology, 45: ruaf036 

Echinomermella matsi

Nematodes (roundworms) just can't get enough of being parasites. Parasitism has independently evolved at least fifteen times among nematodes, with each of them realising their best lives inside the body of another animal (and for some of them, inside plants). But how did they end up like this? Well, the worm being featured in this post may offer some answers (while raising many others).

Left: A bundle of Echinomermella matsi inside of an opened sea urchin. Right: An E. matsi juvenile viewed under a microscope. Photos from Fig. 2 of the paper.

Echinomermella matsi is a species of roundworm that parasitises sea urchins. It can grow fairly large and is very noticeable, opening up a heavily infected sea urchin is like looking at a bowl of udon noodles. In some urchin populations, these worms can be rather common, and they exert a demanding toll on their hosts, so much so that they can end up castrating the sea urchin as they divert resources away from their host's gonads. But sometimes, mere castration is not enough, and this worm can also shorten the urchin's lifespan.

All this affects the population dynamics of sea urchins, which has flow on effects on the rest of the intertidal ecosystem as sea urchins are often major grazers of kelp and seaweed in such environments. Given the impact that this nematode can have on urchin populations, there is some interest in figure out what makes this worm ticks, and this study took a genomic approach to that question.

It has previously been thought that Echinomermella and similar parasitic nematodes might be marine relatives of mermithid nematodes, which are a family of roundworms that infect insects on land. But subsequent studies have found that their resemblance is merely superficial. While like the mermithids, they can take up a lot of room in their host's body cavity, that is hardly a unique characteristic. Turns out, Echinomermella and related worms seem to belong in a group of nematodes that have all independently become parasites of marine invertebrates.

So how does our urchin-dwelling friend fit into all this? The researchers' analyses found that Echinomermella belong to a larger group of nematode worms called Plectida which contains both parasitic and free-living species, with some species found in the sea and others in freshwater. The parasitic species within that group infect all kinds of marine organisms, even single-celled foraminiferans.  The closest living relative of Echinomermella is a nematode called Trophomera, a parasitoid worn that lives in a range of different deep sea invertebrates including crustaceans and bivalves.

Since Echinomermella is evolutionarily close to both free-living and parasitic worms, that can provide us with some clues as to how it evolved to be parasitic. One of the hypothesis is that the ancestor of Echinomermella and other parasitic plectids started out as marine nematodes. Their long slender bodies would allow them to squeeze into all kinds of tight spaces, including the insides of other animals. And because the body cavity of marine invertebrates contains fluid which is almost at the same solute concentration as seawater, it seems natural for those worms to squeeze in there to escape from the bustle of the outside world. Once they're in, they're protected from potential predators and other sources of danger, all while bathing in delicious nutrient-rich fluids.

There are multiple paths to parasitism, many of which have been explored and mastered by nematodes. In a dangerous and uncertain world, perhaps shacking up in the cozy wet insides of other creatures is the best way to live when you are just a worm.

Reference:

Kirangwa, J., King, E., Collins, J., Bates, A., Blaxter, M., & Holovachov, O. (2026). The genome of an enigmatic sea urchin parasite Echinomermella matsi Jones & Hagen, 1987 resolves its place among other invertebrate parasitic nematodes. G3: Genes, Genomes, Genetics 16: jkag084.

Lathraea squamaria

When it comes to parasites, for most people it is a case of "out of sight, out of mind", and that especially applies to holoparasitic plants that spend most of their lives hidden underground or inside the tissue of their host plant, and only make their presence known when their flowers emerge from the ground or their host. Lathraea squamaria is a parasitic plant that infects many different types of deciduous trees including beech, hazel, aspen, and ash. Also known as the common toothwort, it is found in ravine forests and spends most of its life underground as rhizomes clinging to the roots of its host.

Left: A whole Lathraea squamaria plant, Centre: L. squamaria flowers in bloom, Right: Cross-section of a L. squamaria fruit with developing seeds.
Photos from Fig. 1, Fig. 3, and Fig. 5 of the paper.

While the word "parasite" often has a negative connotation, there are many parasites that perform important ecosystem services, and L. squamaria is no different. When few other plants are flowering, the toothwort blooms, providing bees with an important early-season food source when food for such pollinators are relatively scarce. They also release nutrient-rich saps that are savoured by small invertebrates and other plants in the forest.  While common toothwort has been studied for almost two centuries, because it exists almost entirely underground, many of the most basic aspects of this plant's biology and structure remain a mystery.

This post features a study where researchers set out to provide a detailed description of this parasite, particularly the haustoria, a very important part of parasitic plant as this is the organ they use to cling to and dig into their host. They collected L. squamaria from parts of the Czech Republic and Ukraine, sampling toothworts at different stages of development, from newly sprouted rhizomes to fully matured fruiting plants. Throughout the toothwort's life, the only part of this parasite which emerges aboveground are its flowers, which are mainly pollinated by specialised bees that can tolerate the parasite's extremely alkaline nectar. While the nectar of most flowers is generally slightly acidic, nectar from the common toothwort has a pH level of 11.5, which is comparable to soap.

