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.