"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

July 14, 2021

Echinophthirius horridus

Lice are common parasites of mammals. Humans alone are host to three different species of lice, and it's not just humans or land mammals that can get infected with lice. Pinnipeds - seals and sea lions - also have to contend with being covered in those ectoparasites. Unlike many other ectoparasites in the sea which have been bestowed with the name of "lice" such as salmon lice, tongue-biter lice, or whale lice (all of which are crustaceans), seal lice are true lice, in that they are parasitic insects belonging to the order called Phthiraptera.

Left: An adult seal louse, Right: two opened seal lice eggs (nits) glued to a strand of seal fur hair
From Fig. 1 of the paper

When the ancestors of modern pinnipeds took to the sea some time in the Oligocene about 30 million years ago, the lice followed them into the water, and in the process, they have to deal with all the challenges associated with living on an aquatic host. Seal lice belong to a family of lice called the Echinophthiridae and they have some specialised adaptations for living on hosts that spend most of their time immersed in sea water. This include elongated spiracles (the opening insects use to breathe) with mechanism for closing, a dense covering of spines and scales, and stout clamp-like claws that allow them to grip tightly onto their hosts' fur.

Blood-sucking arthropods such as ticks, fleas, and lice are often responsible for transmitting a wide variety of parasites and pathogens. And it seems that seal lice can also play a similar role in the sea. While performing routine diagnostics on 54 harbour seals and a very heavily-infected grey seal pup that were hospitalised at the Sealcentre Piteterburn (a seal rehabilitation and research centre in Netherlands), a group of scientists were able to use that opportunity to collect a massive number of seal lice from those marine mammals. They ended up collecting 200 lice from the harbour seals, and another 1000 from that one very heavily infested seal pup.

Those researchers divided the lice into batches of 1-20 lice, based on the individual host that they came from (the lice from the heavily infected seal pups were divided into multiple batches of 15 lice), then grind them up, and examined the lice slurry by subjecting it to polymerase chain reactions that amplified the DNA of known seal parasites and pathogens.

The DNA analyses showed that the seal heartworm (Acanthocheilonema spirocauda) was the most commonly found parasite, with it being detected in about one-third of the lice samples. While most people would associate "heartworm" with the dog heartworm (Dirofilaria immitis), that species is just one out of many different filarial roundworms that live in the heart of mammals. The findings of this study corroborates with previously published research which have found heartworm larvae dwelling in the gut of seal lice, demonstrating that these ectoparasitic insects play a key role in the transmission and life cycle of these nematodes.

Alongside the heartworm, there were also some bacterial pathogens lurking in those lice. Some of the lice from the grey seal pup were also carrying Anaplasma phagocytophilum, the bacteria which causes tick-borne fever and as their name indicates, are usually carried by ticks. Additionally, a few of the lice from that seal pup and some of the harbour seals were also carrying a species of Mycoplasma bacteria. This microbe is commonly found in seals and other marine mammals, but when it gets transmitted to humans, it is also associated with a disease known as "seal fingers". However, unlike the heartworm, it is unclear if the lice actually play a role in the transmission of these bacterial pathogens, or if they were incidental infections that simply came with living on a seal host.

It is worth noting that while pinnipeds had retained an heirloom of their terrestrial ancestry in the form of lice, another group of marine mammals - the whales - have acquired their own unique suite of ectoparasites which are unlike that of any other mammals. They have "whale lice" which are actually crustaceans in the same group as sandhoppers, along with pennellid copepods - a family of parasitic copepods that usually infect fish, with the exception of one species which has evolved to parasitise whales.

So why are there no "true lice" on whales? Well, for all their adeptness at clinging to their host, lice ultimately depend on the presence of hair or similar structures to hang onto their host. When a seal dives underwater, the layer of fur forms a covering that the lice can shelter underneath. But no such shelter exists on the smooth, hair-free surface of a whale. As a result, while whales have escaped the lice (and have picked up other parasites in the process), pinnipeds have kept their fur, and along with it, their lice and the worms that they carry.


