"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

April 11, 2024

Anoplocephala gorillae

Tapeworms are found in all kinds of vertebrate animals, and while their life cycles and transmission usually rely upon parasitised prey being eaten by predatory final hosts, some tapeworms have evolved ways to infect herbivorous animals as well. Anoplocephala is a genus of tapeworms that parasitise a wide range of herbivorous mammals including elephants, rhinos, hyrax, zebras, and more. The most well-studied species is Anoplocephala perfoliata because it happens to be a parasite of horses, and heavy infection with that tapeworm can cause gastrointestinal diseases. But the species featured in this post are found in a close relative of humans, specifically the Mountain Gorilla (Gorilla beringei beringei), and its name is Anoplocephala gorillae.

Left: Anterior of four Anoplocephala gorillae with their scolices (attachment organ) visible. Right: Proglottids (reproductive segments) of Anoplocephala gorillae collected from faecal samples.
Photos of the parasite from Figure 2 and Figure 4 of the paper

This post is about a study which took place at the Volcanoes National Park (VoNP), in the Rwandan part of the Virunga Massif - a complex of protected areas spanning the borders of Rwanda, Uganda and the Democratic Republic of the Congo (DRC). The aim of the study was to examine the epidemiology of tapeworms in mountain gorillas, and to improve the diagnostic tools for detecting such parasites. To do so, researchers examined faecal samples which were collected by park personnel and Gorilla Doctors veterinarians from groups of habituated gorillas in the VoNP. Whenever possible, each of those samples were identified to specific gorilla individuals, allowing veterinarians to keep track of each gorilla's health and parasite status.

Researchers estimated the abundance of tapeworms in the gorillas by counting the number of tapeworm eggs in each gram of faeces. Generally speaking, more eggs means more worms, but egg production varies between individual worms at different times, so multiple samples needed to be taken to ensure a more accurate count. Out of the 1500 samples they examined, about seven percent had egg counts of over a thousand eggs per gram of faeces, though the average was much lower at 384 eggs per gram. While A. gorillae seems to dominate the tapeworm fauna of these gorillas, the faeces of one gorilla, an infant male named Inkingi, also had another tapeworm species in the genus Bertiella. It is relatively easy to distinguish the eggs from those two different tapeworms - Anoplocephala has quadrangular or triangular-shaped eggs with flat sides and thick shells, whereas Bertiella has spherical eggs with thin shells

In addition to those faecal samples, any gorillas that had died were retrieved from the wild and necropsied as a part of the local veterinary surveillance program. For the purpose of this study, five deceased gorillas that were recovered between 2015–2018 were necropsied and examined for tapeworms. In total, 53 A. gorillae tapeworms were collected, and they varied in size from 1.5 to 13 cm long. Most of them were found in the small intestine, but there were also some in the caecum and colon.

So how do the gorillas end up with all those tapeworms in the first place? While the eggs are released into the environment packaged in the gorilla's faeces, they cannot infect the gorillas directly. Like other tapeworms, they have to go through an intermediate host, which as mentioned earlier, is usually a prey animal. But since gorillas are herbivores, how can tapeworms gain entry into their guts? 

Based on what is known for other Anoplocephalidae tapeworms, gorillas become infected by swallowing mites that are parasitised by the tapeworm's larvae. These mites are tiny, barely pinhead-size, thus can be easily swallowed among a mouthful of foliage. While the prevalence of Anoplocephala among mites might be extremely low, like other herbivorous mammals, gorillas go through a lot of plant matter, eating 18-45 kilograms of vegetation a day. So just a few infected mite sprinkled in would be enough to ensure that the gorillas get infected,

While the deceased gorillas that were necropsied in this study had large numbers of tapeworms dwelling in their gut, they were all in good condition, and had died from other sources of trauma rather than disease. So in contrast to A. perfoliata which can cause major pathologies in horses, A. gorillae is content with a more peaceful existence, just living quietly as a part of the gorilla's regular gut symbiont fauna.


March 9, 2024

Veneriserva pygoclava

There are many ways to become a parasite, and there are parasites with vastly different ancestries that end up joining the same path on the road of parasitism. In some cases, sharing the same path can also mean adopting a certain shape. This post is about Veneriserva pygoclava, a worm that lives inside a worm, more specifically it is a polychaete worm that has evolved to parasitise another type of polychaete worm which are commonly called "sea mice".

Top left: Ventral view of an infected Aphrodita longipalpa with a Veneriserva pygoclava parasite inside. Bottom Left: MicroCT scan image of an Aphrodita longipalpa with Veneriserva pygoclava female highlighted in yellow and juvenile highlighted in blue. Right: A female Veneriserva pygoclava (top) and a male (bottom).
Photos from Fig. 1 and Fig. 3 of the paper

The genus name of this parasitic polychaete translates into "Venus' servant" though this worm is certainly a servant for nobody but itself. You'd think that living inside the body of another animal would restrict how big it can get, but the female Veneriserva grows to about seven centimetres long, which is twice as long as its host. Surprisingly enough, being longer than the host is not unusual among these kinds of parasitic polychaete worms. Despite its size and the amount of space it occupies within the host, it does not seem to cause any injuries or damage to the host's internal organs.