As with other plants, once they are pollinated and produce mature fruit, they need to disperse their seeds, and like other holoparasitic plants, its seeds are transported by ants. Lathraea squamaria recruits their service by providing them with a treat, in the form of a tiny tag on each seed called an elaisome. This fleshy tag is irresistible to ants, which carries the whole seed back to its nest to rip off this tasty morsel. The ant then tosses the seed into the nest's communal refuse pile, but by doing so, they also end up planting the seed where it can sprout and find a new host.

The seed begin sprouting when exposed to chemicals which are extruded by the host tree's roots, and when the toothworth initially emerges from its seed as a rhizome, it can extend itself quite a long way beneath the ground in search for a host, before latching itself around the roots of a tree. Once it does, it sends out fine tendrils that tap into the host's vascular system, not just to draw nutrients, but also to send proteins, hormones, and signalling molecules to the host plant to make it more compliant. But sometimes they can also get a bit too overzealous and end up wrapping those tendrils around their fellow parasites, or even themselves. This has also been observed in other parasitic plants.

While called the "common" toothwort, researchers have found that they're not found in just any ravine forest, but mostly in mature forests which have not been disturbed. This is comparable to some horsehair worms which help keep trout well-fed with drowning crickets - some of these worms are only found in old growth forests. While one is a plant and the other a worm, both of those parasite provide important ecosystem services. For all that those parasites may take from their hosts, their actions also help support many other inhabitants of such forests.

Reference:

Krasylenko, Y., Teixeira-Costa, L., Sosnovsky, Y., Šamajová, O., Ovečka, M., Sytschak, Horielov, O., Brücknerová, N., Polláková, K.  & Ioannou, E. (2026). Persephone’s Flower: Morphology and anatomy across the life cycle of the forest root holoparasite Lathraea squamaria (Orobanchaceae). Flora 337: 152948.

Sorochytrium milnesiophthora

Tardigrades (also known as "water bears") have built up a reputation for being pretty much invulnerable (even though Bdelloid rotifers does a lot of tardigrades' tricks and more), but while they can survive being launched into space, they are at the mercy of some fungi that can turn their little bodies into living incubators. One such fungi is a chytrid fungus named Sorochytrium milnesiophthora. And while most people who have heard of chytrid only know about the two species which infects frogs and other amphibians, most chytrids are actually parasites of invertebrates and algae, with some playing important ecosystem roles such as controlling harmful algal blooms and converting otherwise inedible phytoplankton into food for zooplankton, like water fleas

Left: A tardigrade infected with Sorochytrium milnesiophthora, Top Right: S. milnesiophthora zoosporangia (indicated by arrow) in the body of an infected tardigrade, Bottom Right: S. milnesiophthora grown on nutrient agar.
Photos from Figure 1 of the paper

In this study, researchers found S. milnesiophthora growing in tardigrades that were living amongst salted shield lichens collected from Kuljunmaa Island, Finland. Samples of the lichen were dried out for a year, then rehydrated and shaken to isolate the infected tardigrades and extract their fungal cells. When samples of the fungus were grown on a petri dish filled with nutrient agar, it grew into a type of lumpy yellow mould. But S. milnesiophthora takes on a very different form upon encountering a tardigrade.

The story of a water bear's fungal downfall begins with a single zoospore. Unlike most fungi, Sorochytrium and other chytrid-type fungi produce spores which have a flagellum-like tail that allows them to swim through water. Upon contact with the unfortunate tardigrade, the spore fires a tube which punctures the victim's body wall and invades its interior. At this stage, the fungal cells look like round balls which are indistinguishable from regular cells that float around in the tardigrade's body cavity. But over the course of a week, these fungal cells start proliferating throughout the water bear's body, which develops a reddish-brown colour. Once the host dies from being completely taken over by growing fungal cells, S. milnesiophthora squirts newly produced zoospores out of the carcass to begin the infection cycle anew

Aside from its ability to take down the seemingly invulnerable tardigrades, S. milnesiophthora also has something else which makes it stands out - an internal transcribed spacer gene that is 55 thousand megabases long, which is among the longest known for any organisms. This section of DNA, also known as ITS, is commonly used for identifying and classifying different species of organisms, and can serve as a DNA "barcode" to identify different types of fungi. Because it is a relatively short sequence, traces of it can be easily amplified and detected in a sample. Hence they are also often used in metabarcoding, a molecular technique for detecting DNA from a whole bunch of different organisms in a single sample, often used for detecting microorganisms which would be difficult and laborious to find using conventional microscopy. 

But because the ITS gene of S. milnesiophthora is so long, this messes with the primers which are used for metabarcoding, as they were designed for amplifying short sections of DNA. Which means S. milnesiophthora would have been missed by conventional metabarcoding since the primers can't amplify its DNA for detection. So Sorochytrium and other similar fungi might actually be more common than previously thought. Case in point, while examining tardigrades for Sorochytrium, the researchers also found two other species of fungi along with two species of water molds associated with the water bears.