June 16, 2021

Allokepon hendersoni

Crabs have some pretty scary parasites infecting them. They range from worms that use them as vehicles to complete their complex life cycles, to parasitic dinoflagellates that turn their muscles into bitter slurry, and on top of those, there are also other crustaceans that can take over their body, and in some cases, castrate them in the process. These body-snatching crustaceans come in two main types - bopyrids and rhizocephalans. 

Top: Bopyrid isopod and an infected crab, Bottom: Rhizocephalan barnacle with infected crab.
Photos modified from the graphical abstract of the paper

Bopyrids are parasitic isopods in the same suborder as the infamous tongue biter parasite, but instead of going into a fish's mouth, they go inside the body of crabs and make themselves at home, often causing a characteristic bulge on the infected crab's carapace. And then, you have the rhizocephalans, which are freaky barnacles that have a body composed of a network of roots which wrap themselves around the crab's internal organs.

Each of them inflict their own respective flavour of pain on their crab hosts.

This study looked at the effects that these parasitic crustaceans have on the two-spotted swimming crab (Charybdis bimaculata), which is host to both parasitic isopods and barnacles. Here representing the bopyrid isopods, we have a species named Allokepon hendersoni. And fronting for the barnacles, is an as yet undescribed species of rhizocephalan. As hinted at earlier, these two body-snatcher parasites seem to have different effects on the crabs - but what exactly are they?

When scientists compared infected crabs with uninfected crabs, they found the effects to be most pronounced in male crabs, with both species of parasites causing a reduction in weight and claw size of their hosts. This is most likely due to the energetic drain associated with hosting these crustaceans, since they can grow to alarmingly large sizes when compared with their hosts. 

While reducing the claw size may leave the host crab less able to compete with uninfected males, on another hand (or claw as the case may be), it would not be in the interest of the parasite for its host to be getting into too many fights and risk injuries anyway, so it can be a beneficial side-effect from the parasite's perspective

But there were some changes which were more specific to particular parasite species. Male crabs infected with Allokepon had a narrower abdominal flap (the triangular flap on the "belly" of a crab). In male crabs, this flap would usually serve to protect the gonopods - which are specialised appendages that arthropods use in reproduction - but given the crab is already hosting such a demanding resident in its body, it wouldn't be getting up to any of that any time soon.

In contrast, the rhizocephalan barnacle had the opposite effect on the crab and widened that flap - this is part of a whole suite of changes that these parasites induce in male hosts. Male crabs that are infected by rhizocephalans develop characteristics which are associated with female crabs in both appearance and behaviour. In female crabs, the wider abdominal flap serves to cradle and brood the eggs before they hatch. So the barnacle essentially "feminise" the male crabs so that they can become better babysitters for the barnacle's offspring.

Fortunately for this crab population, infection rate was very low. Of the 2601 crabs the scientists examined, only 14 were infected with the isopod, and 21 infected with the rhizocephalan barnacle, though the isopod seems to have a preference for infecting male crabs, whereas the barnacle was less discriminate.

But if you are the unfortunate crab that gets infected, you are in for a bad time either way.

Corral, J. M., Henmi, Y., & Itani, G. (2021). Differences in the parasitic effects of a bopyrid isopod and rhizocephalan barnacle on the portunid crab, Charybdis bimaculata. Parasitology International, 81: 102283.

May 18, 2021

Anisakis physeteris

Being on top of the food chain sounds like it'd be pretty awesome - all the other animals in the ecosystem are potentially your food and nothing else hunts you. In reality, it also means that there are many parasites out there that see you as prime real estate, a nice place to settle down and start a family. And there's no way for you to avoid them since many of those parasites would be climbing their way up the food chain via the prey animals you have been eating. Nowhere is that more obvious than in the ocean.

