"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

January 11, 2018

Riggia puyensis

It is no secret that I am a big fan of parasitic isopods, especially those in the Cymothoidae family - the most well-known of which is the tongue biter parasite, and my love for these adorable crustaceans has even manifest itself in some of my artwork. But while the tongue-biters are no doubt the most (in)famous representatives of that family, to the extent that they even made an appearance on an episode of the Colbert Report, it is their less well-known cousins - the belly-dwellers/burrowers - that turn the horror factor up a notch (or four, or eleven) and as a result, really earned my adoration.

Left: Adult female Riggia puyensis (scale bar = 10 mm), Right: Adult make Riggia puyensis (scale bar = 1 mm)
From Fig. 3 and Fig. 9 of the paper

Imagine if the chest-burster xenomorph from Aliens didn't just explode through your ribcage and leave you for dead - instead, it stays inside your torso for the rest of your life, laying a steady stream of eggs that trickle out through a small(ish) hole in you belly. That's how these belly-dwelling isopod live their lives. So let's kick off the year with a recently described species of these belly-dwellers!

I've previously written a post about a species of belly-dweller call Artysone trysibia which lives in the body cavity of an armoured catfish from the Amazon. This post features Riggia puyensis, which is quite similar to A. trysibia in that it was also found to be parasitising armoured catfish, specifically two species from the Bobonaza River and Puyo River in central Ecuador - Chaetostoma breve and Chaetostoma microps - both of which are better known as suckermouth armoured catfish.

Most of the R. puyensis specimens that the scientists found in this study were females, but the scientists did come across three male specimens which were clinging to the limbs of the female isopods. These male isopods are comparatively tiny reaching only one-tenth the length of the adult female R. puyenesis. The small size and relative rarity of males is par for the course for Riggia. In other studies on this genus of parasite, male isopods are rarely found, if at all. It is possible that this is because the mating strategy of the male isopod is to scoot in, mate with the larger female, then go off and find another infected host.

Riggia puyensis inside its host, from Fig. 2 of the paper
In this study, each infected fish was only parasitised by a single female isopod - which is probably just as well since R. puyensis is quite large in relation to the host. The female R. puyensis reaches over an inch in length and considering one of the host catfish is a species that grows to about four inches long at most, that parasite is a hefty load to be carrying around. It would be like having a corgi living inside you.

So it may seem rather surprising that the survival of these fish does not seem to be compromised by the parasite. In fact, a previous study have shown that the parasite may in fact enhance the infected fish's growth. But this parasite-induced growth spurt comes at a price - after all, there is no free lunch in nature and for the gain in body growth, the parasite incurs a severe penalty on the fish's reproductive functions. A study on bonefish parasitised by Riggia paranensis found that infected fish has reduced level of sex hormones and undeveloped gonads.

So Riggia render its fish host impotent in order to free up more resources for body growth, and a bigger host means more for the parasite to consume. So while a chest-bursting xenomorph invokes a more immediate visceral reaction, the way that R. puyensis and other parasitic castrators modify their hosts' body to fuel their own reproduction presents a more existential form of lingering horror.

Reference:
Haro, C. R., Montes, M. M., Marcotegui, P., & Martorelli, S. R. (2017). Riggia puyensis n. sp.(Isopoda: Cymothoidae) parasitizing Chaetostoma breve and Chaetostoma microps (Siluriformes: Loricariidae) from Ecuador. Acta Tropica 166: 328-335.

December 30, 2017

Zombifying fungi, Hitch-hiking parasites, and making the most out of your hosts

It's been another year and as usual there were many interesting studies on various parasites that were published in peer-reviewed journals - far more than what I ended up writing about for the blog. While papers about parasites are usually published in Parasitology journals - as one would expect - because parasitism is a life style rather than a taxonomic group, there were also many studies that were published in various evolutionary, ecological, and multi-disciplinary journals.

So I've tried to browse widely to find papers which would make for an interesting story and can be written up in a reasonable timeframe. So what are some of the highlights from 2017?


Of the papers that I did manage to write up, some of them were on fungi that infect and zombify insects and other terrestrial arthropod, there are ants, beetles, even millipedes that have fallen under their spells - admittedly, I do have a soft spot for those fungi, so in a way I have fallen for them too.

And the fungi did not have a monopoly on the insect killing business - this year, the blog featured two separate studies on parasitic nematodes that turn an insect's innards into a soupy baby food for the next generation of killer worms. They have many ways of doing so - the main way is with help of a bacterial ally, but some species also have an arsenal of toxins.

But microbes are not the only allies that are enlisted by parasites, one post featured a flea that hitch a ride on earwigs to reach bats. Being able to arrive at a new host is a vital part of any parasite's life-cycle, and while that bat flea uses an earwig to get there, there are many other ways to accomplish that end. This year there were blog posts on two turtle parasites - a copepod and a blood fluke - which have evolved very different ways of reaching their marine reptile hosts amidst the oceanic expanse.