Water bears are charming-looking critters, and it may seem sad to see them die such a brutal death, but all this tardigrade slaying does serve an important ecosystem function. In addition to regulating the tardigrade population to make room for other microscopic animals, S. milnesiophthora may inadvertently be providing food for the ecosystem that exists on lichens. By converting water bears into zoospores, Sorochytrium could be providing other microscopic animals, such as rotifers, with a free meal. While deadly to their hosts, for everything else, chytrid zoospores can be tasty snacks for filter-feeding animals which relish those flagellated spores, as they are more nutritious than those animals' usual fare of algae, bacteria, and organic detritus,

There are vibrant ecosystems everywhere for those with the eyes (and microscope) to see. So next time you see a patch of moss or lichen, take a moment to appreciate the drama of life, death, and rebirth being played out in those microscopic worlds.

Reference:

Dirks, A. C., Vecchi, M., Orozco-Quime, M., Schwarz, E., Calhim, S., & James, T. Y. (2026). Rediscovery of the tardigrade pathogen Sorochytrium milnesiophthora, a blastoclad boasting the longest-known fungal internal transcribed spacer. Mycologia 118: 623-637.

Myxidium anatidum

The bile duct of a bird is probably the last place you'd expect to find a microscopic, parasitic relative of jellyfish, but that is exactly what the study featured in this post is all about. The parasite in question is a myxozoan, a type of single-celled parasites with complex life cycles which are found in a range of vertebrate animals. Evolutionary speaking, they're technically animals, but for whatever reason, over the course of their evolution, they had abandoned having bodies made of multiple cells to become the only known group of unicellular animals.

Left: Histology section show Myxidium anatidum spores in the bile duct, Centre: Myxidium anatidum myxospore under brightfield (top) and Nomarski Interference Contrast, Right: Photo of a bald eagle.
Photos from the paper (left), this paper (centre), and US Fish and Wildlife Service

There are over 2000 known species of myxozoan and most of them are usually found in fish, though there are also some species that infect amphibians and turtles. The myxozoan parasite being featured in this host was found in a rather unexpected host - a bald eagle, of all things. This eagle was found in Western Canada and was found in very poor condition prior to its death. The parasite's spores were found throughout the eagle's bile duct. While that may sound concerning, the parasite clearly played no role in the eagle's death. That's because based on the bird's condition when it was found and the toxicology findings, the eagle had died from lead poisoning. 

So why would a bald eagle be harbouring a type of parasite which is usually associated with fish and amphibians? While it is true that most myxozoan tend to be found in cold-blooded aquatic or semi-aquatic animals, some discoveries over the last decades have shown warm-blooded land-dwelling animals are not beyond the reach of these single-celled jellyfish cousins, and that includes the parasite found in the eagle's bile duct - Myxidium anatidum, a species that seems to specialise on our feathered friends

Myxidium anatidum was initially described back in 2008 from seven species of ducks (hence the "anatidum" species name) collected from across different parts of the United States. Much like with that bald eagle, the parasite resided in the bile duct and most of those infected ducks had died of other causes. This discovery overturned a lot of the previous assumptions about myxozoans and what they're capable of infecting, but since then there hasn't been any new findings about this peculiar parasite, which is still surrounded in mysteries. One such mysteries is the parasite's life cycle.

For other myxozoans, they have a complex life cycle which involves an asexual stage living in a vertebrate animal host, and the sexual stage living in either segmented worms or bryozoans (moss animals). But it is currently unknown how M. anatidum infects its feathery host, nor what invertebrate it infects for the sexual stage of its life cycle. 

When M. anatidum was initially discovered, researchers examined worms and fish from one of the ponds where an infected duck was collected, and while they did find some which were infected with myxozoans, none of them had M. anatidum. In nature, the prevalence of myxozoans in their worm hosts can be rather low, so it is possible that they had simply missed the infected worms, or the parasite was only present in worms during certain seasons, or perhaps the ducks have picked up the infection from elsewhere. Given what is known about myxozoan life cycles, since many ducks are dabblers, it is conceivable that they might have acquired the parasite through swallowing infected worms which had been hiding in the muck. 

But in that case, how did a bald eagle end up contracting this parasite? Bald eagles primarily eat fish, so it is possible that M. anatidum may have been using fish as a type of "paratenic host" - an animal that serves as an optional stopover that can potentially carry the parasite to its nominal host. It's kind of like going on a side quest which could help you complete the main goal. While the use of paratenic hosts is common among other parasites with complex life cycles, it has not been reported for myxozoans. But then again, myxozoans haven't been reported in bald eagles until now.

Since lead poisoning was what actually led to the infected eagle's death, there may be many other healthier bald eagles flying around with M. anatidum lurking in their bile duct. Between the eagles and the ducks, this humble parasite is clocking up some frequent flyer miles which its fish-infecting cousins could never hope to match. 