Living this endoparasitic lifestyle requires some specialised adaptations, and over the course of its evolution, Veneriserva has ended up with a body plan which is very similar to that of tapeworms. Despite both being called "worms", tapeworm and polychaete worms are from entirely separate animal phyla and their path to this "tapeworm body plan" (for the lack of a better term) were very different.

Tapeworms evolved from free-living flatworms, which are fairly simple animals, at least in terms of their body plan. A flatworm has no body cavity, its gut is more or less a blind-end sac (with some branches in larger flatworms), and it doesn't even have a circulatory system. If anything, in order to adapt to a parasitic lifestyle, tapeworms have evolved to become more complex than their free-living ancestors. Over the course of the tapeworm's evolution, they have gained a new attachment organ - the scolex - which is a heavily modified head, while the rest of the body has become an efficient conveyor belt of reproductive organs. These parasitic flatworms have even evolved a brand new type of "skin" called a tegument which allows it to absorb nutrients as well as protect itself against the host's enzymes, and some tapeworms even have the most complex central nervous system among all the flatworms, enabling them to navigate and maneuver in the dark, fleshy tunnels that are their host's intestinal tract.

In contrast, polychaetes are segmented worms, and are actually more similar to us in their body plan, equipped with a full body cavity, muscular gastrointestinal tract, and a closed circulatory system with blood vessels. But Veneriserva has abandoned much of that, because when you're living inside another animal, being built like a tapeworm seems to be the way to go.

Veneriserva does have a mouth of sorts, but it is not connected to any digestive tract to speak of. In fact, the digestive tract has been reduced down to a throat with a blind-end. Instead, the mouth of Veneriserva serves as a grabber to hold the parasite in place, functioning much like a tapeworm's scolex. Additionally, Veneriserva has also evolved its own version of the tapeworm's tegument, which is covered in fine microscopic finger-like projects (rather like the lining of your small intestine, just inside out) allowing it to absorb nutrients through its skin. There are also patches of cilia on the skin which may serve to stir the host's bodily fluids in order to bring more nutrients into contact with the parasite's skin.

However, when it comes to sex, there is one key difference between Veneriserva and tapeworms. Tapeworms are hermaphroditic - any tapeworm can mate with any other individual of the same species, or even with itself if it is desperate and alone. In contrast Veneriserva have separate female and male sexes which are clearly distinguishable - male worms are tiny compared to their much larger partners (see accompanying photo).

This "attachment organ + loads of gonads" type of body plan that tapeworms and Veneriserva have both independently evolved is also found in other internal parasites. For example, acanthocephalans - thorny-headed worms - are parasitic worms which live in the gastrointestinal tract of vertebrate animals, and are somewhat related to rotifers. Despite being in a different phylum, they share some key anatomical similarities to tapeworms, with their own version of the tegument, a body dominated by gonads, and a prickly anchor at its "head" to stay attached to the host's intestinal wall. Another example is Thyonicola, the parasitic snail which uses a thin stalk to attach itself to the intestines of its sea cucumber host, while the rest of the body is simply a long tube of reproductive organs and developing eggs. There are even some parasitic dinoflagellates that have evolved to resemble tapeworms. 

Judging from how common this "tapeworm-style" anatomy is across different parasite groups, it seems that when you are an internal parasite, you have to get into shape - and that shape happens to be that of a tapeworm.


February 12, 2024

Ascarophis globuligera

To land-dwelling humans, deep sea hydrothermal vents would seem like a vision of hell, amidst the crushing darkness you have plumes of superheated water, mixed with noxious sulfides, erupting from fissures on the seafloor. But for many deep sea animals, this "hell" is in fact a vibrant oasis in the middle of the abyss. This lively habitat is made possible thanks to bacteria that are able to extract energy from the sulphurous waters billowing from those vents. In the absence of sunlight, these chemoautotrophs form the foundation of the food chain. Some tube worms have been able to co-opt the power of these bacteria by housing the microbes in their gills, enabling them to grow to enormous sizes. Their tubes form dense, forest-like habitats for many other animals including other polychaete worms, fishes, crustaceans, and molluscs. This sets the stage for all kinds of complex ecological interactions, and that includes parasitism.

Left: Anterior of Ascarophis globuligera from Fig. 6 of the paper, Right: Photo of Thermarces cerberus (pink vent fish) by Dr Lauren Dykman, used with permission 

This post is about a paper reporting on three newly described species of Ascarophis nematodes that have been found in the guts of some deep sea hydrothermal vent fishes. Some of those worms were collected as a part of a larger study which focused on looking for parasites from hydrothermal vent animals, and along with freshly caught specimens, the researchers also examined preserved fishes collected by past expeditions. While they only managed to recover a few specimens of Ascarophis nematodes, some of which were fragmentary, those were enough to provide a scientific description for three different species - A. justinei, A. globuligera, and A. monofilamentosa.