While the size of those parasites are minuscule compared to their rather large host, other parasites can reach alarmingly large sizes in proportion to their host. Some parasite take up so much space that they represent a major drain on their host's resources, and become parasitic castrators. The rhizocephalans is one such example and when it comes to body-snatching, these parasitic barnacles give the insect-zombifying fungi a run for their conidia. There's a good reason for being so imposing upon their host, as the more space they take up, the more eggs they can produce.

Those barnacles have a network of tendrils that can squeeze through the nooks and crannies of the host's body, but if the host doesn't provide you with a space, then you have to make your own, as with the case of a parasitic snail that lives in the spines of a sea urchin. Often, getting through life as a parasite is all about making the most out of the living condition that you've been dealt with. Whether you happen to be parasitic plant that spends your whole life underground except when it comes to flowering, or flukes living in the brain of some endangered fishes, or a seal parasite that has found itself living in the gut of a penguin.

Amidst the zombifying fungi and body-snatching barnacles, it is important to remember that not all parasites are nearly so deadly or harmful to their host. In fact, one of the post featured a downright benign parasite - a fungus that live as an external hyperparasite on bat flies, which are themselves parasites of bats. There was even a post featuring a parasite that live in the gut of cat fleas and helps it reach maturity more quickly to start producing more baby fleas - after all, more fleas means more hosts for that parasite.

Both of those parasites happen to be parasitic on ectoparasitic blood-sucking insect - so it looks like those hyperparasite are showing those insect killers mentioned earlier in this post that there is more than one way to make the most of your host

So that does it for 2017! As I hinted at the start of the post, there is only so many papers I can possibly cover in one year - let's hope there's more to come next year so I can continue to bring you more parasite stories! Meanwhile, I often tweet about the parasitology (and other) papers that I didn't get to write up as a full post at @The_Episiarch - so you can go there to see more.

In August, I was also interviewed for the In Situ Science podcast where I talk about parasites (for a bit anyway, we ended up talking about sciart, social media, and many other things), and of course, those who follow my work online for long enough (especially on Twitter) would also know that in addition to science, I also do art, and sometimes my science intersects with my art to create... Parasite Monster Girls? Since I do plan on continuing to draw Parasite Monster Girls in 2018, I guess in addition to blog posts about parasites, that's another form of parasitological content that you can look forward to seeing from me in the new year...

See you in 2018!

December 12, 2017

Megadenus atrae

A few months ago, I wrote about a snail that forms galls in the spines of sea urchins, and while most people might not think of snails as parasites - let alone parasites that live on animals like sea urchins, sea stars, and sea cucumbers - the parasite-host relationship of snails and echinoderms actually goes back hundreds of millions of years. There are fossils of snail boreholes and galls on ancient echinoderms. In fact, they are probably one of the few examples of parasitism that leaves a clear trace in the fossil record. If a sea cucumber is to write a parasitology textbook, most of it would be devoted to snails.

(1) A pair of Megadenus atrae - female on the left, male on the right; (2) Drawing of a M, atrae showing the proboscis (pr) and the pseudopallium (pp) cut away to show the shell (sh); (3) The shell of M. atrae - the larger ones are the female snail
Photos from Fig. 1 of the paper

Most of these parasites are from a family of snails call Eulimidae and the study that this blog post is covering was focused on a species call Megadenus atrae.  This parasitic snail has a few peculiar features when compared with the kind of snails that most people would be more familiar with. The shell is mostly wrapped up in a fleshy hood call the pseudopallium with only the tip visible, and it also has a giant sucker-like proboscis which it uses to cling to its host.

While other parasitic snails may simply attach to the skin or reside in the spines of their echinoderm hosts, this snails hangs out at a very specific spot - M. atrae lives in the cloaca of Holothuria atra - the black sea cucumber.

As strange as it may seem to us land-lubbers, the sea cucumber's butt is a popular hangout or gateway for many animals. There's the pearlfish which inserts its slim body into the sea cucumber through the echinoderm's cloaca and uses it as a kind of living shelter (some species also nibble on the sea cucumber's gonads while it is in there). There are also various crustaceans that are perfectly at home in a sea cucumber's butt. It is at this prime piece of real estate that M. atrae spends its adult life

In this study researchers collected black sea cucumbers from the chain of islands known as the Nansei Islands which stretches from the southern tip of Japan to the north eastern part of Taiwan, and recorded the presence of this parasitic snail. The snail is not particularly abundant, it was only found at two of the seven island sites they sampled from, and even on a reef flat at Kuroshima where they were most common, it was only found in one out of every ten sea cucumbers. Megadenus atrae has also been reported from other parts of the world including New Caledonia, India, and Australia. And in those other studies, the prevalence of this snail range from one in ten sea cucumbers to as few as one in a thousand.

Given that this parasitic snail is sparsely distributed in the sea cucumber population, this presents some challenges when it comes to reproduction - the likelihood of a larval snail encountering a host which is already occupied by another M. atrae is low enough, but the chance of that snail being of the compatible sex is even lower. Unlike other symbionts like pea crabs which can leave their host for a booty call, the only mobile stage of M. atrae is when it is a free-drifting immature larva. Once they are in a sea cucumber's butt, they are there for life

While it is possible that the snail can send out some kind of pheromone to recruit other M. atrae to settle in their host, how can they guarantee the new arrival would be of the suitable sex? After all there's no dating apps for snails living in a sea cucumber's butt.

