"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 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.

December 17, 2020

Ophiocordyceps sinensis

Ophiocordyceps is a genus of fungi that is probably most well-known for their abilities to usurp and manipulate the behaviour of ants, which gave rise to their more commonly known name - the "zombie ant fungi". But aside from the ant-infecting species, the genus Ophiocordyceps also contains another very well-known insect-zombifying fungus - Ophiocordyceps sinensis, more commonly known as the "caterpillar fungus" - which infects the caterpillars of ghost moths.

Left: O. sinensis fruiting body emerging from a caterpillar, photo by Zhu Liang Yang from here
Right: Ghost moth (top) adult, and (bottom) caterpillar stage, photos from here

While the reputation of the ant-infecting Ophiocordyceps species were built upon their ability to control their host's mind, the roots of O. sinensis' fame is based on the fungus' prized medicinal properties, which has been known and documented for centuries in China where it is known as dōng chóng xià cǎo (冬蟲夏草: which translates into "winter worm, summer grass). It also made an appearance in Moyashimon, a manga (and subsequently, anime) about microbes. Unfortunately, in recent decades, this fungus is currently under threat from a combination of climate change and over-harvesting.

Despite being highly valued and extensively studied for its pharmaceutical potential, the natural ecology of this fungus is not all that well-understood. For example, it is not entirely clear as to how this fungus actually infects its caterpillar host in the first place. Attempts to cultivate the fungus in artificial settings to alleviate harvesting pressure on wild populations have been met with limited success, in terms of producing them on a commercially-viable level.

The host of O. sinensis are ghost moth caterpillars, which live underground munching on the roots of plants. So unlike the ant-infecting zombie fungi that can simply scatter their spores around areas where their ant hosts are likely to walk by, such means of dispersal would be ineffective for reaching caterpillars that spend their entire time underground. Furthermore when scientists examine the soil around fruiting bodies of O. sinensis, the concentration of spores was fairly low, and in any case, they don't seem to disperse very far, with most of the spores found within 20 cm of the fungus fruiting body.

But some of these zombie insect fungi also live a secret double life. When they are not infecting and zombifying or mummifying insects, some of those fungi moonlight as plant symbionts called endophytes. They dwell out of sight within plant tissue, and in some cases providing the plants with various benefits. So perhaps O. sinensis is also leading this double life too? If so, that might be a way through which they are coming into contact with their soil-dwelling caterpillar hosts. 

A group of scientists in China set out to investigate this ecological puzzle at Mount Gongga, in the Sichuan province of China. First of all, they ascertain whether O. sinensis is indeed spending part of its life cycle dwelling as endophytes in the tissue of plants. To do that, they collected plants from areas where the caterpillar fungus was found at the Yanzigous valley, and extracted DNA from the leaves and root tissues of those plants. They then used Quantitative PCR to screen for the presence of O. sinesis. Of the 115 species of plants that were examined, O. sinensis was present in about half of them, across 18 different plant families

Secondly, they also investigated the caterpillars' diet to determine whether they have been eating any of those O.sinesis-positive plants. The scientists collected the caterpillars' gut content, extracted the genetic material they contained, and amplified key sections of DNA that can be used as genetic markers to detect and distinguish different types of plants. From that, they found that those ghost moth caterpillars munched on plants from at least 22 different families, and of the plants that were on the caterpillar's menu, 12 of them had the endophytic stage of O. sinesis in their roots. 

So this might mean that instead of relying upon those spores coming into direct contact with the caterpillars, the way that this fungus completes its life cycle is by using its spores to infect a plant, become established in the plant tissue, then wait for a hungry, hungry caterpillar to come by.

Infecting the host via hiding in their food or prey item (also known as trophic transmission) is a transmission strategy that is usually associated with parasitic worms with complex life-cycles. But here we have a fungus that seem to have convergently evolved this way of reaching its host. While in this case, the hosts (plants and caterpillars) are very different to those that parasitic worms usually infect, functionally it is the same - the hosts become infected through what they eat. Additionally, many of those aforementioned parasitic worms can alter the behaviour and/or appearance of a prey to make it more attractive to a potential host. Can O. sinensis do the same to their host plants to make them more attractive to those soil-dwelling caterpillars?

Given that there are many other fungi which also infect subterraneans insects - this transmission mode might be more common than previously thought, with a wide range of fungi secretly living this double life of being both friends to plants and killers of bugs.