The three species differed slightly in which fish species they infect - A. justinei is found in both the pink vent fish and a species of viviparous brotula, whereas A. globuligera has only been found in the pink vent fish, and A. monofilamentosa lives in a species of zoarchid fish named Pyrolycus manusanus. While it is not possible to conduct experimental infections to work out exactly how these nematodes transmit between hosts, their life cycles can be inferred based on what is known about other Ascarophis species which are found in shallower waters. This usually involves a crustacean, often amphipods, serving as the intermediate host for the parasite's larvae. Amphipods are plentiful around hydrothermal vents, and these crustaceans are eaten by a range of animals including deep sea fishes such as the pink vent fish, making them the ideal vehicle for Ascarophis to complete its life cycle.

The need for Ascarophis to reach an amphipod host may explain why each of those Ascarophis species has differently shaped eggs. For example, A. justinei has eggs which are regular, ovoid shape rather similar to other known species of Ascarophis, but the eggs of A. globuligera have a bulge on their side (which gave the species its name), and A. monofilamentosa eggs have a long filament dangling from them which is about fifteen times the length of the egg itself. These differently shaped eggs could mean slightly different transmission strategies. The extra ornament on the eggs of A. globuligera might serve to entice a hungry amphipod by resembling something edible (as with some tapeworm eggs that infect crustaceans by mimicking diatoms), or in the case of A. monofilamentosa, its long filament may prevent the eggs from drifting away into the empty abyss by wrapping them around a structure, or entangle them around something which might get eaten by an amphipod.

Some Ascarophis species are actually known to take a shortcut with their life-cycles -  instead of waiting for a fish host to come along, they become sexually mature and start laying eggs inside the amphipod, bypassing the need to enter a fish host to complete their life cycles. It is unknown whether any of the newly described deep sea species are capable of doing this, but in an ephemeral habitat like hydrothermal vents, it would be useful to have such an option as insurance.

There are many biomes on this planet which are completely inhospitable to humans. But that does not stop them from being as rich and vibrant as those that we are more familiar with, and wherever there is a thriving ecosystem, you will find parasites taking part in its web of interactions.

Moravec, F., Dykman, L. N., & Davis, D. B. (2024). Three new species of Ascarophis van Beneden, 1871 (Nematoda: Cystidicolidae) from deep-sea hydrothermal vent fishes of the Pacific Ocean. Systematic Parasitology 101: 2.

January 7, 2024

Prosthogonimus cuneatus

Prosthogonimus is a genus of flukes that live in a special part of a bird's anatomy. It is usually found either in the Bursa of Fabricius, an organ that only birds have, or in the oviduct, and it's this latter location which lend this parasite its common name, the oviduct fluke. This fluke is found all over the world in many different species of birds, and while it doesn't seem to cause much issues for wild birds, it presents a major problem for the poultry industry.

Left: Dragonflies Sympetrum vulgatum (top) and Sympetrum depressiusculum (bottom), Right: A metacercaria cyst of Prosthogonimus cuneatus. Photos from Fig. 2 and Fig. 3 of the paper

Since this fluke lives by clinging to and feeding on the surface of mucosal membranes, its activities can leave lesions and cause inflammations, and heavy infection of Prosthogonimus can lead to all kinds of oviduct disorders in chickens. This includes leaking milky discharges from the cloaca, laying soft-shelled or malformed eggs, or even egg peritonitis, where egg yolk material gets displaced into the hen's body cavity, leading to secondary infections and death. In some cases, the fluke can even end up getting bundled into the egg itself, which seems pretty mild compared with what I have mentioned above, but it would nevertheless be a nasty surprise for anyone looking to make an omelette. To make matters worse, there are currently no effective treatments available for getting rid of this fluke once a bird is infected.

So how do birds end up with this peculiar parasite? Prosthogonimus has a multi-host life cycle that takes it across three very different animals - freshwater snails, dragonflies, and birds. In the dragonfly, the larval Prosthogonimus lies in wait as a dormant cyst called a metacercaria, waiting for its host to get eaten by a bird. That is why this parasite is usually associated with free-range chickens, as they have more opportunity to feed on a variety of things. Most studies on Prosthogonimus have focused on the effects it has on the bird hosts, but surprisingly fewer studies have investigated the source of the infection - parasitised dragonflies.

To rectify this oversight, a group of researchers undertook a truly herculean effort to investigate the presence of Prosthogonimus in dragonflies from the Heilongjiang province, China. The researchers collected over TEN THOUSAND dragonflies, composed of 12 different species from 41 locations. They identified each of the dragonflies before dissecting them for Prosthogonimus metacercariae, which are usually located in the abdominal muscles. The researchers noticed that infected dragonflies tend to have softer abdominal muscles, possibly due to injuries caused by the presence of the Prosthogonimus cysts.