Reference:

Perdrizet, U. G., Lockerbie, B., & Bollinger, T. K. (2026). Myxidium anatidum in a Bald Eagle Haliaeetus leucocephalus from Western Canada. Journal of Wildlife Diseases 62: 257-259.

Pinnotheres pholadis

Pea crabs are a type of tiny crabs that live mostly in bivalve shellfish such as mussels, oysters, and scallops. Scientifically known as pinnotherids, they hang out in the mantle cavity, a muscular bag which these molluscs use to pump water in and out of their body. Nestled in this flesh chamber, pea crabs get to enjoy a steady stream of aerated water and delicious phytoplankton, while protected by the bivalve's hard shell (although this does make hooking up a bit tricky). While some of these crabs can take up a fair bit of room in their host's body, they don't tend to feed on the shellfish's tissues. So are they just innocuous house guests hanging out with their bivalve homies?

Top left: A Pinnotheres pholadis pea crab in a scallop's mantle cavity, Top right: A female pea crab,
Bottom left: A male pea crab, Bottom right: A female pea crab with eggs.
Photos from Fig. 2 and 3 of the paper.

Well, the study featured in this post shows that at least when it comes to Pinnotheres pholadis, their presence can have a detrimental effect on their hosts, especially among juvenile shellfish. Researchers in Japan collected both wild and farmed Yesso Scallops from twelve different locations across Mutsu Bay in Japan and examined them for pea crabs. They looked through 881 scallops, and found that close to one-third of them carried pea crabs. When they compared the conditions of the scallops with and without crabs, they found that the crab-endowed juveniles tend to be skinnier and have smaller shells, especially if they housed more than one crab. 

So if the crabs aren't feeding on the scallop's tissue, why would they affect the host's condition? Like other bivalves, scallops feed on algae and other organic detritus in the water, which they suck up and trap in strings of mucus for easy swallowing. But this bundle of nutrient-rich mucus also happens to be the pea crab's favourite food, and the resident crab can intercept the mucus strings before they can be swallowed by the shellfish. So these crabs are literally snatching food out of the scallop's mouth, making them a kind of internal kelptoparasite.

While older and larger scallops might be able to offset their tenant's appetite due to their greater filtration capacity and larger reserves of energy,  juvenile scallops need all the nutrients they can get to fuel their growing body. So they are less able to afford the crab skimming off the top from their phytoplankton slime smoothee. The researchers also found that crabs living in larger scallops tend to be bigger, possibly due to a combination of having more room in the mantle cavity, and also because bigger hosts are able to gather more food from the surrounding water. In this system, the available space with the bivalve places an absolute limit on how big the crabs can get.

So while for humans, having crabs may simply be itchy and embarrassing, for scallops, having crabs when you are young can potentially ruin you for life.

Reference:

Yamazaki, T., Odani, K., Nakayama, R., Watanabe, T., & Kawai, S. (2026). Parasitism by pinnotherid crabs in the Japanese scallop Mizuhopecten yessoensis: first host record and quantitative assessment of host impacts. International Journal for Parasitology: Parasites and Wildlife 29: 101198.

Galactosomum nagasakiense

Galactosomum nagasakiense is a parasite that lives rent-free in fish's brain and give them "Trematode Whirling Disease" (TWD). There are actually quite a number of different fluke species that all infect fish brains, but unlike those other species where hundreds of individual flukes are needed to change how the fish behaves, all it takes is a single Galactosomum sitting snugly in the centre of a fish's brain to send it into a tizzy. Whereas other flukes manipulate the neurotransmitters in the brain of their host, Galactosomum takes a more blunt force approach, as the mechanical pressure exerted by their larval cyst causes the surrounding brain matter to degenerate or undergo necrosis as a by-product of their presence. So this fluke literally gives its fish host brain rot.

From left to right: Line illustration of a Galactosomum nagasakiense cercaria, Photo of Galactosomum Type C, Photo of Galactosomum Type B (top) and Galactosomum cysts in the brain of a tiger buffer (bottom) with a close-up of the cyst (insert), photo of Cerithium dialeucum shell.
Photos and illustration from Fig. 1, 4, 6, 7 of the paper and from the D-PAF (Database of Parasites in Fish and Shellfish)

Galactosomum nagasakiense was first noticed in fish in the 1960s, and the adult fluke lives in the intestine of black-tailed gulls, who presumably appreciate the easy pickings that these brain rotted fish present as they swim in circles near the surface of the water. But where are the fish getting their flukes from? This is a important question because TWD can affect a wide range of fish from anchovies to kingfish, and is known for causing bouts of mass die-offs at fish farms among important aquaculture species such amberjacks and fugu (puffer fish). 

Despite its impact, the full life cycle of G. nagasakiense was unknown until the publication of the study we'll be looking at in this post. Deciphering the complex life cycle of parasites can often be a labour intensive and thankless task which involves a fair amount of informed intuition and luck. Hence for many parasite species which are otherwise well-studied, often the one aspect about them which remains unknown is their full life cycle.