Despite such obstacles, the researchers noticed that these snails were always found in pairs, and always as a female-male pair. They suggested that that this parasitic snail might have a sex determination system which is similar to that of the tongue-biter parasite and a range of other animals call protandry. With a protandric system, the larva starts out life as an immature male. If it settles down alone, it grows into a mature female snail. But if the snail larva happens to settle in a sea cucumber which is already occupied by a mature female, it will grow into a mature male. That way, M. atrae ensures that it will end up with a suitable reproductive partner no matter the circumstance.

So life finds a way, even for a parasitic snail trying to find a life partner amidst a sea of unlikely butts

Reference:
Takano, T., Warén, A., & Kano, Y. (2017). Megadenus atrae n. sp., an endoparasitic eulimid gastropod (Mollusca) from the black sea cucumber Holothuria atra Jaeger (Aspidochirotida: Holothuriidae) in the Indo-West Pacific. Systematic Parasitology 94: 699-709.

November 24, 2017

Corynosoma australe

Most parasites are very picky about what host they infect. Even those that can infect a number of different host species usually parasitise a selected bunch from the same family or order. But sometimes circumstances can bring together unlikely parasite and host pairings. The parasite featured in this post is Corynosoma australe, and it is an acanthocephalan - a group of prickly parasites commonly called thorny-headed worms. Corynosoma australe usually infects pinnipeds, the group of marine mammals that includes seals and sea lions. But in the study featured in this blog post, researchers found this worm living in the gut of a decidedly non-mammalian host - specifically the Magellanic penguin. So how did penguins end up acquiring parasites that usually infect seals?

(A) Adult male Corynosoma australe, (B) Adult female C. australe, (C) spiny proboscis of an adult worm
Photos from Fig 4. of the paper
For this, we need to look at the life-cycle of this parasite. Like other acanthocephalans, C. australe infects an arthropod as their first host, in the case of Corynosoma, this is usually tiny shrimp-like crustaceans called amphipods. For other acanthocephalans, the life-cycle is complete when the infected arthropod is eaten by a vertebrate predator, which can be a mammal, fish, bird, reptile or an amphibian, depending on the species of acanthocephalan in question. But during the life-cycle of C. australe, it also infects what is known as a paratenic host - a host animal which is not vital to the completion of the parasite's life-cycle, but can act as a vehicle to get it to the final host. In this case, the paratenic host is a fish.

The reason why they need a paratenic host is that seals and sea lions do not usually go rummaging through the the mud for tiny thumbnail-size crustaceans. But there are fish that do, and it is those fish that seals and sea lions eat. By using fish as paratenic hosts, C. australe can bridge the ecological gap between tiny amphipods and seals. But having fish as paratenic hosts also open up other possibilities because pinnipeds are not the only marine animal with a taste for fish. This is where penguins enter the story.

Even though taxonomically, birds and mammals are on very different branches of the vertebrate animal tree, because seals and penguins lead comparable life-styles, sometimes they can also end up with similar (or in this case, the same) parasites. In this case, Magellanic penguins end up with what is usually a seal parasite because they have been eating the same fish that the seal usually feed on, and they are physiologically similar enough to seals and sea lions for C.australe to go "Eh, good enough.". In fact, C. australe seems to be a fairly versatile parasite - it has been reported from 16 different types of marine mammals and birds. However, those previous reports also indicate that the parasite can only produce viable eggs while living in pinnipeds, and in the evolutionary game it all comes to nothing if you can't reproduce. Which means while C. australe can stay alive in those non-pinniped hosts, those other hosts are effectively dead ends.

But, this study shows that not only can C. australe survive perfectly fine in penguins, they can also reproduce while living in a bird host. From the samples that the researchers examined, 19 out of the 20 seals and sea lions they looked at were infected with C. australe. In comparison, only 18 out of the 87 penguins they examined were infected. Female worms grew bigger in the gut of Magellanic penguins, yet at same time they did not produce as much eggs as those living in pinnipeds. Also for some currently unknown reason (s), the sex ratio of C. australe in penguins is highly skewed - whereas seals and sea lion have an almost one-to-one ratio of male versus female worms in their guts, females worms vastly outnumbered male worms in the gut of Magellanic penguins.

Judging from egg production and prevalence, Magellanic penguins are not exactly the most ideal or reliable hosts for C. australe. Pinnipeds remain the hosts with the most for C. australe, but at least penguins can serve as a viable (if not ideal) substitute. For C. australe living in penguins, this might be a case of ecological fitting, whereby an organism can survive and (and even thrive) in a habitat which different to the one that it usually live in because it just so happen to have the right set of adaptations that allows it to survive in this new and novel environment.