November 19, 2020

Microgaster godzilla

While there is an oft-mentioned quote by evolutionary biologist JBS Haldane that God has an "Inordinate Fondness For Beetles", it is becoming apparent that a different group of insects may be more deserving of being considered as the chosen ones. A recent study estimated that there are actually 2.5 to 3.2 times as many hymenopterans (the insect order that contains ants, bees and wasps) as there are beetles. Furthermore, much of the diversity within the hymenopterans are parasitic wasps, making those parasitoids the most species-rich group of animal on this planet. 

So rather than beetles, the animal group which the hypothetical Creator is most fond of appears to actually be body-snatching parasitic wasps - a sentiment that I can wholeheartedly endorse. And it is one of those wonderful insects which is being featured in today's post. 

Top: Female adult Microgaster godzilla from Figure 1 of the paper
Bottom: Frames showing the parasitisation process, from the supplementary videos of the paper

This post is about a recently described species of parasitic wasp - Microgaster godzilla - which has been named after that famous King of Monsters, Godzilla. While its species name may have attracted much of the attention - not surprisingly, given it has been named after one of the most famous movie monsters in the world - to me, that is the least interesting thing about this insect. Because unlike those many thousands of parasitic wasps out there, M. godzilla has evolved to use an aquatic insect as its host - a very rare feat among these parasitoids. 

Microgaster godzilla belongs to a subfamily of wasp called Microgastrinae, a diverse group composed of 2000 described species. But Microgasterinae itself belongs to a much larger family of parasitic wasps called the Braconidae which contains 17000 known species, with an estimated 42000 species in total. All braconid wasps have larval stages that develop attached to or inside the body of another insect, and when they are ready to mature into full-fledged adults, the endoparasitoid types come bursting out of the body of their hosts like a xenomorph chest-burster.

But for all their diversity and success in using the bodies of other insects as living incubators for their babies, most parasitic wasps are limited to parasitising terrestrial insects, with only 150 species (0.13% of all known hymenopterans) having been recorded to parasitise aquatic insects. Microgaster godzilla belongs to this very special and exclusive club, going where few other wasps are able to venture. 

The target which M. godzilla is after are the aquatic larvae of the moth Elophila turbata. These water-bone caterpillars feed on floating aquatic plants such as duckweeds. They do so usually by burrowing into the plants' leaves, and the older caterpillars, which have grown too large to burrow into the tiny leaves of those aquatic plants, actually weave a casing around itself from bits of vegetation. So at every stage of the caterpillar's development, not only is it submerged, it is also enclosed in a casing of plant material, one way or the other. 

Microgaster godzilla searches for its target by carefully walking on the leaves of duckweed and other floating vegetation on the water surface. But sometimes, it will take the plunge and dive briefly underwater in its hunt. Once it spots the caterpillar's characteristic case, instead of just forcing its way through with brute force, it annoys the caterpillar leaving its protective shelter. Microgaster godzilla starts tapping incessantly on the caterpillar's case with its antennae, accompanied by some prodding with its stinger-like ovipositor. 

Eventually, all this ruckus coaxes the caterpillar into popping out of its cosy plant bag. As soon as that happens, M. godzilla will pounce on the caterpillars and stab it with its ovipositor, injecting eggs in the process (you can view videos of this via the supplementary material which the paper's authors have provided here and here). 

The extraordinary sets of behaviour adaptations displayed by this tiny wasp, which allows it to do something that few other parasitoid wasps are capable of, is just as fascinating as the power of any movie monsters.


October 21, 2020

Ichthyolepis africana

For most people, tapeworms are among the most recognisable types of parasites. The most commonly known species include the beef tapeworm, pork tapewormdwarf tapeworm, and the flea tapeworm due to their human health and veterinary importance. Even though those tapeworms are all different species with very different life cycles, they all happen to belong to one specific tapeworm order called Cyclophyllidea.

While there are many other tapeworms out there which infect vertebrate animals as their final hosts,  what most people might not realise is that aside from the cyclophyllideans, most branches of the tapeworm trees are actually sleeping with the fishes (in particular, sharks), so to speak. Cyclophyllidean tapeworms are notable in that they have evolved to only use terrestrial vertebrate animals (tetrapods) as their final hosts...well, with one notable exception.