They found three different species of Prosthogonimus in those dragonflies, of which Prosthogonimus cuneatus was the most common. Overall, about 20% of the dragonflies they examined were infected by Prosthogonimus, but it was more common in some species than others. The spotted darter (Sympetrum depressiusculum) was most frequently infected (28.53% prevalence), followed closely by the vagrant darter (Sympetrum vulgatum) (27.86% prevalence) and the autumn darter (Sympetrum frequens) (20.99% prevalence). The highest number of fluke larvae in a single dragonfly goes to an unlucky Sympetrum kunckeli which was packed with 157 Prosthogonimus metacercariae in its abdomen.

But dragonflies are aerial predators - how do they end up being infected with fluke larvae which are shed from freshwater snails? Well, before becoming acrobatic flying hunters, dragonflies spend their early life as underwater predators. But this aquatic life also expose them to Prosthogonimus' waterborne larvae, which are drawn into the dragonfly nymph's body through its respiratory current - in other words, they get sucked through the dragonfly nymph's butt whenever it takes a breath. Even as the dragonflies metamorphose into airborne adults, they carry the legacy from their youth in the form of Prosthogonimus cysts

Overall, the study found that Prosthogonimus was most common in Heihe, which might be due to the presence of large wetlands in the area. Those wetlands are home to high levels of biodiversity which help support the life cycle of this parasite - they provide habitats for numerous snails that can host the asexual stage of Prosthogonimus, along with wild birds that would usually act as the final host for this parasite. Just add dragonflies, which are always common around water bodies, and the circle of life is complete for Prosthogonimus.

Studying and elucidating the life cycles and ecological role of parasites in their natural context is an important part of disease ecology research. Understanding what these parasites actually do in nature can help us prevent them from causing problems in the animals that we raise.

Li, B., Lan, Z., Guo, X. R., Zhang, A. H., Wei, W., Li, Y., Jin, Z. H., Gao, Z. Y., Zhang, X. G., Li, B., Gao, J. F., & Wang, C. R. (2023). Survey of the Prosthogonimus spp. metacercariae infection in the second intermediate host dragonfly in Heilongjiang Province, China. Parasitology Research 122: 2859-2870.

December 14, 2023

Euglenaformis parasitica

This parasite is invisible to the naked eye, can kill its host in 3 days, and it can be found lurking in the waters of rice fields. What I have just described may sound like a nightmare pathogen from a b-horror movie, but it is actually a microscopic flagellated protozoan that has given up a solar-powered life for one fuelled by the blood of its victims. The name of this microscopic monster is Euglenaformis parasitica, and it belongs to a group of otherwise innocuous single-celled critters called Euglenids.

Left: Scanning electron micrograph of Euglenaformis parasitica. Top Right: E. parasitica extracted from an ostracod host. Bottom Right: E. parasitica visible in the appendage of an infected ostracod.
Photos from Fig. 1 and Fig. 7 of the paper

Euglenids are single-celled flagellated organisms often found in freshwater. The most well-known and well-studied genus is Euglena, which is literally the textbook example of the group, appearing in many biology books as an example of a single-celled eukaryote organism. Many euglenids are photosynthetic, and historically they have been treated as sharing affinity with plants (due to their photosynthetic capabilities) or with animals (due to their active flagellum and being able to take in nutrient via heterotrophy), before getting shunted into a group called the "protist" which is just a jumble of different organisms that scientists couldn't classify into plants, animals, or fungi.

Euglena and its kin are mostly free-living, photosynthesizing when the sun's out, absorbing organic matter from the environment when it's dark. But the ancestor of E. parasitica was not content with this mostly peaceful lifestyle, and has evolved to live inside the body of animals. These euglenids were found parasitising ostracods (also known as seed shrimps) and flatworms in a rice field in Ibaraki, Japan.  Ostracods and flatworms belong to two entirely different phyla of animals, and most parasites that infect different phyla of host animals do so at different stages of their complex, multi-stage life cycles. But E. parasitica has just a simple life cycle, which makes it quite remarkable that it is able to adapt to the very different internal environments presented by ostracods and flatworms.

When E. parasitica is in an ostracod, it dwells in the body cavity, swimming in the hemolymph and bathing in its nutrients. Whereas in flatworms, since they don't have any body cavities to speak of, E. parasitica lives in the space between the spongy tissue that forms the bulk of a flatworm's body, burrowing between the cells of the parenchymal tissue. But whether it is in an ostracod or a flatworm, once E. parasitica establishes itself in the host's body, it starts absorbing the literal lifeblood of their host, using it to fuel its exponential growth as it divides and conquers from within. 

What started with just a single or a few E. parasitica soon turns into a swarm. This is particularly noticeable in flatworms - uninfected flatworms are semi-translucent, but infected flatworms darken in colour as their body becomes filled with brownish to blackish granules which are actually rapidly dividing E. parasitica. The same goes for ostracods as their blood becomes saturated with the parasite's progenies. After three days, the insides of the host are completely consumed by the swarm of E. parasitica, which proceeds to exit into the surrounding water, leaving behind an empty husk.  