But there is another twist to this story. While most species of Corynosoma live in marine mammals, it seems that they had evolved from ancestors that originally lived in aquatic birds. So perhaps Corynosoma already has the latent ability to survive in the gut of a bird, and when circumstances brought them together, C. australe was ready. When it comes to this thorny worm, what is good enough for the sea lion is good enough for the penguin.

Reference:
Hernández-Orts, J. S., Brandão, M., Georgieva, S., Raga, J. A., Crespo, E. A., Luque, J. L., & Aznar, F. J. (2017). From mammals back to birds: Host-switch of the acanthocephalan Corynosoma australe from pinnipeds to the Magellanic penguin Spheniscus magellanicus. PloS One 12(10): e0183809.

November 2, 2017

Steinina ctenocephali

Cat fleas (Ctenocephalides felis) is a parasite that everyone would be familiar with one way or the other. It is found worldwide and is the bane of cats, cat owners and basically anyone who does not like getting their blood sucked by tiny insects. But cat fleas are themselves just another animal and are host to their own parasites, such as Steinina ctenocephali; a single-celled parasite that lives in the gut of cat fleas. In that sense I guess one can regard S. ctenocephali as a hyperparasite - a parasite that parasitise a parasite.

(A) Female cat flea infected with feeding stages of Steinina ctenocephali (indicated by white arrow heads), (B) Male cat flea infected with feeding stages of Steinina ctenocephali (indicated by white arrow heads), (C) Scanning electron micrograph (SEM) of the parasite's feeding stage, (D) SEM of oocysts infective stages in a flea's gut wall, (E) oocysts of the parasite as seen through a hematocytometer. [all photos from Fig. 1. of the paper)
Steinina ctenocephali belongs to a group of single-celled "protozoans" call gregarines. They are parasites of arthropod and other invertebrate animals, and despite being single-celled, they are comparatively large, with some species having cells that reach almost a millimetre in length. They also have some rather unusual shapes for a large single-celled organism, with some species shaped like worms and there's even a genus that looks kind of like a rubber chicken. Steinina ctenocephali is not nearly as oddly shaped those species - it is roughly pear-shaped, which is pretty generic for a gregarine. However, far more noteworthy is the way that this parasite has thoroughly integrated itself into the flea's life-cycle.

Fleas are holometabolous insects that undergoes complete metamorphosis. This means much like butterflies and wasps they have larval stage that looks radically different to the adult form.
Newly hatched baby fleas look somewhat like bristly worms with chewing mouth parts and they are not at all equipped for blood-sucking. So what do baby fleas eat? Until they become fully-fledged jumping vampires, they mostly feed on organic detritus - some of that include poop from the adult fleas, which also contain undigested blood.

Steinina ctenocephali uses this cycle of poop-eating and blood-sucking to infect each subsequent generations of cat fleas and propagate in the flea population. In the adult flea, S. ctenocephali attaches to the gut wall as a feeding stage, eventually producing infective spores called oocysts which are released into the environment with the flea's faeces. Then, along come the flea larvae that gobble them up and inoculating themselves with S. ctenocephali. In the flea larva, the parasite take up residence inside the cells, eventually moving into the gut tract when the flea metamorphose into an adult and take its first blood meal.

Being infected with parasites usually carry some kind of cost for the host, in fact that is the very definition of parasitism. But the paper being featured in this post reveals another side to this gregarine-flea interaction. For their study, the researchers obtained flea eggs from a captive colony and raised them in microwells filled with a type of powder which is kind of like baby food for fleas. When the larval fleas hatch, they feed on this powder mixture until they metamorphose into blood-sucking adults. For the experiments, half of the fleas were raised on powder which had S. ctenocephali oocysts mixed in, while the other half were raised on a parasite-free diet.

The researchers did not find any differences in the survival of infected and uninfected fleas, but there was a difference in their growth rate. Parasites usually divert resources away from the host itself, and by doing so reduce the hosts' growth rate. But instead of what one might have expected, the researchers found that fleas raised on food dosed with S. ctenocephali actually grew faster than their uninfected counterparts. The infected fleas became mature adults a few days earlier than uninfected fleas. In fact, the more parasites they've been dosed with, the faster they grew. On average uninfected fleas took about 19 days to reach adulthood, whereas fleas that got a high dose of S. ctenocephali took only 16 days to become adults.

The researchers suggested that this faster development could be due to hormonal manipulation on the part of this (hyper)parasite. The sooner the infected fleas become adult, the sooner it can start pooping S. ctenocephali spores that can go on to infect even more fleas. Alternatively, it could be some kind of compensatory growth response by the fleas, and the cost of this accelerated growth may manifest later in life in other ways (such as reduced egg production or immune function)

Given that S. ctenocephali seems to give its host a competitive edge (at least when it comes to reaching reproductive maturity earlier) over their uninfected counterparts, is it really a parasite? One thing to keep in mind is that parasitism is just a another type of symbiosis. Terms like parasitism, commensalism, and mutualism are just categories that we have come up to place such interactions into some kind of context which are more convenient for our own understanding. But nature does not care about our categories and all symbiotic relationships exist along a gradient - in the natural world the line between friends or foes is fuzzy and may change at any time.