In this study, researchers conducted field sampling on the coast of Tsushima Island near a tiger puffer farm that regularly had cases of TWD. Knowing the typical life cycles of digenean flukes, the source of the infection would most likely be a snail, but which one? There are many sea snails living among the rocks on the coast of the island which are prime candidates as the source of G. nagasakiense. The researchers in this study ended up sampling 1314 cerithioid sea snails, most of which (798) belonged to a species named Cerithium dialeucum, which turned out to be the snail that was serving as crawling Galactosomum factories.

Infections were not common, only 15 out of the 798 snails they sampled were shedding Galactosomum larvae, but they made up for their rarity with productivity. During their peak emergence period which lasted two to three weeks, each snail can pump out 3000 of those wriggling Galactosomum larvae per day, though this number declines to a steady (but persistent) trickle for another ten weeks. In total, each infected snails can release 16000 Galactosomum larvae into the surrounding waters, made possible by the prolific asexual stages of the parasite which has taken over the snail's internal organs. 

The free-swimming larval stage of digenean flukes come in all kinds of shapes and sizes, but Galactosomum stands out for having a thick, rippled tail which is about ten times as long as the larval fluke's actual body. When these wrigglers leave the snail, they swish their tail in a figure 8 motion that would attract the attention of any curious fish.

The researchers also discovered that G. nagasakiense was not the only species of Galactosomum in those snails, they found two other species of big-tailed cercariae, one of which has a sucker-like structure on its massive tail. It is likely that those species also infect fish, but it is unclear whether they also burrow into fish brains the way G. nagasakiense does. In any case, the discovery of these snails as the source of infections, can provide aquaculture managers with ways to limit or control TWD, such as situating the fish farms away from habitats where those snails are likely to be found.

The influence of parasites on their hosts are often overlooked because they are hidden out of sight inside of their hosts, but their impacts cannot be ignored. In this case, all it takes is a tiny fluke to cause some serious headaches for a whole lot of fish and fish farmers.

Reference:

Sugihara, Y., Iwasaki, R., Miyazaki, H., Shirakashi, S., Itoh, N., Nakano, T., Takano, T. & Ogawa, K. (2026). Elucidation of the life cycle of Galactosomum nagasakiense (Heterophyidae), the causative parasite of trematode whirling disease in marine fish, with discovery of congeneric species in the gastropod first intermediate host Cerithium dialeucum. Parasitology International 111:103190.

Limnotrachelobdella okae

Do you like eating fish? Well so do some leeches. Most people only know about the leeches that feed on humans, but there are also numerous other species of leeches out there, and many of them are found in the sea. As the saying goes, "there are plenty of fish in the sea", so they are often the target of these leeches' appetites. And if you're going to be farming fish in the sea, there's a chance that you have inadvertently opened up an all-you-can-eat buffet for some hungry leeches.

Top left: Limnotrachelobdella okae attached to the base of a fish's dorsal fin, Top centre: a leech attached to a fish's tail fin, Top right: a lesion caused by the leech's feeding. Bottom: A mature Limnotrachelobdella okae leech
Photos from Fig. 2 and Fig. 3 of the paper.

The Mi-iuy Croaker (Miichthys miiuy) or 鮸鱼 is a popular food fish in China, where it is considered to be not just tasty and highly nutritious, but may also have medicinal properties. Because of the high demand for this fish, it is farmed in sea cages to ensure continuous supply without threatening wild stocks. But as with farming on land, having a lot of animals in a single place makes them extremely vulnerable to parasites.

In January 2025, there was an outbreak of marine leeches at a croaker farm off the coast of Raoping County in the Guangdong Province. Fish began dying off at the start of January, with fish mortality peaking towards the end of month, before tapering off over the course of February and March. At its peak, the sea cages that housed the croakers were swarming with leeches, and over a hundred thousand kilograms of fish were killed by their feasting.

So what exactly were the slithery killers that dined and dashed at that fish farm? Researchers were able to identify those thirsty blood-suckers as a species of leech called Limnotrachelobdella okae. It belongs to the Piscicolidae family - a group of leeches that mostly specialise in parasitising fish. When the leeches entered the sea cage, they just grab onto whichever part of a fish and start sucking, but they mostly focused on the part of the body which had thin skin and plenty of blood vessels. This included the fins, mouth, the edge of the gill covers, the tail, and the belly of the fish. Limnotrachelobdella okae are big leeches, reaching 12-15 cm in length, and having just a few of those big suckers on a fish would render it anaemic in no time.

When the researchers examined the parasitised fish, they found that not only did these fish have severely depleted blood cells as result of the leech's ravenous appetite, they also suffered internal injuries to their internal organs including their liver, spleen, and kidneys. This is due to depleted blood flow to those organs, because under anaemic conditions, blood flow is preferentially directed to organs with high oxygen demands such as the heart and the brain, and this comes at the expense of the other organs. And unlike our kidneys, the "head kidney" in fish also produces new blood cells, rather like our bone marrow. So the reduction of blood flow into the fish's head kidney also reduces its capacity to replenish that lost blood.

