"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

May 10, 2018

Raillietiella orientalis

The Burmese python is the third largest snake in the world and it is a consummate hunter. It is equally at home swimming through water as it is at climbing trees, and it eats whatever that it can wrap around and swallow. While it is native to Southeast Asia, it has also been introduced to Florida and their presence has caused all kinds of ecological disruptions. Being such a large predator with a broad appetite, many native animals (including alligators) of the Everglades are at risk of becoming python food.

But the Burmese Python may also be affecting the Everglades in a less noticeable manner. On their native range the python is host to a range of parasites, one of which is Raillietiella orientalis, a peculiar-looking creature that belongs to a group of parasite called Pentastomida, more commonly known as "tongue worms".

Left: close-up of the anterior of an adult Raillietiella orientalis adult, Right: a Raillietiella orientalis larva
Photos from Figure 1 of the paper
These parasites are called "tongue worms" not because they live on the tongue, nor because they are "worms" as such. Their name comes from the appearance of the adult pentastomids which are shaped somewhat like a long tongue, and instead of being "worms", they are in fact a lineage of crustaceans that have evolved to live as respiratory tract parasites in terrestrial vertebrates, mostly reptiles. In the Everglades, Burmese Python is host to tongue worms, but where did these parasites come from? Were they brought to Florida by the Burmese python, or were they native to Florida? As it turns out, it was a bit of both.

This blog post features a recently published study where a group of researchers examined the snakes of Florida for pentastomids. The sample they looked at had been collected gradually over the last decade; the Burmese pythons were either roadkills or snakes that were captured and euthanised, and all the native snakes that they examined were roadkills as to minimise the impact their study would have on the native snake fauna. In total, they looked through the lungs of 805 Burmese pythons and 498 indigenous snakes, picking out tongue worms along the way.

From that mountain of dead snakes, researchers discovered that the Burmese pythons in Florida are host to two species of tongue worms - Raillietiella orientalis and Porocephalus crotali - both of which also infect the native Floridan snakes. By examining the DNA of R. orientalis, the researchers determined that the parasite did not originate in Florida. Instead it had arrived as a stowaway in the lungs of Burmese pythons. Elsewhere, R. orientalis infect a wide range of snakes from across many different families, and it seems to have take a liking to Floridan snakes as well. Wherever the Burmese pythons were found, the native snakes in the surrounding areas were also infected with R. orientalis.

But in addition to bringing a new parasite to Florida, the Burmese python has also become acquainted with some of Florida's own native snake parasites. Porocephalus crotali is a parasite which infects the snakes of North and South America and is a Floridan native. It was previously thought that P. crotali can only infect vipers, but now it adds the Burmese python to its list of hosts. The presence of this parasite in those Burmese pythons shows that it wasn't as picky as previously thought. The reason why P. crotali was previously only found in vipers wasn't because they were particularly picky, but the opportunity for it to infect other types of snakes never came up - until the Burmese python arrived.

So what does this mean for the Floridan snake fauna? The short answer is: more parasites. The native snakes are facing a parasite double-punch - not only did the Burmese python added another species of parasite that can infect them, but they would also be dealing with higher prevalence of the native parasite because the Burmese python is acting as an additional breeding ground for P. crotali.

With so many plants and animals (and their parasites) being transported around the world on a daily basis, invasive species have become fixture in many ecosystems. As these invasive species settle into their new habitats, they also end up exchanging parasites with the native species. While this is a scenario which is being played out in many different ecosystems around the world, the ecological impact of these parasite exchanges for most habitats is still largely unknown.

Reference:
Miller, M. A. et al. (2018). Parasite spillover: indirect effects of invasive Burmese pythons. Ecology and Evolution, 8, 830-840.

April 9, 2018

Massospora cicadina

Periodical cicadas spend most of their lives as juveniles (also known as nymphs), living underground and sucking juices from tree roots. Depending on the species, they keep to this subterranean existence for 13 or 17 years before finally emerging into daylight. And they do so simultaneously in massive numbers. These newly emerged nymphs will climb on to a nearby tree to moult into winged adults. The life of an adult cicada is short and over in about a month. During this period they sing their hearts out and mate until they drop to produce the next generation of cicada nymphs which will return to the soil. But the cicadas aren't the only ones to get busy during this period. Scattered across the landscape are the spores of Massospora cicadina, and for over a decade they have been waiting patiently for the cicadas' return.
Cicadas with and without Masspora infection, note uninfected male cicada which still has the genitalia of an infected female cicada attached. Photos from Figure 1 and 2 of the paper
Massospora cicadina is a parasitic fungus that targets all seven known species of periodical cicadas, and its effects on the host are devastating. Once infected, the cicada is done for - the fungal infection turns the cicada's abdomen into a chalky mass of spores. Surprisingly, despite missing a big chunk of itself, an infected cicada carries on as if it is business as usual - these diseased cicadas keep flying, singing and mating like their uninfected counterparts. But surely there must be more going on beneath that exterior of surprising normality.