Reference:
Alarcón, M. E., Jara-f, A., Briones, R. C., Dubey, A. K., & Slamovits, C. H. (2017). Gregarine infection accelerates larval development of the cat flea Ctenocephalides felis (Bouché). Parasitology 144: 419-425.

October 6, 2017

Arthrophaga myriapodina

The forests around Ithaca, New York is the scene of an arthropod murder mystery. The killer seems to cover their track well and leave no obvious clues behind - aside from the dried, empty husk of dead millipedes clinging to the top of fence posts, branches, and fallen logs. So who or what is the macabre killer leaving the desiccated corpses of millipedes in prominent places? There are pathogens with similar modus operandi that infect and mummify insects; most of them are fungi, and a few of them have been previously featured on this blog, the most well-known example being the "zombie ant fungus". So what is the identity of this millipede killer?

(A) Typical posture of zombified millipedes infected with Arthrophaga myriapodina, (B, C) fungal structures erupting from between the segments of zombified millipedes. Photos from Fig. 3 of the paper

To find out, a group of scientists collected zombified millipedes and examined their fungal infection in detail using microscopes and by sequencing specific sections of their DNA which are used to identify and distinguish different fungi species. With this, they were able to identify and describe the zombie millipede fungus - they named it Arthrophaga myriapodina. This fungus that belongs to a group called the Entomophorales - a group of fungi consisting mostly of insect killers. For example a few months ago, I wrote about another entomophorale fungus that zombifies soldier beetles.

But A. myriapodina is the first species of that group documented to target millipedes. And while this study is the first time that this fungus has been formally described in detail and given a scientific name, such "zombie millipedes" have been known from as long ago as 1886, with some specimens stored in herbarium collections dating back from the early 20th century.

Given this millipede-infecting fungus has had such a long, but under-studied history, these scientists compared their freshly collected zombie millipedes with similar specimens held in museum collections, along with photographs of similar zombified millipedes hosted on sites such as Flickr, BugGuide, iNaturalist and other online photo-sharing sites. Through the combination of collecting fresh specimens, examining museum collections, and searching for online photos, they were able to establish that this fungus is found throughout Northeastern North America, with a few sighting from Texas and California.

As mentioned above, A. myriapodina has a modus operandi similar to many fungi that infect insects. The fungal spores find their way into the host's body and proliferate, eventually taking over the host entirely. When the fungus is ready to reproduce, it changes the host's behaviour so that it would carry it to a position that maximise spore dispersal. For A. myriapodina, this means anywhere elevated, whether it is the top of a fallen log, tree branches, or bridge abutments. Once in position, the fungus  emerge from the zombified millipedes in the form of powdery masses that seep out from between the segments. After they have dispersed their spores, the remaining fungal mass withers away, leaving an empty corpse and a fairy ring of infective spores.

The climbing behaviour that A. myriapodina induces in millipedes is comparable to those caused by zombie ant fungi. It is also a remarkable example of convergent evolution with a group of viruses known as baculoviruses which infect caterpillars and cause them to climb to their deaths. Those viruses induces a syndrome called Wipfelkrankheit or "treetop disease" that makes infected caterpillar climb to a high place before melting their bodies and raining droplets of virus-laden caterpillar goo into the forest canopy.

The emergence of zombie millipedes also seems to be weather dependent, because they are typically sighted a day or two after a bout of heavy rain. Perhaps heavy inundation acts as a trigger for the fungus to produce its spores. More research is needed to understand how rainfall and other seasonal pattern affects the life-cycle and outbreak of this fungal killer.

Reference:
Hodge, K. T., Hajek, A. E., & Gryganskyi, A. (2017). The first entomophthoralean killing millipedes, Arthrophaga myriapodina n. gen. n. sp., causes climbing before host death. Journal of Invertebrate Pathology 149: 135-140.

P.S. Some of you might know through my activities on Twitter (@The_Episiarch) that when I'm not writing these posts on new scientific papers about parasites, I also do illustrations, many of which are inspired by parasites and for the last two years I have been doing a series of illustrations known as "Parasite Monster Girls". So in keeping with the theme of this post, my most recent piece is Cordelia - a Parasite Monster Girl version of Cordyceps-infected zombie ants.

September 8, 2017

Sylon hippolytes

Some of you might have heard of the infamous parasitic barnacle Sacculina carcini which infects crabs and take over their bodies. These barnacles are true body-snatchers in every sense - they divert the host's resources for their own growth and reproduction, and by doing so they end up castrating their host. Additionally, some can also can alter their host's behaviour, making them unwitting babysitters for their eventual spawn.

(A) Shrimp infected with Sylon hippolytes; (B) Internal structure of infected shrimp show its nervous system (n) and the interna (i) and externa (e) of Sylon; (C) Close-up internal view; (D) Close-up internal view with colour-marking.
Photos from Fig 2 of the paper
While it sounds like a gruesome fate for the host, parasitic castration is a very clever way for a parasite to get the most out of the host without killing it. While the host can no longer reproduce, and thus dead from a evolutionary perspective, it doesn't need its reproductive organ to stay alive, but instead it now serves as a walking life support system for the parasite - a walking dead. So just how big can these body snatchers get in comparison with their host?