So why did the outbreak happen when it did? Well, it might have something to do with leech's temperature preference. Limnotrachelobdella okae seems to prefer water at a chilly 5-10°C. When the researchers kept some of the leeches in aquariums set at different levels of salinity and temperature, they found that the leeches thrived in cold, salty water, but if it gets above 20°C they start dying in droves. This might explain why the swarm of leeches suddenly appeared in January, since that is the coldest month in China. As the year progressed and the weather got progressively warmer, the number of leeches declined and their threat ended once the water temperature reached 20.5°C in April.

Parasites are just like any other organisms in the environment, their activities are deeply tied with climatic conditions. So understanding parasite ecology can help us better prepare for their emergence. Because sometimes winter doesn't just bring cold wind and snow, it could also be bringing leeches in tow.

Reference:

Che, S., Mo, Z., Zhang, H., Tang, H., Dan, X., & Li, Y. (2026). First report of marine leech Limnotrachelobdella okae infestation in miiuy croaker (Miichthys miiuy): an emerging threat to Chinese mariculture. Veterinary Parasitology, 341:110637.

Gordius wulingensis

Horsehair worms are a menace to crickets (as well as mantis, millipedes, and centipedes) everywhere. Imagine being a cricket going about your day when you happen upon a protein-rich snack in the form of a smaller invertebrate. As you munch on this treat, you have also unknowingly ingested a parasitic larva. Over the course of the next few months, this parasite will grow so large that it fills up your entire abdomen, displacing all your internal organs. And when it comes slithering out of your belly, it is several times the length of your body. Life can be rough for crickets.

Left: Free-living adults Gordius wulingensis in shallow water pool, Top Right: Cave cricket with multiple emerging worms, Bottom Right: Adult worm among rocks. Photos from Figure 7 of the paper

Hairworms are mostly associated with aquatic environments such as streams and rivers, though there are also some species which are found among wet soil. If you are observant (and lucky), you may spot adult hairworms crawling or swimming out in the open. But the hairworm featured in this post is found in near total darkness, sheltered in limestone caves where it parasitises the caves' inhabitants.

Gordius wulingensis is a species of hairworm which can be found in karstic caves throughout the Hunan province in China. It was first discovered in 2014 at Jinji cave (金鸡洞) in Yongshun county, but before it could be properly described, those initial specimens were lost, and it wasn't until 2019 that the worm was rediscovered, and researchers began searching more extensively for this hairworm across Hunan province. The worms described in this particular study were collected from caves at the Wuling Mountains. 

As mentioned at the start of this post, hairworms found in the outside world parasitise a wide range of arthropods, but since G. wulingensis is found only in caves, its host choice is more limited. Given crickets are common hosts for hairworms aboveground, a cave-dwelling cricket would be the ideal hosts for G. wulingensis. In this case, the unlucky chosen one happens to be a species of camel cricket.

Compared with the house or field crickets that most people are more familiar with, camel crickets look rather gangly, with their long slender legs and antennae. They are often called cave crickets because of their tendency to live in caves, along with similar human-made structures such as abandoned mines, sewers and wells. But this cave-dwelling habit also brings them in contact with G. wulingensis, which turns these leggy crickets into living incubators.

Currently, only the adult stages of G. wulingensis have been found, and while the rest of the life cycle is presently unknown, it is probably similar to that of other hairworms - with the larval stage in small invertebrates, and the adult worm developing in and emerging from larger arthropods. While most hairworms are brown in colour, G. wulingensis is entirely white and when shined under light, it gives off a rainbow iridescence. It might be tempting to think that this might have something to do with its lightless habitat, there are other hairworms living aboveground which also have pale complexions.

The emergence of hairworms from their hosts usually follow a seasonal pattern, but seasons don't really mean as much when you're living in a dark cave, and researchers found adult G. wulingensis all year round in the caves, with worms in puddles, soil, and sometimes even crawling up the cave walls. These adult hairworms can live for up to 3 months, which gives them plenty of time to find a mate and produce the next generation. 

Caves and other subterranean environments are largely unexplored, and the fauna living in those caves constitute a part of Earth's hidden biodiversity that remains unaccounted for. But the parasites that infect those cave-dwellers presents an even deeper layer of hidden biodiversity, which goes to show that in biology, you can always go deeper

Reference:

Zou, Y. Z., Huang, J., Xiang, H. Y., Li, S., Tang, Y., & Liu, Z. X. (2025). A new species of Gordius (Nematomorpha, Gordiidae) from the karstic caves in the Wuling Mountains, Central China. Zoosystematics and Evolution 101: 907-918.

Cymbasoma sp.