The oceans are filled with parasites - not that you'd necessarily know since the vast majority of them are hidden out of plain sight within the body of their hosts. Many of them are parasitic worms that treat the oceanic food web like a transit system, using predator-prey interactions to get from one host to another. This post is about a study on two nematodes that cross path inside some oceanic squids

Left (a, c): Lappetascaris larva (top) embedded in squid mantle muscle, (bottom) viewed under the microscope.
Right (b, d): Anisakis physeteris larva (top) in squid testis, (bottom) viewed under the microscope
Photos from Figure 1 of the paper

A group of researchers from Italy looked at parasitic roundworms that are found in the umbrella squid and the reverse jewel squid. Both of them belong to a group of squid called the "cock-eyed squids", which are commonly found in the mesopelagic zone. The squid that the researchers examined were caught as by-catch from commercial trawling vessels that were operating off the coast of Italy and Naples, and every squid that they looked at were infected with some kind of nematode larvae. 

Most of the nematodes were of a genus called Lappetascaris, along with another species which was identified as Anisakis physeteris. While both of those parasites look superficially similar and sometimes co-infect the same squid, there are some key life history and life cycle differences between them. 

For parasites, a host is not a single homogenous entity, but a collection of different microhabitats, and each parasite species has their own taste when it comes to fine-scale real estate. In this case, the researchers found that A. physeteris mostly settled in the squid's testis whereas Lappetascaris preferred embedding itself in the firm mantle musculature (the part of squid which are sold on the market as "squid tubes").

But these worms don't just differ in the part of the squid they prefer, but also which species of squid they infect. While Lappetascaris was found in both the umbrella squid and the reverse jewel squid, A. physeteris was choosier, and was only found in the umbrella squid. Finally, the two worms complete their life cycles in totally different animals. Lappetascaris reaches maturity in the gut of large teleost fishes such as swordfish and billfish, whereas A. physeteris needs to get into the stomach of a sperm whale - as denoted by its species name (the genus name for sperm whale is Physeter).

This may explain why A. physeteris was only found in the umbrella squid. Compared with the reverse jewel squid, umbrella squid venture into much deeper water which overlaps with the sperm whale's usual hangouts. And this exposes them to infective stages that are being released from sperm whales which have hundreds and thousands of adult Anisakis worms in their gut.

While the popular perception of the sperm whale often depict them as duking it out with the giant squid, the majority of their diet is composed of more modestly sized cephalopods, and the umbrella squid seems to form a major part of their diet. That's not to say umbrella squid is not on the menu of other large oceanic predators like swordfish and billfish too (hence it is also infected by Lappetascaris larvae), but if you are a parasite that is looking for the ideal ride to get you into the belly of a sperm whale, you can't do much better than the umbrella squid. 

What about the Lappetascaris which are sharing that squid with A. physeteris? Well they better hope a swordfish would come along and snatched it up before it ends up in the belly of a marine mammal - an environment that it is ill-equipped to live in.

So while these two worms may sometimes meet in the same squid, they eventually have to go their separate ways - and reaching their respective final hosts would unfortunately spell doom for the other worm in the shared squid. As for the sperm whales, a belly full of yummy squid must inevitably lead to a stomach full of wriggly worms.


April 21, 2021

Pterobdellina vernadskyi

If you've ever been out hiking in the wilderness, you would know that there is no shortage of tiny animals out there that love nothing more than to feast on your blood. They range from mosquitoes to midges, from fleas to ticks, and of course, let's not forget about leeches - a group of animals so synonymous with blood-sucking that its name is also used as a term for exploiting the life blood of others.

But leeches aren't just found out in the bushes, there are hundreds of species of blood-sucking leeches that are actually aquatic, feeding mostly on amphibians and fishes. In fact, spare a thought for the fishes, which have a whole family of jawless leeches called Piscicolidae that are after their blood.