Sacculina carcini and related parasites belong to a group of very unusual parasitic barnacles call Rhizocephala. Their bodies consist of a network of roots call the interna which wrap around the host's organs, and a bulbous reproductive organ call the externa which sticks out of the host's abdomen. In a previous post I wrote about a study which used micro-CT scans to look at how the these parasites' roots are distributed around the host's organs. In the study featured in this post, a group of scientists compared the anatomy of two different rhizocephalan species and how it relates to their reproductive strategies

The two species they compared were Sylon hippolytes which infects the shrimp Pandalina brevirostris, and Peltogaster (featured in a previous post on this blog) which infect hermit crabs. They collected specimens of both parasites (and their hosts), and prepared them for scanning. After putting the prepared specimens through the micro-CT scanner, they used special software to calculate the parasites' volume and were able to construct a 3D computer model of each parasite along with the internal anatomy of their hosts.

Additionally, they also counted the number of eggs produced by each parasite, and for both species, it seems bigger hosts means more parasite eggs. Both Peltogaster and Sylon grew to about the same size in proportion to their respective hosts (17.78% for Peltogaster, 18.07% for Sylon). But the key difference lies in how much of that mass is distributed between reproductive externa versus the interna root system in the host's body.

The shrimp-infecting Sylon devoted the bare minimum to its interna which is only about 2.5% of the volume of its externa. In contrast, the interna root system of the hermit crab-infecting Peltogaster was about one-fifth of the volume of its externa. So why is there such a massive difference between those two species since they parasitise the host in a similar way? The answer lies in their respective reproductive investments. The Sylon specimens measured in this study had about 1400 to over 22000 eggs, and to produce all those eggs Sylon has to devote a lot more of its mass to its reproductive tissue. In contrast, Peltogaster produced a comparative modest number of eggs, only 371 to 4580.

But why does Sylon put so much into egg production while leaving the bare minimum to the part of its body which is actually embedded in the host? The main reason is that Sylon only gets one shot at breeding - it only ever produce a single brood in its lifetime before it withers away, so it has to make the most of it by having a massive externa. In contrast, once Peltogaster becomes established in a host, it spawns repeatedly and grow a new externa each breeding season, and in order to do so, it needs to invest in a robust network of tendrils which will stay in the host for good.

In this sense, Sylon has a "YOLO" approach to host exploitation and reproduction, whereas Peltogaster is in it for the long haul and so devote more of itself to establishing an extensive root system inside the host. This also has important consequences for the host as well since both parasites places such a massive burden on their hosts - while the demanding presence of Sylon will eventually come to pass, Peltogaster is a persistent body-snatcher that's going to stick around for quite a while.

Reference:
Nagler, C., Hörnig, M. K., Haug, J. T., Noever, C., Høeg, J. T., & Glenner, H. (2017). The bigger, the better? Volume measurements of parasites and hosts: Parasitic barnacles (Cirripedia, Rhizocephala) and their decapod hosts. PloS One 12(7): e0179958.

August 16, 2017

Sabinella troglodytes

Snails are host to a wide range of parasites, especially parasitic flukes that turn snails into clone factories to pump out streams of parasite larvae. But there are species of snails which are parasites themselves, and many of them are parasites of echinoderms - the phylum of animal which includes the likes of seastars, sea urchins, and sea cucumbers.

Left: live adult Sabinella troglodytes, Centre top: snail eggs inside a spine gall, Centre bottom: snail feeding on spine gall,
Right top: Female snail feeding on top of a spine gall, Right bottom: A spine gall showing feeding scar from snail
From Figure 1 and 2 of the paper
This post features Sabinella troglodytes - a parasitic snail found off the coast of Brazil which lives on the body of the slate pencil urchin Eucidaris tribuloides. The paper featured in this post presented a description of its life-cycle and other natural history observation of this gastropod. Parasites tend to be very specific about what part of their host's body they live on, and if there's one thing that sea urchins are known for, it is their spines, and that's what S. troglodytes feed and live on

Most molluscs have a rasping organ call a radula which they use while feeding to scrape away at their food. In the vampire snail this has been modified into something like a syringe which they can use to stab into a fish to drink their blood. In predatory whelks, the radula is used like a file to rasp away at the hard shell of their prey (usually another mollusc) to access the soft, gooey centre. But that is not how S. troglodytes feed on its sea urchin host. Unlike most of its gastropod relatives, S. troglodyte has lost its radula - so how can it bore into the spine of a sea urchin to reach its tasty core? Based on their observations, the researchers who conducted this study concluded S. troglodyte is secreting some kind of corrosive substance to eat through the tough walls of the spine in order to gain access to all that soft internal spine tissue.