There are still a lot of mysteries surrounding this flagellated organism, such as how it is able to make use of such radically different hosts as flatworms and seed shrimps, or how it enters the host's body in the first place. Does it somehow bore through the body wall, or perhaps it tricks the host into eating it, and then burrows through the digestive tract to other parts of the body? If so, it won't be the only parasite to use that trick. There are also questions about its evolutionary origin. Euglenaformis parasitica's close relatives are photosynthetic euglenids, so what made it abandon a solar-powered life in favour of living and reproducing in the bodies of small aquatic animals? Understanding that process would provide us with another clue as to how various different organisms ending up following the path of parasitism.

Kato, K., Yahata, K., & Nakayama, T. (2023). Taxonomy of a New Parasitic Euglenid, Euglenaformis parasitica sp. nov.(Euglenales, Euglenaceae) in Ostracods and Rhabdocoels. Protist 174: 125967.

November 14, 2023

Stylops ater

Strepisptera is an order of parasitic insects with some very unique characteristics.They are also known as twisted wing parasites, based on the twisted hindwings on the male parasite. They infect many different orders of insects, but mostly target wasps and bees where they up take up residency in the host's abdomen. If you know what to look for, you can immediately spot their presence. In fact, there's even a special term for describing bees and wasps that are parasitised - they get "stylopized".

Top: A male Stylops ater (indicated by red arrow) attempting to mate with a female in a bee's abdomen.
Bottom left: Female Stylops ater adult (indicated by red arrow) in a bee's abdomen,
Bottom right: Male Stylops ater pupa casing (indicated by red arrow) in a bee's abdomen. 
From Fig. 1 of the paper

And it's not just the hindwings of stresipterans that are a bit twisted, these insects have extreme sexual dimorphism, so much so that if you didn't know any better, you'd think the females and males are completely different types of animals. The female stresipteran looks like a grub, and she spends her entire life inside the abdomen of the host, with just her head partially poking out from between the segments of the host's abdomen.

In contrast, the males have a pair of giant compound eyes, prominent branched antennae and the "twisted wings" that give this group of insects its name. They have a short and frantic adulthood - after emerging from the host, he only lives for a few hours and his sole mission in life is to find and mate with an elusive female strepisteran, hidden away in the abdomen of a host insect. And he does the deed with an appendage that entomologist Tom Houslay once vividly called a "stabby cock dagger". The technical term for this form of mating is "hypodermic insemination" - where the male basically stabs and inject his sperm into the female, and the sperm somehow find their way to the eggs. Strespiterans are not alone in having this type of appendage, male bed bugs also have a stabby cock dagger - but that's another story.

The study being featured in this post focuses on Stylops ater, a species which parasitises Andrena vaga, the grey-backed mining bee. Unlike the honeybees that most people are familiar with, these are solitary bees, with no castes. And while they do gather into an aggregation to nest, each bee just builds and looks after their own nest. The researchers examined a population of these bees in Lower Saxony, Germany. They sampled over two periods, during late winter, when all 508 bees they looked at were stylopized, and late spring, when they only managed to find two stylopized bees out of a total of 150.

Almost two-third of the stylopized bees were female, but these parasites seem to prefer hosts that are of the same sex as themselves. Since female bees live longer and can provide more nutrients than male bees, this works out well for the life history of female Stylops as it gives them more time and nutrients to grow her brood. After mating, the female Stylops can release up to 7000 offspring, which crawl off to find other bees. While each larva is merely 0.2 millimetre long, they can traverse long distances by hitching a ride on the hair, pollen sacks, or even the crop of bees, to end up in a new bee nest, filled with fresh hosts.

While most bees only hosted a single parasite, some had two or three, and the researchers did find one very unlucky bee that was harbouring four Stylops in its abdomen. But even a single Stylops can take a severe toll on its host. In fact, this parasite is so demanding that it wouldn't grow as big if it had to share its host with another Stylops. As a result, bees infected with Stylops are unable to develop eggs or only produce poorly developed eggs.

But aside from effectively sterilising the bee, Stylops also tinkers its host's biological clock, making it emerge out of hibernation a few weeks earlier than uninfected bees - hence why the researchers found so many stylopized bees in late winter. Making the bees such early risers ensures that there will be plenty of female Stylops around for the male Stylops to find, which will be emerging at that time to live out their extremely short lives. It also gives the female Stylops' larvae more time to develop, so they will be able to crawl off in time to find new hosts in the bee's brood cells. This type of behaviour manipulation is comparable to what's found in Sphaerularia vespae, a nematode that alters the seasonal biological clock of hornet queens.

In order to make these changes to the bee's internal clock, Stylops would have to manipulate the host's hormones, but this also results in some side effects on the bee's body. Female bees that get stylopized tend to have a hairier back, skinnier legs, and the hairs on said legs become shorter and more sparse. In short, they take on characteristics that are more similar to that of regular male bees.