A group of researchers investigated if Massospora is doing more to cicadas than just robbing their booties. In particular, they were interested in whether Massospora is altering the cicada's behaviour, as many other insect-infecting fungi are known to do. Since the mid-1990s, they have been spending hundreds of hours documenting the behaviour of both infected and uninfected cicadas. They also collected some of those cicadas and kept them in captivity for closer observations, and played recordings of male cicada songs to them to see how they responded.

There are two ways that cicadas can get infected with Massospora, and how they do so determines what kind of infection they end up with. If a cicada brushed up against some Massospora spores while emerging as a nymph, they end up with what's called a Stage I infection. However, if they picked up the fungus by coming into contact with an infected adult cicada, they would end up with a Stage II infection. Both are equally bad for the cicada, but there are some key differences between them.

Cicadas with Stage I infection tend to crawl around a lot more and leave behind a trail of contagious spores wherever they go. In contrast, those with Stage II infection fly around more often. But aside from that there are also other key behavioural differences, and it relates to what all these cicadas have emerged for - mating. Male cicadas with Stage I infection respond to mating calls the way that female cicadas usually do - with wings flicks that are the cicada's equivalent of "Hey, I'm interested - come and get me!" Any amorous cicadas that respond to this gesture and mate with the infected male also end up contracting the deadly fungus. However those with Stage II infections simply ignored those calls and kept to themselves.

This behavioural change in the infected cicada is more sophisticated that simply turning the male cicada to a "female phenotype". Aside from responding to calls with wing flicks, these male cicadas still behave like other males. The fungus merely added another behavioural response to their repertoire. So what about those with Stage II infection? Why don't they get in on the action?

The spores produced by Stage I infections immediately contagious, so it spreads through the cicada population through physical contact (such as mating). Meanwhile, Stage II infections produce a different type of spores that cannot infect cicadas right away, but can stay dormant and viable in the soil for decades. These spores lie in wait for a future brood of cicadas to emerge, infecting the nymphs as they crawl out of the soil.

In this case, the fungus doesn't need the host to be flirty and rub carapace with other cicadas, they just need it to be a diligent little crop-duster that sprinkle fungal spores all over the landscape. By doing so, Massospora is well-prepared for the next emergence event, when the festival of frantic cicadas and fungal booty-snatchers can start all over again.

Cooley, J. R., Marshall, D. C., & Hill, K. B. (2018). A specialized fungal parasite (Massospora cicadina) hijacks the sexual signals of periodical cicadas (Hemiptera: Cicadidae: Magicicada). Scientific Reports 8(1), 1432.

March 8, 2018

Gyrinicola batrachiensis

As far as parasitic nematodes go, pinworms are comparatively benign. Whereas Ascaris roundworms go tearing through your organs and can block up your intestine, and hookworms are basically gut-dwelling vampires that drink your blood, for the most part, pinworms just give you an itchy bottom. But the human pinworm (Enterobius vermicularis) is only one out of about 850 described species of pinworms. Pinworms belong to the order Oxyurida and they are found in the hindgut of various insects, reptiles, amphibians, fish, birds, and mammals, and as mentioned above, they don't usually cause their host much trouble - all they really want to do is munch on bacteria, and it just so happen that the hindgut of some animals, especially those that include plants as a significant part of their diet, is heaven for the kind of bacteria that pinworms crave.

Adult female G. batrachiensis on the left, adult male G. batrachiensis on the right
Left photo is from Fig. 1 of this paper and the right photo is from Fig. 1 of this paper
Gyrinicola batrachiensis is a species of pinworm that infects amphibians and it has been reported from 18 species of frog and toad. But G. batrachiensis only survive in the gut of their host during the tadpole stage. Once a tadpole begins metamorphosing into an adult, it become uninhabitable for G. batrachiensis. Reason being that while most tadpoles are algae-feeding herbivores with a long coiled gut, frogs and toads have relatively a short hindgut and are strictly carnivorous - so the complete opposite of what a pinworm needs. From the pinworm's perspective, this puts a definitive time limit on how long its cozy oasis will last before it transforms into a barren wasteland. In the study featured in today's blog, a group of researchers investigated how this parasite respond to living in tadpoles of different frog species, and whether there are some tadpoles that are more of a pinworm magnet than others.