But this parasitic snail is not content to simply just feed on the sea urchin, they also alter the urchin's spines to make it a more comfortable home. Sabinella troglodytes is one of many species of gall-forming snails that parasitise echinoderms. As their name indicates, the slate pencil urchin is covered in straight, pencil-shaped spines - but the spines housing S. troglodytes look almost  like fattened tubers. Much like how gall-wasps can induce bulbous growths on their host trees, these gall-forming snails can cause growth abnormalities in the sea urchin's tissue. This is also somewhat comparable to Accacoelium contortum, a parasitic fluke that lives on the gills of ocean sunfish while wrapped in a cosy little flesh bag made out of the host's tissue.

It is currently unknown how S. troglodytes alters the sea urchin's spines, but it could be due to some other components in the snail's saliva - in addition to corrosive agent to erode the sea urchin's spine, it might also be spitting out growth factors that alters the tissue of the spine. In addition to being a cosy place to feed and hide from threats, these galls seem to be a bit of a love nest for S. troglodytes during the summer months. The researchers noted that between December and February, almost all the galls were mostly occupied by snail couples (consisting of a female and her smaller male mate) which have settled down to raise a brood of eggs. But for the rest of the year, the galls were filled with juvenile snails which had probably inherited the gall from their parents.

While the spines of many sea urchins are straight and narrow, they are not immune to tampering by the right parasite. Sabinella troglodytes shows that with a little biological renovation, one can turn even something like a sea urchin's spine into a cosy home suitable for raising a healthy brood.

Reference:
The gall-former Sabinella troglodytes (caenogastropoda: Eulimidae) and its association with Eucidaris tribuloides (Echinodermata: Echinoidea). Journal of Conchology 42: 371-377.

July 27, 2017

Myriophora alexandrae

Millipedes are known for secreting some really noxious chemicals to ward off its enemies. So much so that other animals are also known to co-opt the millipede's chemical cocktails for their own use.
For example lemurs and capuchin monkeys are known to smear millipede juice on themselves as a mosquito repellent. But there is no perfect defence and the millipede have some adversaries that are not deterred by its defensive secretion, some of which are phorid flies in the Myriophora genus - which are specialised millipede hunters.

a) Myriophora alexandrae laying an egg at the base of a millipede's antenna, (b) Myriophora communis (insert: ovipositor),
c) Millipede excreting defensive fluid while a Myriophora harwoodi feeds on it, (d) Myriophora maggot and dead host
Photos from Fig. 2 of this paper
Phoridae is a family of flies also known as "scuttle flies" - they are mostly parasitoids and there are thousands of species found all over the world. Some species are well-known for their ant-decapitation trick. Despite being rather common, very little is known about most of these parasitoids - this is possibly because of the daunting number of species, their small size, and general lack of research interest beyond a handful of species which either have potential as a mean of biological control, or are pest of cultivated insects like bees.

With so many different species of phorids out there, perhaps it is not surprising that they have evolved to target a wide range of terrestrial invertebrates - and some of them specialised in hunting millipedes, undeterred by their chemical defences. So how do Myriophora flies track down their millipede hosts? To find out, a group of researchers conducted a series of experiments at a research station in Costa Rica to determine what it is about millipedes that these flies find so attractive. They wanted to figure out whether it is the defensive juices secreted by the millipedes or the sight of the millipedes themselves. So they collected some millipedes, kept them for a few days to ensure they're parasitoid free to start with, and used them to set up an experiment in luring phorid flies.

They end up presenting the flies with the following: (1) millipede juice dabbed on pieces of paper (which they obtain by lightly zapping the millipedes), (2) dead millipede smeared in millipede juice, and (3) millipedes which have been cleared of any millipede juice by zapping them until they run out of defensive secretions.

They found that the Myriophora flies were rather attracted to paper cards dabbed with millipede juice and millipedes that were smeared with their own defensive secretions. In contrast, the flies completely ignored perfectly intact millipedes that were completely juiced out. So it seems that the scent of a millipede is far more important than the sight of one for these flies.

So not only are Myriphora not deterred by the millipede's noxious chemicals, they are actually attracted to it. But millipede juice is a complex chemical cocktail - which is the exact compound that the flies are homing in on? After further analysis, the researchers determined that the compound in millipede juice most responsible for attracted these parasitoids is a chemical called 2-methoxy-3-methyl-1,4-benzoquinone. That alone was enough to bring the flies to the yard. But when that is combined with another compound found in millipede juice called 2-methyl-1,4-benzoquinone, this cocktail was three times more attractive to Myriophora flies than that first compound by itself.

But a millipede is a well-protected target - even if you can get past its noxious secretions, it also has some formidable armour platings. But there are gaps in its armoured segments and Myriophora has a specialist weapon to exploit those gaps. This parasitoid fly has an ovipositor which is shaped somewhat like a thin stiletto - when it lands on a millipede, Myriophora stabs its ovipositor at spots like the base of the antenna, the gap between the head and the rest of the body, the vulnerable underbelly of the body segment, or between the plates covering the millipede's butt.