Left (top) Antarctic toothfish with P. vernadskyi leeches on its body surface, and (bottom) in its mouth. Photos by Gennadiy Shandikov from Fig. 1 of the paper.
Right: Two live specimens of P. vernadskyi (the left leech has a spermatophore in its clitellum) from Fig. 2 of the paper
For fish, there is no escape from these leeches as they are found in a wide range of aquatic habitats, ranging from freshwater to the marine environment. They target a wide range of hosts, from trout and carp, to rays and sharks. Even in the inky depths of the deep sea, there are hungry leeches waiting for a tasty fish to swim by, and it is one of these deep sea leeches that is featured in today's post.

This post is about a newly described species of fish leech - Pterobdellina vernadskyi - which has been found parasitising the Antarctic toothfish (Dissostichus mawsoni) in the cold dark waters of Antarctica. The researchers who described this species collected them from fish that were caught by longline commercial fishing vessels - Antarctic toothfish are highly valued on the commercial market, where they are often sold alongside the Patagonian toothfish as "Chilean Sea Bass".

While most of the fish that the researchers encountered only had one or two leeches, some were afflicted with ten or more, and one unlucky fish was covered in twenty eight leeches all over its body. They tend to favour attaching to either the dorsal surface of the fish, or inside the mouth, where they are more sheltered. And P. vernadskyi can grow rather large compared to other fish leeches, reaching about 8 cm in length - so roughly the size of your finger.

Aside from its sheer size, another thing that differentiates it from other leeches are series of distinct, zig-zag ridges along its back and fin-like projections along its sides. It is not entirely clear what purpose those structures serve for the leech, though there are other deep sea ectoparasites which also have some unusual external structures. The researchers suggested that perhaps they serve some kind of sensory function that allows to leech to find their host, or they might be adaptations to the low oxygen levels of its environment, increasing the leech's surface area so it can absorb more oxygen from the surrounding waters.

In additional to those external features, it is worth mentioning that this leech's host, the Antarctic toothfish, is notable for producing antifreeze glycoproteins in its blood, which allows it to dwell in such frigid waters. But this additive would surely have some implications for the digestive system and physiology of P. vernadskyi compared with other fish leech that feed on hosts with more conventional blood.

Since the Antarctic toothfish can be found dwelling as deep as 2600 m below sea level, this would make P. vernadskyi the deepest Antarctican leech that has ever been recorded. However, it is NOT the deepest depths that a leech has ever ventured. That title goes to Johanssonia extrema which has been found in the hadal zone over 8700 m below sea level in the Kuril–Kamchatka Trench, where the waters are still and the pressures are crushingly immense.

Pterobdellina vernadskyi is just one out of two dozen different species of fish leeches that have been recorded from Antarctica, and there are a number of other leeches which have been reported from deep sea habitats. It would be safe to say that P. venadskyi, and other marine leeches that have been described in the scientific literature, represents only the tip of the iceberg. Where there are fish, there are leeches.

Utevsky, А., Solod, R., & Utevsky, S. (2021). A new deep-sea fish leech of the bipolar genus Pterobdellina stat. rev.(Hirudinea: Piscicolidae) parasitic on the Antarctic toothfish Dissostichus mawsoni (Perciformes: Nototheniidae). Marine Biodiversity 51: 15.

March 18, 2021

Elicilacunosis dharmadii

Tapeworms are found in the guts of every class of vertebrate animals. And even though the tapeworms that most people are familiar with infect terrestrial animals - such as the beef tapeworm and pork tapeworm which both infect mammalian hosts for each stage of their respective life cycles - the true ancestral home of these parasites are actually elasmobranch fishes (sharks and rays). And it is within those cartilaginous fishes that we find tapeworms with some of the most interesting adaptations found among parasitic worms.

This post is about a new study on some tapeworms living in the guts of two species of eagle rays from opposite sides of the globe. Though they are separate by vast geographical distance, they both have one very special feature in common.