So next time you are out and about, keep an eye out for a bee that looks a bit different from the rest. It might be flying under the influence of a parasite tucked away in its abdomen, looking to make a rendezvous with her short-lived partner.


October 10, 2023

Atriophallophorus winterbourni

In Lake Alexandrina of New Zealand lives a species of tiny freshwater snail called Potamopyrgus antipodarum. These snails are capable of alternating between sexual and asexual reproduction and can be extremely abundant. So much so that they have become invasive in many other parts of the world. Outside of their original home, they are free to proliferate to their heart's content. But back in New Zealand, these snails don't always have things go their way. They are held back by a whole menagerie of flukes which parasitise them - at least 20 different species in fact.

Top: Photo of the snail Potamopyrgus antipodarum by Michal Maňas, used under Creative Commons (CC BY-SA 4.0) license. Bottom: The metacercariae cysts of Atriophallophorus winterbourni, from Figure 1 of the paper.

These flukes have a range of different life cycles, but all of them use P. antipodarum as a site of asexual reproduction - converting the snail's insides into a clone factory and rendering it sterile in the process. These flukes might be the reason why these snails continue to engage in sex every now and then, despite asexual reproduction being much more efficient. Sex is necessary to maintain genetic variations - the key ingredient in the evolutionary arms race against all those flukes.

Researchers who have been studying these snails and their flukes noticed that while all 20 species of those parasite are essentially body snatchers that take over the insides of their unwitting host, one species - Atriophallophorus winterbourni - goes beyond simply messing with their host's physiology and seems to be influencing the snail's behaviour too. Snails infected with A. winterbourni tend to be found in the shallow areas of the lake. Among snails collected from the shallow water margin of the lake, they represent 95% of the infections. Is this because those areas just happen to be hot spots for snails to get infected with A. winterbourni? Or are these flukes actually coaxing the snails into hanging out in the shallows?

To figure out if there's something special about A. winterbourni, researchers compared snails infected with A. winterbourni with those that were infected with a different species of fluke - Notocotylus - to see if such behavioural change is simply a side-effect of fluke infection, or if it is something specific to A. winterbourni. The researchers did this by setting up a series of ten 5 metres long tubular mesh cages that stretched across different depth clines in the lake, from less than 0.8 metres at the shallow end to 2.8 metres at the deep end, with different sections of the cage corresponding to different levels of water depth. Using snails collected from two high infection prevalence sites at the lake, they added about 800 snails to the deepest section of each cage, and the snails were allowed to freely roam between the different sections. After eleven days, samples of snails were randomly collected from each depth level and examined for parasites.

There are some key differences in the life cycles of A. winterbourni and Notocotylus that makes them good for comparisons. Just like other flukes, A. winterbourni undergoes asexual reproduction inside the snail host, producing a whole load of clonal larvae. But unlike many other flukes, these clonal larvae stay in the snail and transform into cysts, where they wait to be eaten by a duck hungry for snails. In contrast, snails that are infected with Notocotylus release those clonal larvae into the surrounding waters, and they do so continuously over the course of about 8 months. These larvae attach themselves to vegetation or the shells of other snails, and are transmitted to grazing ducks that accidentally ingest them. Therefore, unlike A. winterbourni, their transmission is largely decoupled from the snail's own movement and behaviour.

So after those eleven days of allowing infected snails to roam in the cages, what did the researchers find? Well, snails infected with A. winterbourni were heavily distributed towards the shallow end, with over a third of the snails in that section being infected, which is over three times higher than the expected background infection level (11%). In the deepest section of the cage, A. winterbourni-infected snails were rare, representing only 3-5% of the snails in that section, and some of them were immature infections. In contrast, those infected with Notocotylus were found to have distributed themselves fairly evenly across the entire depth cline. It is unclear what exactly A. winterbourni is doing to the snails that makes them favour shallow water, but more importantly, why would they do this? What's in it for the fluke? Well, the final hosts for A. winterbourni are dabbling ducks that only feed in the shallow parts of the lake. So in order for A. winterbourni to make a successful rendezvous with its final host and complete its life cycle, it will have to prod its snail host into the shallows.

Atriophallophorus winterbourni belong to a family of flukes called Microphallidae, and there are a few other species in this family which are also known host manipulators. For example, Gynaecotyla adunca is a species that infects marine mudsnails, and it coaxes its mudsnail host into stranding itself onto beaches, which brings them closer to the crustaceans that serve as the next host in the parasite's life cycle. There's also Microphallus papillorobustus, which infects little sand shrimps (amphipods), and it alters their behaviour in a number of different ways that makes them more visible to hungry birds. Even though not all members of Microphallidae are host manipulators, it's a trait that does seem pretty common in this fluke family. Sometimes, in order to complete a life cycle, you just have to drag that snail to where you need it to be.


September 8, 2023

Rhizolepas sp.