By far the most important task that a parasite needs to accomplish during its limited time in the host is reproduction. Gyrinicola batrachiensis can reproduce in two different ways: (1) the asexual way, which result in thick-shelled eggs that are release to the outside world and infect other tadpoles, or (2) via sexual reproduction which produce a mix of both thick-shelled eggs and thin-shelled eggs. Those thin-shelled eggs never leave the tadpole, instead they are "autoinfective" - which means they hatch right there in the tadpole's gut and starts growing. So while those thin-shelled eggs won't survive the rigours of the outside world, but are good for filling up the tadpole's gut with more worms in a relatively short period. Each of those egg types have their own purposes, so how does G. batrachiensis balance between producing those two different types of eggs?

Of the five different species of frogs and toads that the researchers examined, one species stood out as being the best host for G. batrachiensis - the tadpoles of the Southern leopard frog (Rana sphenocephala). Leopard frog tadpoles are much larger than those of other four species they looked at, and it takes between 8 to 13 weeks for the tadpole to reach adulthood, comparing with the tadpoles of the other species which can complete development in as little as 4 weeks. With more space and time to grow, the pinworms living in leopard frog tadpoles could afford to invest time and resources towards growing bigger instead of rushing to pump out eggs before their time runs out. In the long run, bigger worms can produce more eggs - but the pinworms living in the tadpoles of those other frog species don't have that luxury.

Additionally the researchers found that only the pinworms in leopard frog tadpoles produced the autoinfective thin-shelled eggs. While pinworms in the tadpoles of other frog species have to focus on producing thick-shelled eggs to infect new tadpoles before their limited time run out, those in the gut of leopard frog tadpoles have more time and room to work with - so they might as well make the most of it by producing some autoinfective, thin-shelled eggs to fill up the tadpole's gut with more of its own offspring and get a head start on producing the next generation.

But while the leopard frog tadpole seems to provide G. batrachiensis with the ideal environment, it is not the species which is most commonly infected with G. batrachiensis. Once those thick-shelled eggs leave the tadpole, they sink to the bottom of ponds where they wait to get sucked up by an unwary tadpole - and they don't get to chose which tadpole they end up in. For this study, the researchers found that pinworms were most commonly found in the tadpoles of Blanchard's cricket frog (Acris blanchardi). In contrast, the tadpoles of the narrow-mouthed toad (Gastrophryne olivacea) found in the same pond managed to stay worm-free.

So why does one species seem to be a pinworm magnet while the other manage to stay clean even though they are living in the same environment? This might something to do with how they eat. Tadpoles of the Blanchard cricket frog feed by scrapping algae off the bottom of ponds with their mouth. In the process, they also suck up some of those thick-shelled pinworm eggs that are lurking amidst the muck. In contrast, the tadpoles of narrow-mouthed toad feed by slurping tiny plants and animals off the water's surface, so they don't come anywhere near those pinworm eggs. While G. batrachiensis might not always end up in their ideal host, they always try to make the most of it.

Reference:
Pierce, C. C., Shannon, R. P., & Bolek, M. G. (2018). Distribution and reproductive plasticity of Gyrinicola batrachiensis (Oxyuroidea: Pharyngodonidae) in tadpoles of five anuran species. Parasitology Research 117:461-470.

February 12, 2018

Neocyamus physeteris

Today we're featuring a guest post by Sean O’Callaghan - a student from 4th year class of the Applied Freshwater and Marine Biology' degree programme at the Galway-Mayo Institute of Technology in Ireland. This class is being taught by lecturer Dr. Katie O’Dwyer, who has previous written guest posts about salp-riding crustaceans and ladybird STI on this blog. This post was written as an assignment on writing a blog post about a parasite, and has been selected to appear as a guest post for this blog. Anyway, I'll let Sean take it from here.

Sperm whales are the largest toothed animal alive and they are capable of diving down to depths of 1200 m to feast on cephalopods (including the planet's largest cephalopods, the colossal and giant squids), but despite their size and abilities, these leviathans can fall victim to a range of cunning ectoparasites, including…Whale Lice!

Line drawing of adult female Neocyamus physeteris from Fig. 2 of this paper, SEM photograph from Fig. 2 of this paper
Three species of whale lice are known to target sperm whales, and from this trio there is a divide of preference between male and female whales. Neocyamus physeteris is one such example - they would rather live on a female whale than a male one. While the exact reasoning behind why there is such a divide in parasite species targeting opposite sexes, the answer may be due to the habits of male whales, which frequent the polar waters more often than the females who seek out the warmer waters around temperate zones.

Whale lice are not really lice in a taxonomic sense. Instead, they are classed as amphipods, crustaceans related to the so-called "lawn shrimps" which are found in some back gardens, but with more specialised features for hanging on to a free-swimming whale. Neocyamus physeteris’ body is flattened like a leaf but largely segmented and have legs tipped with hooked edges that act like crustacean crampons to ensure a consistently ample footing. Otherwise the lice would find itself cast adrift without a home or food supply to die alone in the deep. They also possess sharpened mandibles to munch through the host whales epidermis (top skin layer) while for breathing it has two pairs of gills lining its underside towards the front half of the body. Neocyamus physeteris’ head is quite small in comparison to the rest of its body and is dotted with a pair of tiny eyes along with two antennae. Their white colouration almost gives off a dandruff-like appearance against the whale’s darker complexion (though they would be well camouflaged on Moby Dick if it had existed and was also female!).