Floating amidst the ocean's plankton is a tiny monster, it has no mouth and it must mate, after which it will give birth to a new generation of little monsters that will grow within the bodies of worms. Everything about this tiny crustacean sounds like a science fiction monster, starting with the group's name - Monstrilloida, meaning "tiny monster" - coined by a scientist who found their life cycle and appearance to be delightfully bizarre.

Left: Copepodid stage of a female Cymbasoma dissected from a Haplosyllis worm, Right: Adult stage of a female Cymbasoma 
Photos from Fig. 2 and Fig. 4 of the paper

Adult monstrilloid are free-swimming and they don't feed, but as juveniles, they live as parasites that can grow inside various marine invertebrates including snails, mussels, and polychaete worms. In polychaete worms, they can grow pretty large in relation to their host, and when they reach adulthood, they bust out of the host like it's a novelty birthday cake. In that way, their life cycles are comparable to the hairworms that parasitise crickets and mantids. 

Unlike other planktonic copepods that often swim by flicking their long antennae, the antennae of adult monstrilloids are fixed, so instead they have powerful swimming legs that allow them to kick their way through the water.  And while the adult stage of these weird little crustaceans are sometimes found in plankton trawl samples, their juvenile stage are much more elusive. Out of the 195 known species of monstrilloids the parasitic juvenile stage has only been identified for seven species, since they are hidden away in the bodies of their hosts. As a result, it has been over a century since anyone has investigated those parasitic juveniles in detail.

In this study, scientists in Japan were examining pieces of sponge that had been washed up on Tancha Beach at Okinawa Island. Those sponges turned out to be home for hundreds of Haplosyllis polychaete worms, but the worms themselves were also occupied by monstrilloids. This was also the case for sponge worms from Diamond Beach on another part of the island, which turned out to be an absolute haven for the little monsters, with over half of the worms hosting monstrilloids. This abundance of monstrilloids at Okinawa Island presented an amazing opportunity for scientists to get a better look at the parasitic stage of these copepods. 

In order to find out more about these enigmatic crustaceans, scientists first had to coax the host worms out of their spongey home, and they did that by taking chunks of the sponges and kept them in water without aeration. As oxygen level dropped, the worms were forced to abandon their sponge to seek more oxygenated water, at which point they could be collected and examined under the microscope. Monstrilloids are relatively large and highly visible as the bulk of the copepod stretches out the worm's body wall to transparency. 

Among these sponge-dwelling polychaete worms, the scientists found the larvae of two monstrilloid genera - Cymbasoma and Monstrilla, the former is coloured pale pink while the latter is teal green, but only the female copepods are so eye-catching due to their ovaries. The males are colourless and transparent. These larvae also live up to the monstrilloid name - they are banana-shaped, with a single eye, enclosed in a translucent sheath, and have a pair of long feeding tubes which it uses to slurp up nutrients from the host's body. When they reach maturity, the copepod uses those same tubes to make its exit by tearing a hole through the worm's body wall. Once free of the host's body, the monstrilloid shrugs off its juvenile exoskeleton to transform into an adult and takes its place among the zooplankton. 

In order to complete its life cycle, monstrilloids have to survive in three very different environments - the open ocean as adults, the sea floor (briefly) as nauplii, and inside the body of animals as juveniles. In the words of one of the scientists who study these little monsters, they are simply an awesome group of crustaceans.

Reference:

Emelianenko, V., Savchenko, A. S., Husnik, F., & Huys, R. (2025). Live observations of endoparasitic stages of Cymbasoma sp. and Monstrilla sp.(Copepoda: Monstrilloida) utilizing sympatric sponge-associated Haplosyllis spp.(Polychaeta: Syllidae) in Okinawa. Plankton and Benthos Research 20(Spec): s235-s241.

Dolops discoidalis

The electric eel, also known as poraquê, is a formidable animal. Not only is it capable of stunning its prey with an electrifying shock, it can leap out of the water to deliver a powerful jolt to any larger animals (including humans) unwise enough to approach it. Anyone would think twice about laying a finger on one of those living tasers. And yet, there's Dolops discoidalis, a humble little fish louse which makes its living by clinging to and sucking blood from these slippery shockers.

Top left: an adult Dolops discoirdalis, Top right: A D. discoidalis on the skin of an electric eel. Bottom: A electric eel in an observation tank, with a hand wearing protective glove reaching towards it with a pair of tweezers.
Photos from Fig. 1 of the paper

Dolops discoidalis is a branchiuran - a group of ectoparasitic crustaceans which are related to the endoparasitic tongue worms. Branchiurans are commonly called "fish lice" and they cling onto the skin of their slippery hosts with all kinds of gnarly equipment. Some of them such as Argulus, have a pair of massive suckers which have been modified from the crustacean's mouthpart. But in the case of Dolops, they have a pair of stout, sickle-like hooks - and they'll need those to cling to a smooth-skinned fish like the electric eel.