Parasitism has evolved a few different times in barnacles. Most parasitic barnacles belong to a group called the rhizocephalans, which are body-snatchers of decapod crustaceans like crabs and shrimps. Aside from them, there are two other known genera of parasitic barnacles: Anelasma squalicola - which is the bane of deep sea Squaliform sharks, and then there's the barnacle being featured in today's post - Rhizolepas, a rare little crustacean that parasitises seafloor-dwelling aphroditid scale worms. Both of them belong to a group called Thoracicalcarea, which happens to be a sister group to the rhizocephalans.

Left: Rhizolepas in situ attached to its scale worm host. Right: Rhizolepas removed from the host, showing its entire anatomy.
Photos from Figure 1 of the paper.

Rhizolepas has a general shape that broadly resembles typical stalked barnacles that can be found attached to piers or drifting debris, but it lacks the feeding legs that those barnacles use to filter food particles out of the water. Instead, it has a dense network of roots at its base that extend deep into the host's body which it uses to suck up nutrients directly from the host.

This blog post covers a recent study on Rhizolepas, and it's about time too because the last time anyone managed to collected a specimen of this little barnacle was back in 1960. The Rhizolepas specimen in this study was collected during a trawl in the seas off Kagoshima, southern Japan. Out of the ten Laetmonice scale-worms that were collected by the trawl, only ONE of them was infected with Rhizolepas. This provided an amazing opportunity to find out more about this rare little barnacle, so the scientists carefully removed the barnacle from its scale worm host and preserved it in high-grade ethanol for further DNA analyses.

How did Rhizolepas get to be the way it is now? Looking at its morphology is of relatively limited value - evolving towards parasitism does weird things to an organism's body. It is a process that turns copepods into fleshy blobs, and transform snails into sausages. So trying to work out the evolutionary origin of something like Rhizolepas based on its anatomy is an exercise in futility. But while its anatomy may have been modified beyond recognition, its evolutionary history is recorded in its DNA.

DNA analysis revealed that Rhizolepas' closest relatives are Octolasmis - a genus of goose barnacles that spend their lives attached to all kinds of different animals, including the shells and gills of crabs and the skin of sea snakes. The study also found another barnacle called Rugilepas, is actually nested among the various species of Octolasmis, and it provides a perfect transitional model for how Rhizolepas might have evolved from a regular stalked barnacle into a fully committed parasite.

Rugilepas lives on sea urchins, but they don't simply attach to their host, their presence induces a gall on the sea urchin's body which snugly encases the barnacle. However, unlike other gall-inducing animals in sea urchins, Rugilepas is walled off from the urchin's internal anatomy, and doesn't draw any nutrients from its host. Furthermore, while its feeding limbs are significantly reduced, they are not completely useless like those in Rhizolepas and Anelasma. So between Octolasmis and Rugilepas, we can get an ideal of the evolutionary steps that Rhizolepas might have taken on its path to becoming a parasite of scale worms

Based on its level of DNA divergence from other barnacles, Rhizolepas is estimated to have originated about 19 million years ago, during the Miocene. Given the external part of this barnacle no longer performs its ancestral function of feeding, the potential next step in their evolution would be to get rid of any dangly parts altogether, and become completely internalised within the host like their rhizocephalan cousins.

Barnacles are particularly pre-adapted for flirting with or even becoming completely committed to a parasitic lifestyle. Even among non-parasitic barnacles, these crustaceans are remarkably versatile in attaching to different living substrates, from sponges and corals, to whales and turtles. Perhaps this versatility gives barnacles an advantage in taking the next step from a mere hitch-hiker into a full-blown parasite. Since the oldest known barnacles date back to the mid-Carboniferous period around 330 million years ago, who knows what other marine animals they might have attached to or even parasitised throughout Earth's history?


August 10, 2023

Bothrigaster variolaris

Student guest post time! One of the assessments that I set for students in my ZOOL329 Evolutionary Parasitology class is for them to summarise and 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) here on the Parasite of the Day blog. So from the class of 2023, here’s a post by Nikita Sheelah, about a bird of prey with too many flukes.

To dare to do what hasn’t been done before has been the driving force behind many advancements in society, such as the creation of vaccines, anime, or the ground-breaking Reese’s Peanut Butter Cups. Being the first in recorded history to do something different essentially immortalises people in the history books, which often carries incredible pride and achievement. This seems to be the case for a group of trematode flukes (Bothrigaster variolaris) which infected a snail kite (Rostrhamus sociabilis), and made their way into the bird’s air sacs, causing the snail kite’s fatal end. 

Left: Snail Kite, photo taken by Bernard DuPont, used under Creative Commons (CC BY-SA 2.0) license. 
Right: Bothrigaster variolaris fluke from Fig. 6 of the paper. Centre Insert: Bothrigaster variolaris fluke on the pericardium of the Snail kite's heart from Fig. 1 of the paper

“Big deal,” you say, “trematodes infect air sacs in birds all the time.” And you’re right! Death from trematodes infecting air sacs is fairly common,  but this has mostly been reported in Passeriformes; birds known to be more susceptible to these parasites. It has even been reported in snail kites themselves, but that was in Florida rather than South America. Every continent needs its own firsts, after all. 