Left: Scolex of Ichthyolepis, Right: Two of the host species, Mormyrus caschive (top), Marcusenius senegalensis (bottom)
Photo of Ichthyolepis scolex from Fig. 2 of the paper, Photos of elephantfishes by John P. Sullivan and Christian Fry

A recently published study described a very unusual species of cyclophyllidean tapeworm, which has made the switch from living in terrestrial vertebrates to living in the gut of a bony fish. To put things into perspective, finding an adult cyclophyllidean tapeworm in a teleost fish is like finding a pig living in the middle of the open ocean. The host of this special tapeworm are elephantfishes (Mormyridae) - a family of small electrical fish found in Africa, which are notable for their ability to generate electric fields, and their unusually large brains.

This species of intrepid parasite has been named Ichthyolepis africana, and the adult tapeworm dwells in the host's intestine, just behind the opening to the stomach, where it hangs in place using its formidable crown of hooks and four muscular suckers. Based on the phylogenetic analyses that scientists have conducted, the closest living relatives of this tapeworm are found in birds - specifically swifts, of all things.

And as if infecting a species of electric fish wasn't enough for this special tapeworm, I. africana was found in not just one, but SIX different species of elephantfishes, distributed across different parts of the African continent, including Senegal, Egypt, Sudan, and South Africa. And wherever they were found, they were present in between 36-63% of the elephantfish population that the scientists sampled. Its ubiquity and abundance shows that Ichthyolepis has had a long and well-established co-evolutionary relationship with this group of freshwater fish.

But how did it get there in the first place? Why and how did the ancestor of this tapeworm make the switch from living in a group of small birds to the gut of electric fishes - two lineages that have been separated by over 420 million years of divergent evolution?

A clue can be found with the animals that host this tapeworm's closest living relatives - which are swifts. Swifts belong to a family of birds called Apodidae, As their name implies, they are swift flyers with fantastic aerial manoeuvrability, which they use to snatch flying insects out of the sky. Tapeworms usually infect their vertebrate final host by having larval stages that develop in the bodies of prey animals that their final hosts feed on. So those insects would have served as marvellous vehicles for tapeworms which infect those birds.

But aerial hunting is not the only way for an animal to eat insects. Any insects that fell into a water body would have made a handy snack for many aquatic animals, and elephantfish - which usually feed on invertebrates such as small crustaceans and aquatic insects - would have eagerly hoovered up those morsels from above.

While most of those tapeworm larvae - which were adapted to the warm, cosy intestine of a bird - would have perish when they ended up in the gut of an elephantfish, an aberrant few might have had mutations which allow them to survive in such a unfamiliar environment, giving them a survival advantage. Over evolutionary time, surviving in an elephantfish's gut might have evolved into a viable alternative pathway to maturity, and the ancestors of Ichthyolepis might have found the conditions inside to be hospitable enough to abandon the bird host, and took up long-term residency in the gut of those electric fishes.

This type of host-switching or host-jumping across quite disparate host animal lineages has happened in other parasites too. In 2017, I wrote a post about a thorny-headed worm which has established itself in both seals and penguins - simply because they feed on the same prey (fish). Despite being in completely different classes of vertebrate animals, they were exposed to the same parasite via what they ate.

To some people, it may seem that spending your life living inside the body of another animal would relegate you to an evolutionary dead end. But the evolutionary histories of many different parasite lineages tell an entirely different story. It seems that when the right opportunities present themselves, parasites have often been ready to seize the moment, and make an evolutionary leap to take on new hosts, and beyond.

Scholz, T., Tavakol, S., & Luus-Powell, W. J. (2020). First adult cyclophyllidean tapeworm (Cestoda) from teleost fishes: host switching beyond tetrapods in Africa. International Journal for Parasitology 50: 561-568

September 22, 2020

Parapulex chephrenis

Here's the second student guest posts from the third year Evolutionary Parasitology unit (ZOOL329) class of 2020. This post was written by Patra Petrohilos and it is about the social life of Egyptian Spiny Mouse and how that relates to their fleas. (you can also read a previous post about how a muscle-dwelling worm survives under a cover of snow here).

It doesn’t require a particularly vivid imagination to appreciate that being eaten by fleas is not exactly the most stress-free experience for an animal. Neither (to the surprise of introverts nowhere) is being bullied into submission by the resident bossy boots in your social group. Surely, then, it would logically follow that being bullied by your peers AND preyed upon by parasites at the same time would be the most stressful option of all? That’s certainly what some researchers thought – and were stunned to discover that the answer was not quite what they expected.