Top: SEM photo of Tapeworm Elicilacunosis dharmdaii, Bottom Left: SEM close-up of a Caulobothrium multispelaeum proglottid, Bottom Right: SEM close-up of C. multispelaeum mid-body, showing the bacteria-harbouring grooves.
Photos from Fig 1 and 2 of the paper

Elicilacunosis dharmadii is a tapeworm living in the gut of banded eagle rays (Aetomylaeus nichofii) which can be found off the northern coast of Borneo. For all intents and purposes, it's a pretty standard looking tapeworm, with a scolex (the attachment organ) armed with suckers, followed by a body composed of a chain of segment-like reproductive organs called proglottids. But in addition to those default tapeworm features, it also has a long, deep groove running along the length of its larger, more mature proglottids which makes them look kind of like tiny hotdog buns.

And the grooves are not merely simple slits on the tapeworm's body - the edges of the grooves are covered in microscopic, finger-like projections which extend to the inner cavities as well, lining the sides like layers of shag carpet. And nestled snugly amongst the strands of these microscopic, tapeworm-borne shag carpet are colonies of bacteria. In fact those grooves are filled with so much bacteria that they are practically spilling over the edges.

But E. dharmadii is not the only tapeworm living out its life with pockets full of microbes - on the other side of the globe, there is another, unrelated species of tapeworm which has also evolved these groovy bacterial hot pockets. Caulobothrium multispelaeum is a tapeworm which is found in the gut of duckbill eagle rays (Aetomylaeus bovinus) from the waters of Senegal in the eastern Atlantic Ocean. Much like E. dharmadii, there is a bacteria-filled groove running along its body, but the grooves of C. multispelaeum are even deeper and more pronounced.

Though both of these tapeworm share this unique feature, they actually belong to entirely different orders - E. dahmadii is in the Lecanicephalidea order while C. multispelaeum is currently assigned to the "Tetraphyllidea" order - a mixed bag of tapeworms known for having varied and uniquely shaped scolex structures. They also carry different type of bacteria as well - E. dharmadii carries spherical, coccoid-type bacteria whereas C. multispelaeum hosts rod-shaped, bacilliform bacteria.

The researchers who observed this symbiosis suggested that this partnership may have come about because the bacteria is able to digest the tapeworms' metabolic by-product, and in turn produce enzymes that help break down carbohydrate and protein in the ray's gut content, making them easier for the host tapeworm to absorb. So how do these tapeworms recruit their bacterial pals in the first place?

Given that tapeworms live in the digestive tract of vertebrate animals - an environment that is filled with all sorts of bacteria in great abundance - it is most likely that the tapeworms pick the bacteria for their starter culture from what's around them when they initially enter into the host's intestine.

This would make them comparable to the symbiosis that Hawaiian bobtail squids have with their symbiotic bioluminescent Vibrio fischeri bacteria. Previous studies have shown that when the squid is still a hatchling, it has to choose the right bacteria from among the plethora of different bacteria floating in the surrounding waters. But once the right bioluminescent bacteria has been selected, this starter culture of bacteria in turn also influences the development of the light organs which house them. Perhaps it is possible that the bacteria also do something similar in the development of those grooves on the tapeworm's body.

Okay, all of the above sounds really neat - but why does it exist though? No other known tapeworms have these peculiar bacteria pockets, and this feature is not even found in other species which are closely related to these bacteria-packing tapeworms. And these two tapeworms have independently evolved their bacterial partnerships on their own. The only other thing they have in common is that they both infect eagle rays - is there something about living in eagle rays that lead to tapeworms evolving this feature?

While most people would think of tapeworms as being quite large parasites since some of the human-infecting species such as the broad fish tapeworm and the beef tapeworm can reach up to 10 metres in length, these eagle ray tapeworms are actually quite small. The adult worms grow to only 0.5 to 3.5 millimetres in total length, and are some of the smallest known tapeworms found in elasmobranch fishes. So maybe because they are so tiny, they need some help from bacteria to obtain sufficient nutrients? But then again, there are also other tiny tapeworms living in eagle rays that don't have such partnerships with bacteria.