A group of researchers were collecting electric eels at the Xingu River to study their physiology and behaviour when they noticed that the eels were not alone. The six electric eels they collected were all covered in little fish lice, which the researchers identified as belonging to a species called Dolops discoidalis, and it is the first ectoparasite to be reported from the electric eel. Each of those fish lice was about the size of your fingernails, and they scuttled freely over the eel's skin.

Electric eels are usually solitary animals, but sometimes they get together to hunt in packs, which may provide opportunities for these parasitic crustaceans to jump onto new hosts. Dolops discoidalis is widely distributed across the Amazon Basin, living on many different types of fishes, but it seem to have a preference for smooth scaleless fish, such as the spotted tiger shovelnose catfish.

It is unclear why D. discoidalis isn't affected by their host's powerful electrical discharges, but then again the electric eel is not the only electrical fish to host external parasites. Torpedo rays are known to harbour tiny blood-sucking isopods which are kind of like "ticks of the sea", and they also sometimes fall victim to parasitic snails that slowly creep up on those electrifying sea pancakes to suck their blood. So there might be other species of ectoparasites living on the electric eel which we don't know about yet. It's just that handling a slippery fish that can shock you with electricity can be extremely tricky, let alone trying to study the tiny things living on it.

So while being a living taser is certainly useful for getting a feed or scaring off any would-be predators, it is no deterrent to much tinier creatures which might see you as just another big, moveable feast.

Reference:

Silva, J. O. S., Sousa, L., de Paula, E. A., Takemoto, R. M., & Carvalho, L. N. (2025). First documented case of ectoparasitism in electric eel: Dolops discoidalis (Branchiura: Argulidae) infesting Electrophorus voltai (Teleostei: Gymnotidae). Parasitology International 109:103114.

Cynomorium songaricum

Deserts can be challenging environments to live in, doubly so when you are a parasitic plant that has to latch onto the roots of a specific host plant to live. Cynomorium songaricum is an endangered holoparasitic plant living in the deserts of northwest China, and it parasitises nitre bushes. Nitre bushes are known for their edible, slightly salty fruits, but C. songaricum is also prized for its culinary and medicinal value. In China, the fruits of this holoparasitic plant are known as "锁阳" and are used in traditional Chinese medicine.

Top left: A Cynomorium songaricum plant, Top right: Ants on the stem of a C. songaricum plant, Bottom left: Beetles feeding on the stem of a C. songaricum plant, Bottom right: C. songaricum seeds collected from the nest of Messor desertora ants.
Photos from Fig. 1 and 4 of the paper.

Despite its important cultural value, as is often the case with parasitic plants, very little is known about its ecology or how it propagates. Cynomorium songaricum is a root parasite, which means its dust-like seeds have to either come in contact with or at least be very close to its host's roots in order to germinate. And the roots of its host are located about three metres underground beneath the dry desert sand - so how do C. songaricum's tiny seeds reach all the way down there?

To find out, scientists from Inner Mongolia University conducted a series of studies in the eastern part of the Tengger Desert and the Badain Jaran Desert in Inner Mongolia. Over multiple days, these scientists observed the C. songaricum plants on rotating shifts during daytime and throughout the night, and when it got too cold at night to observe the holoparasites in person, remote cameras were used to keep an eye on the activities around the plants. They also collected samples from some of those plants, which were used for feeding experiments involving C. songaricum seeds and various insects. 

Like many other holoparasitic plants, C. songaricum has stinky flowers that attract flies to serve as pollinators. But when it comes to its seeds, it offers up something sweeter, which makes them attractive to hungry desert insects. And the main customers for what C. songaricum's offerings seems to be beetles and ants. The beetles eat the pulpy material around the seeds and then poop the seeds out after a day or two, which are then buried by wind. That way of reaching the host plant is a bit hit-or-miss since there's no guarantee that the seeds would be buried anywhere near the host plant's roots. But beetles are messy eaters, and in the process, they also drop some of the seeds onto the desert sand. 

That's when C. songaricum solicits help from another common desert insect. Each seed has a little fleshy tag on it called an elaisome, and it turns out this little tag attracts the attention of desert ants, which considers the elaisome to be a tasty snack. So as with all things the ants find tasty, they haul the seeds back to the larder of their nest, which works out exactly in C. sonagrisum's favour. Because it just so happens that those ants often make their homes around nitre bushes, and these nests can extend up to three metres underground - placing them right on the same level as the nitre bush's roots. So by taking the C. songaricum seeds back to the nest, the ants also inadvertently plant them in the strike zone of the host plant's roots

So that's how a parasitic plant is able to disperse its seeds across a wide, sandy desert - with the help of some little friends. To most observers, a desert may seem empty and barren. But if you take a closer look, you will find that it can be a place which is full of life and connections. 

Reference:

Wang, Z., Guan, H., Li, B., Zhang, Q., Chen, Q., Wang, D., He, K., Jin, Z., & Chen, G. (2025). Endozoochory by the cooperation between beetles and ants in the holoparasitic plant Cynomorium songaricum in the deserts of Northwest China. PloS One, 20: e0319087.

Myxobolus medusae

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.

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.