Photo of spiny mouse from here, photo of Parapulex flea from here

Before we get any further, you may be wondering how exactly one measures the stress levels of an animal. I’m so glad you asked. Turns out, when we get stressed our bodies produce this stuff called glucocorticoids – which is such a long clunky word that I’ll just refer to it from here on in as GC. In the short term (let’s say we see a predator across the street) this is a good thing – a short burst of GC takes the energy that we’d usually spend on boring things like digesting food and diverts it to more useful activities – like running away from predators. But in the long term (let’s say we are trapped in a cage with that predator for a year) it is a very bad thing. Too much GC can do all kinds of awful things, wreaking havoc on our immune system and our fertility. Scientists can measure how much GC an animal is producing (and therefore how stressed out it is) by analysing its poo. It’s all pretty glamorous.

These particular scientists were interested in how two different negative experiences (parasitism and social interaction) interact to affect an animal’s stress levels. They decided to investigate this by studying the Egyptian spiny mouse (Acomys cahirinus) – an incredibly social little fella that is found living in groups of one male and multiple females. Within this little society, one of those females usually stakes a claim to “Queen Bee” of the group. Bizarrely, they are also especially attractive to one particular species of flea (Parapulex chephrenis), who for some reason steer clear of all other mouse species in favour of this one.

Once they had gathered their mice, the scientists split the females into two groups. The first consisted of pairs of mice, two to a cage. As tends to happen in these situations, one of the pair invariably emerged as the bossier one. This two-mouse hierarchy was well and truly established after a week, by which time the submissive one knew her place well enough to not even attempt to rock the social boat. The second group was divided into single ladies. Each mouse in this group got an entire cage to herself (and peace from any potential bickering over petty things like food).

They then divided the groups further. Half of the paired mice and half of the single ladies were infected with P. chephrenis fleas, while the other half were left flea-free. For a brief period, a male was also added to each cage (just long enough to do the kinds of things that male mice like to do with female mice) and then mouse poo was collected at various points so the scientists could gauge each mouse’s stress levels.

To their amazement, the single mice were more stressed than their paired up counterparts – even the ones being dominated by the bossy boots cagemates. Apparently company is so important to such a social species that being alone is more traumatic than being at the bottom of the pecking order. But even more astoundingly, it was the mice who were not only solitary but also flea-free that were more stressed out than anyone!

It’s possible that flea infestation made these already-anxious solitary mice more likely to indulge in a bit of grooming (a behaviour that tends to soothe rodents), but regardless – it’s fascinating that the results were the exact opposite of expected. Rather than one stressful thing exacerbating the other (like adding Carolina Reaper chili peppers to an already hot sauce would) they almost seemed to cancel each other out (like adding yogurt to a vindaloo curry).

So what’s the moral of the story? If you’re an Egyptian spiny mouse, even having awful, flea infested friends that bully you is better than having no friends at all. And for those poor waifs who don’t have friends - any distraction is preferable to the loneliness of a solitary life. Even when that distraction is being eaten by fleas.


This post was written by Patra Petrohilos

September 15, 2020

Trichinella britovi

It's time for some student guest posts! One of the assessments 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. For the class of 2020, two students' posts were selected. So to kick things off, here's a post written by Anna Clemann, and it's all about how a muscle-infecting nematode survives under cover through winter. 

Photo of Trichinella britovi from this paper
We’ve all heard the stories of people lost in the snow building ‘snow caves’ to survive the cold temperatures. Turns out the nematode Trichinella britovi, a small parasitic worm which have larvae that are found in the striated muscles of carnivorous animals, also survives better in ‘snow cave’ type conditions. 

Trichinella britovi can be found in a number of different hosts, with many scavenger species acting as carriers or reservoir hosts that themselves do not experience much ill effects from the parasite, but can be a source of infection for other host species. Trichinella britovi larvae are transmitted when the striated muscle (where the larval worm resides) of an infected host is consumed by another animal. This parasite has adapted to surviving in the decaying muscle of hosts via engaging in anaerobic metabolism, so they can survive in tissue that has little oxygen for long periods of time. 

A recent study has found that temperature and humidity also play a major role in the chances of survival for T. britovi. If a carcass infested with T. britovi is frozen, they can survive for up to several months in the muscles, which increase their chances of being ingested by another host. Researchers from Italy and Latvia decided to test whether the chance of survival for T. britovi was better if the infested carcass was buried under snow or above the snow. 

The researchers conducted their study on two carnivore scavengers, fox and raccoon dog. First, they placed the animal carcasses in a scavenger-proof netted mesh box that was surrounded by snow. They then divided the box into two sections and placed one set of the carcasses on each side. One side was filled almost to the top with snow while the other side was left exposed (see image below). 