There are certainly a lot of unanswered questions posed by these two little tapeworms, and in fact, that's the case for the vast majority of these marine parasites. Out of over a thousand species of tapeworms which have been described from sharks and rays, the full life cycle has only been described for FOUR of them. Compared with the handful of tapeworm species which are of medical and economic importance, the ecology and evolutionary adaptations for the vast majority of these parasites are still poorly known and not well-understood. 

It is a vast wormy world out there, with many mysteries left unsolved.


February 15, 2021

Endovermis seisuiae

Polychaete worms are common in the marine environment, living in just about every habitat ranging from the seashores, to the open ocean, the deep sea, next to boiling hot hydrothermal vents, or even on mounds of methane ice. The type of polychaete worms which most people are familiar with are beachworms and sandworms that live inside sand or mud burrows on the seashore, and are often collected by anglers who use them as bait for fishing. But the polychaete worm that is featured in today's post does not live in sand burrows - instead, it has evolved to live inside another polychaete worm, wearing them almost like someone wearing a mascot costume.

Endovermis seisuiae inside its scaleworm (Lepidonotus sp.) host (from Fig. 1 of the paper)

Endovermis seisuiae is very appropriately named since "Endovermis" basically means "inside worm". There are only 19 other species of polychaete worms that are known to have evolved this macabre life-style, and most of them belong to either the Oenonidae family or the Dorvilleidae family. But Endovermis hails from the Phyllodocidae family, a group of polychaete worms which are mostly free-living predators, or dwell in tubes which have been vacated by tubeworms.

But Endovermis has taken this lifestyle to a truly galaxy brain direction  - why settle for living in a tube created by another polychaete worm, when you can live inside the polychaete worm itself? The hosts of this parasitic polychaete are scaleworms, which are polychaete worms known for having iridescent scales. In this study, the researchers found E. seisuiae living inside of two species - Aphrodita sp. and Lepidonotus sp. - both were located at over 200 metres below sea level off the coast of the Wakayama Prefecture in Japan.

Endovermis can grow alarmingly large in comparison with its host. The two parasitised scale worms which the researchers found were 14 mm and 27 mm long, while the Endovermis living in each of them grew to 13 mm and 21 mm long respectively (depending on the host species). In both scale worms, Endovermis grew to be about as long as the host itself, though the scaleworm hosts have wider bodies than the parasites. So it is a very cosy fit for the parasite, and it takes up substantial room in the host. In fact, those scaleworms caught the researchers' attention in the first place because they noticed something squirming around inside their body cavity. This size parity between Endovermis to its scaleworm host would be like if you find out that there is a whippet living inside the body of a greyhound. 

So how does a worm like that get inside a host which isn't that much bigger than itself? There were no obvious scars on the body of the scaleworm as you would expect if a full-size Endovermis had simply tunnelled its way into the host's body. Since Endovermis produces tiny eggs which are only about 0.1 mm wide, the researchers suggested that it might enter the host as a microscopic larva, drifting into their body via the nephridial canals - which are the equivalent of kidneys in some invertebrate animals. Once inside, it would sit in the body cavity, feeding on the host's body fluids or even internal organs, and eventually getting to be almost as big as the host itself.

In nature, sometimes you get surprise bonus content for a worm - which is also another worm. Simply more worm for your worm.

Jimi, N., Kimura, T., Ogawa, A., & Kajihara, H. (2021). Alien worm in worm: a new genus of endoparasitic polychaete (Phyllodocidae, Annelida) from scale worms (Aphroditidae and Polynoidae, Annelida). Systematics and Biodiversity 19: 13-21.