So how did this even happen? Let me explain. Snail kites, as you might have ingenuously guessed from the name, eat snails! Apple snails (Pomacea spp.), to be precise. Trematodes in the Cyclocoelidae family use snails as their hosts for the larval stage, meaning when those snails are eaten, little baby trematodes get to grow up into a mature adult in the body of whatever ate the snail (usually birds). So, much like eating too many candy apples can rot your teeth with cavities, the snail kite indulged in too many infected apple snails and rotted their insides. With flukes. Not cavities. And the insides weren’t rotten, just parasitised. That wasn’t that great of an analogy, actually. 

A wildlife rehabilitation hospital brought this male adult snail kite into their care and did their best to help him, but he passed shortly after arrival. Immediately afterwards, a necropsy was performed to poke and prod at his insides, taking tissue samples and collecting the flukes. Not the most dignified funeral rites, but it’s all in the name of science, because over 200 flukes were counted in the bird! Thirty-five were collected for DNA analysis and were identified to be in a distinct clade within the Cyclocoelidae family. The physical characteristics of the flukes backed this up, especially the ventral sucker, which is characteristic to the genus Bothrigaster within that family.

Researchers concluded that the bird most likely died from suffocation due to the obstruction by the parasites, as well as lesions in the respiratory tissue. They also noted a mature trematode in one of the wing bones, which is a pretty uncommon spot for a parasitic flukes to be. What an adventurer!

So, these ambitious Cyclocoelidae made history by being the first reported trematodes to have caused death by air sac infection in snail kites in south America. Realistically, this may happen more than we think, and has probably been happening for quite some time, but being the first trematodes to be written about in this sense is a pretty big feat! Their mothers must be so proud. 


This post was written by Nikita Sheelah

July 11, 2023

Diexanthema hakuhomaruae

The study in this post takes us to one of the darkest corners of the deep sea, over 7000 m below sea level in the Kuril-Kamchatka Trench, located in the northwestern Pacific. Living in this dark and oppressive environment are isopods called Eugerdella kurabyssalis. And despite the crushing pressure, these crustaceans like it just fine, in fact they are the most abundant isopod down in those depths. But such success and abundance can also attract the attention of parasites, and this post is about a newly described parasitic copepod called Diexanthema hakuhomaruae.

Left: Diexanthema hakuhomaruae (indicated by white arrow) attached to the leg of its Eugerdella kurabyssalis isopod host. Right: Close-up of D. hakuhomaruae, the arrow indicating the copepod's ovaries. Photos from Figure 1 of the paper

Those who are familiar with this blog would know that parasitic copepods come in all kinds of shapes  that would defy most people's idea of what a crustacean is "supposed" to look like. And D. hakuhomaruae is no different - its tiny body is ROUND and if anything, it looks almost like a legless tick. And much like a tick, D. hakuhomaruae attaches itself stubbornly to the leg of its host.

Diexanthema hakuhomaruae belongs to the Nicothoidae family, a group of parasitic copepods that contains about 140 known species. They live on a variety of crustacean hosts, including tanaidaceans, ostracods, amphipods, cumaceans, mysid shrimps, and lobsters. Most of them have a rotund, almost spherical body, greatly reduced or no legs at all, and a specialised mouthpart that ends in a sucker with syringe-like mandibles. And much like the ticks that they resemble, these copepods feed by stabbing their mouth syringe into their host's body and sucking up that crustacean blood (hemolymph) on tap. Some species such as Choniomyzon infaltus are specialised egg parasites - their balloon-shaped bodies allow them to hide amidst broods of their hosts and feed on their eggs without being discovered.

There are currently six other known species of Diexanthema, all of them are parasites of deep sea isopods. And Diexanthema is not alone in its preference - there are other nicothoid copepods that have also been found parasitising deep sea isopods. What makes D. hakuhomaruae special is that it is the first to be found from the Hadal Zone. All other Diexanthema species have been reported from depths of 1300 to 3500 metres below sea level, but none of them had gone down as deep as D. hakuhomaruae.

It is unknown whether D. hakuhomaruae feeds on the host's fluid or if it is an egg parasite, or how it even completes itself life cycle in the hadal zone - as you can imagine, discovering and describing such parasites in an environment like the deep sea is challenging enough as it is. Studying the life style and ecology of these deep sea parasites with current technology is next to impossible. Even so, this description shows that parasitism is indeed ubiquitous on this planet, and wherever you find life, you can be sure that some of them will be parasites

Kakui, K., Fukuchi, J., & Ohta, M. (2023). Diexanthema hakuhomaruae sp. nov.(Copepoda: Siphonostomatoida: Nicothoidae) from the Hadal Zone in the Northwestern Pacific, with an 18S Molecular Phylogeny. Acta Parasitologica 68: 413-419.