A picture depicting the experimental set-up, taken from Fig. 1 of the paper.

Over the course of the study (112 days) the researchers collected muscle samples from all the carcasses and recorded the temperature and humidity for both environments over key periods of time. The muscle from all carcasses were fed to lab mice and researchers then looked at the prevalence of T britovi larvae surviving and reproducing within the mice. Through that, they found that T. britovi survived better if they were buried in the snow! 

Researchers found little difference in the reproduction capacity of T. britovi in the mice from the carcasses which are beneath and above the snow in the first two months of the experiment. However, during the last 42 days of the study, mice that were fed muscle from exposed carcasses above the snow (which were subjected to more temperature and humidity variation) showed a 100% reduction in T. britovi infection, meaning the worms they were fed with were not infecting them at all. While mice fed with muscles from carcasses that were buried also had a reduction in larvae reproduction, but at least some were successful in establishing infection in those final weeks up to the end of the experiment. 

The researchers found that the difference in temperature and humidity above snow and below snow were enough to provide a better environment for T. britovi over a longer period. They also noted that below the snow, the variation in temperature was 5.5 times lower than above the snow, producing a more stable and warmer environment. This was further confirmed when the researchers found that the extent of rotting in the muscle (which was more in the buried carcasses than the unburied) was not detrimental to T. britovi reproductive capacity. 

 So, life is better if you’re buried alive, at least if you’re a Trichinella britovi larvae. 


This post was written by Anna Clemann

August 24, 2020

Hexametra angusticaecoides

In 2016, a group of crested geckoes in captivity suddenly got sick soon after they were transferred to a terrarium that previously held some Madagascan mossy geckoes. Within a short period of time, the geckoes started dying of a mysterious illness. Of the ten that were held in that terrarium, only a single gecko survived.

Photos of Hexametra emerging from moribund geckoes from Fig 1, 3, and 4 of the paper

When the dead geckoes were dissected, it was found that they had massive worms that were tearing their ways through the their innards. Some of the worms had even started emerging through the skin.
One gecko was lucky enough to receive treatment in time to save it. It was initially given fenbendazole and pyrantel - two commonly used medications for treating parasitic worm infections - but they had no effects. So a surgical procedure was performed on the lizard to remove the deadly nematodes from under its skin, followed by a dose of levamisole. About two weeks after the surgery, the surviving gecko managed to recover to full health.

Between the nine dead gecko and the sole survivor, over 50 worms were retrieved. The worm in question was Hexametra angusticaecoides - a parasitic nematode which commonly infects reptiles. Some of you who have  been following this blog for a while might recognise the genus Hexametra from a post back in 2016 where another species of that parasite was found in the body of a captive false coral snake.

So how did a bunch of crested geckos ended up with all these worms? Tracking down the original source of infection was a bit tricky, given some were sourced from a breeder in Canada, while other were sourced from a pet shop in Germany. Furthermore, they had been kept separately in different terrariums until they were combined into a single enclosure, soon after which the worms began appearing. However, it could be that particular terrarium which was responsible for the worms. Prior to housing the crested geckoes, that enclosure had been occupied by some wild-caught Madagascan mossy geckoes, Uroplatus sikorae.  

The harm caused by Hexametra to its host is likely due to its relatively large size, and its tendency to move around within the host's body instead of staying in place. Additionally, since the crested gecko is an exotic host for the parasite, this pairing would not have happened naturally. Hosts which have had a history of coevolution with their parasites would have also evolved mechanisms for tolerating or offsetting some of the more harmful effects of the parasite. But hosts which have never encountered those parasites would have no such adaptations and are thus exposed to the full effect of the parasite's presence. On top of that, the stress of captivity might have made the geckoes less able to tolerate any kind of parasitic infection.

The exotic pet trade, in addition to driving the poaching, smuggling and distribution of wildlife worldwide, also bring together animals which would otherwise never come into contact with each other, along with their many parasites. Like the majority of parasites, there is simply insufficient basic information about the ecology, life cycle, and life history for most reptile parasites, let alone what effects they might have if they end up in hosts which they have never encountered before.

Barton, D. P., Martelli, P., Luk, W., Zhu, X., & Shamsi, S. (2020). Infection of Hexametra angusticaecoides Chabaud & Brygoo, 1960 (Nematoda: Ascarididae) in a population of captive crested geckoes, Correlophus ciliatus Guichenot (Reptilia: Diplodactylidae). Parasitology, 147: 673-680.