January 21, 2021

Pseudoacanthocephalus toshimai

Parasites with complex life-cycles often use predator-prey interactions to facilitate their transmission. They have larval stages which infect the body of prey animals, where they wait to be eaten by predators that act as the parasite's final host. But the thing about relying on such interactions to reach their destinations, is that they don't always end up where they are supposed to.

Left: Adult P. toshimai in a fish's gut, Centre: Adult P. toshimai in a frog's gut, Right: Larval P. toshimai from a woodlouse
Photos from the graphical abstract of the paper

Pseudoacanthocephalus toshimai is a thorny-headed worm which is found in Hokkaidō, in the northern part of Japan. The adult stage of this parasitic worm usually infects amphibians such as the Ezo brown frog and the Ezo salamander, while the larval stage parasitises a species of woodlouse called Ligidium japonicum. While it is primarily an amphibian parasite, P. toshimai is sometimes also found in a range of stream fishes. So how does an amphibian parasite end up in the belly of a fish? 

A pair of researchers from Asahikawa Medical University conducted a survey on the prevalence and abundance of P. toshimai at the mountain streams of the Ishikari River around the Kamikawa basin. They caught both fish and amphibians, and examined their guts for the presence of P. toshimai. Of the 174 stream fish that they caught, 56 were infected with P. toshimai, all of them were salmonids and were all from one specific stream. The infected salmonid species included the iwanaDolly Varden troutmasu salmon, and rainbow trout.

While P. toshimai appears to be fairly common among those salmonids, they were only present in relatively low numbers. On average, each fish was infected with only two or three worms, and none of the female worms carried any eggs. In contrast, the researchers found the parasite to be much more abundant in amphibians. About two-thirds of the salamanders in their sample were infected with P. toshimai, with an average of about four worms per host. Additionally, all the frogs that they examined were infected, with each frog harbouring an average of about five worms. The highest number of worms recorded from a single host was a salamander which had 22 P. toshimai in its gut. Furthermore, all the female worms in those amphibians were brimming with mature eggs, all ready to go.

So while the fish's gut is a hospitable enough environment for the parasite to grow into an adult worm, it is lacking a certain je ne sais quoi that the female worms need to start producing eggs and complete the life-cycle. It is not entirely clear what exactly that might be - it could be that the fish's gut does not produce the right type of nutrients for egg production, or there is simply not enough mating opportunities for the parasite in the gut of a fish - since they are not as commonly nor heavily infected as the amphibians. Either way those salmonids are ultimately dead-end hosts for P. toshimai. So how are the worms ending up in those fish in the first place?

This is where we have to consider the other animal involved in the parasite's life-cycle which is the woodlouse. Woodlice - also known as slaters - are terrestrial crustaceans commonly found under rocks and among leaf litter. As mentioned above, P. toshimai uses a species of woodlouse as intermediate host, where their eggs develop into larval stages known as cystacanths. Since those crustaceans are commonly eaten by frogs and salamanders, they also act as a vehicle to transport the parasite to its final host.

The researchers noticed that P. toshimai is only ever found in fish from one particular stream which is surrounded by bushes. These bushes are habitats for woodlice and amphibians which are the usual hosts for P. toshimai, and provide the necessary conditions for the parasite to complete its life-cycle. But every now and then, instead of getting eaten by a frog or a salamander, an infected woodlouse would fall into the stream, and become a tasty snack for a hungry fish. Indeed, the researchers did find a few woodlice in some of the fishes that they caught. 

This study shows that for parasites with complex life-cycles, things don't always work out the way that they are supposed to. Even when all the necessary condition are present and accounted for, once in a while, your intermediate host might get knocked into a stream, and you end up in the belly of a fish.

Nakao, M., & Sasaki, M. (2020). Frequent infections of mountain stream fish with the amphibian acanthocephalan, Pseudoacanthocephalus toshimai (Acanthocephala: Echinorhynchidae). Parasitology International 81: 102262.