The parasitoid sticks its ovipositor between those gaps in the millipede's armour, and delivers a deadly payload in the form of an egg. Once the egg hatches inside the millipede, the newborn maggot has a hearty appetite and a growth rate to match. Within five days, it will finish cleaning out the millipede host from the inside, leaving behind only an empty husk and the millipede's hindgut. The maggot will then crawl from its host's empty corpse to pupate and eventually emerge as an adult fly, ready to bring up a new generation of millipede-wreckers

So for the millipedes, while those defensive cocktails are great for fending off everything else, there is no perfect defence - the very thing that protects it against some many predators is also the very thing that brings in parasitoid flies that will eat them alive.

Reference:
Hash, J. M., Millar, J. G., Heraty, J. M., Harwood, J. F., & Brown, B. V. (2017). Millipede Defensive Compounds Are a Double-Edged Sword: Natural History of the Millipede-Parasitic Genus Myriophora Brown (Diptera: Phoridae). Journal of Chemical Ecology 43: 198-206.

July 10, 2017

Anoplocephala manubriata

Tapeworms are a very diverse group of parasitic worms. There are about 6000 described species and they infect a wide range of different vertebrate animals including fish, amphibians, mammals, reptiles, and birds. But even though there are so many different tapeworm species, the one thing they all have in common is that the adult worm lives in the intestine of their vertebrate host. So it would be no surprise that a large animal like an elephant would be host to tapeworms, and the species that is featured in the study that we will be covering in this blog post is Anoplocephala manubriata.

Top left: Adult Anocepgala manubriata tapeworm
Top right, bottom left: close-up of scolex and suckers
Bottom right: tapeworm egg containing oncosphere
Photo from Fig. 1 and 3 of the paper
Despite being an elephant parasite, these tapeworms are not as big as you might think. Many people think that big host means big parasites, and while some parasites in large animals can reach massive sizes, but that is not always the case. Instead of being infected by big parasites, many large animals are often host to parasites that are not much bigger than related species infecting smaller hosts.

For example, the Great White Shark is infected by a species of tiny tapeworm which measures just a few millimetres long, but what they lack in size they make up for in numbers, and a single shark can be infected by thousands of them. While A. manubriata grows to a respectable size for a tapeworm (4.6 cm–7.4 cm long and 0.7 cm to 1.8 cm wide), it is nowhere near the size of the infamous broad fish tapeworm which can reach the alarming length of over 10 metres long.

The tapeworms described in this study were retrieved from a young male elephant that died at the Udawalawe Elephant Transit Home in Sri Lanka. Anoplocephala manubriata has very muscular suckers on its scolex which allows it to keep a firm grip on the host intestinal wall. But this is not so great for the elephant - the suction from the tapeworms' suckers essentially end up leaving hickeys on the elephant's intestinal mucosa, which is not a particularly healthy place for an elephant to get love bites, especially if they have been left there by a bunch of tapeworms. Indeed, the elephant that was necropsied in this study was found to have multiple lesions and ulcers on the gut lining as a result of these parasitic love bites. This tapeworm seems to be far more common among younger elephants than adults, possibly because older elephants have more developed immune systems, and have build up some kind of resistant towards these parasites.

Tapeworms have complex life-cycles, and before the adult worm ends up in the intestine of the final host, they have to first develop as larval stages in smaller animals - usually an invertebrate, in some case a small vertebrate animal - and these small animals are usually the prey species of the final host. That is why the final host for many species of tapeworms are often predatory animals or at least animals that include smaller animals in their diet.  But what about elephants though? They are not usually known for eating bugs or other small animals, and the other tapeworms in the Anoplocephala genus are parasites that infect horses, zebras, and rhinoceros - all herbivorous mammals. So how does A. manubriata finds its way into these giant herbivorous animals?

A previous study found that A. manubriata actually uses orbatid mites as an intermediate host. Orbatid mites are minuscule arachnids that live among soil and litters - they are very tiny, and most species are less than one millimetre long. But being so tiny means that the elephant can easily swallow them inadvertently along with their usual fodder. Branches and leaves that have been in contact with soil can inadvertently pick up some of these tiny mites, and at least a few of those would be infected with  A. manubriata larvae. But there is also another way through which elephant can end up with A. manubriata. Elephants that have gastrointestinal problems also have a habit of eating dirt, possibly as a way of self-medication, as seen in other animal. However, while trying to cure themselves of one ill, they end up ingesting soil mites and inflicting another different ill upon themselves.

Like many parasites, A. manubriata is a key part of the ecosystem, and the life-cycle of this tapeworm, which involves both the elephants and soil mites, reveals the hidden ecological connection between one of the planet's largest living land animal and one of its smallest.

Reference:
Perera, K. U. E., Wickramasinghe, S., Perera, B. V. P., Bandara, K. B. A., & Rajapakse, R. P. V. J. (2017). Redescription and molecular characterization of Anoplocephala manubriata, Railliet et al., 1914 (Cestoda: Anoplocephalidae) from a Sri Lankan wild elephant (Elephas maximus). Parasitology International 66: 279-286.