They are so intertwined with their host that their life cycle that they lack a free-swimming larval phase or active transmission to other whales, offering limited opportunities to move between hosts (unless during social activities where the whales may rub against one another). So it is fair to say that they live, feed and breed on top of their own biological ark, from the sea's clear surface waters to dark depths of the twilight zone, quite a dependent but extreme lifestyle!

Like most whale lice, little is known about the habits of N. physeteris, but it is so specialised for its life-style that whenever the whale dies, the lice would also kick the can as they require a live host. Hanging onto a host may not seem like an exciting lifestyle, but it is a highly beneficial strategy (for the lice at least). Given its tendency to devour sperm whale skin mainly in areas that are sheltered from water movements like the genital slits, body creases or injured skin, this allows the lice to take advantage of a lifetime supply of renewable food. In other words, the lice won’t starve while on a whale, however there will be an increase demand for firm footholds as the parasite population increases, so the species' overall success is not necessarily always good for the individual louse. The whale probably doesn’t suffer too badly when only a handful of lice are present however a colony must surely be highly irritating to say the least.

The strain imposed on N. physeteris at different depths due to the varying degrees of pressure imposed between the surface and abyss would far exceed our own limits. Undoubtedly there must be a risk posed by potential fishy predators on occasion given the lack of cover afforded by a whale’s skin. However, the benefits appear to outweigh the risks - otherwise they would cease to exist as a species. There is still much to learn about these fascinating parasites but until new means of studying the movements and behaviours of these small, somewhat inconspicuous amphipods on top of a large mobile host like a sperm whale are developed, it could take a while to unravel the intricacies of this skin serrating invertebrate!

References
Hermosilla, C., Silva, L.M.R., Prieto, R., Kleinertz, S., Taubert, A. and Silva, M.A. (2015). Endo- and ectoparasites of large whales (Cetartiodactyla: Balaenopteridae, Physeteridae): Overcoming difficulties in obtaining appropriate samples by non- and minimally-invasive methods. International Journal for Parasitology: Parasites and Wildlife. 4, 414-420.

Leung, Y. (1967) An illustrated key to the species of whale-lice (Amphipoda, Cyamidae), ectoparasites of Cetacea, with a guide to the literature. Crustaceana 12, 279-291.

Oliver, G. and Trilles, J.P. (2000). Crustacés parasites et épizoítes du cachalot, Physeter catodon Linnaeus, 1758 (Cetacea, Odontoceti), dans le golfe du lion (Méditerranánée occidentale). Parasite. 7, 311-321.

This post was written by Sean O’Callaghan

February 1, 2018

Glyptapanteles sp.

Today we're featuring a guest post by Niamh Dalton - a student from 4th year class of the Applied Freshwater and Marine Biology' degree programme at the Galway-Mayo Institute of Technology in Ireland. This class is being taught by lecturer Dr. Katie O’Dwyerwho has previous written guest posts about salp-riding crustaceans and ladybird STI on this blog. This post was written as an assignment on writing a blog post about a parasite, and has been selected to appear as a guest post for this blog. Anyway, I'll let Niamh take it from here.

Wasps in adult form are terrifying, right? Humans automatically associate the sight of wasps with sudden panic in the fear of getting a minor sting. What do we really have to be afraid of? After briefly studying the life-cycle of a species of wasp, Glyptapanteles, I assure you it’s not adult wasps we should be frantically sprinting away from, it’s their babies.

Glyptapanteles cocoon being watched over by their caterpillar guardian, from Fig. 1 of the paper
Glyptapanteles wasps are parasitoids, a group of parasites that inevitably kill their host.  Adult females, after mating, will inject their eggs into a live caterpillar. The caterpillar will act as a surrogate womb, giving the eggs a chance to develop into mature larvae as they feed of its bodily fluids. The larvae eventually break through the skin of the caterpillar to complete pupation, meanwhile the caterpillar is still living and undergoes mind control by the parasite, becoming a modified bodyguard and surrogate parent until the larvae break out and fly away, leaving the caterpillar to die of starvation.

As spine chilling as this process is, a team of scientists were particularly interested in this survival technique and they constructed an experiment to investigate the behaviour modifications inflicted by the parasite on their host.

It all begins with a female wasp injecting approximately 80 eggs into the body cavity of a caterpillar using an ovipositor or egg layer. Each egg hatches into a larva in the the caterpillar’s body, feeding only off the bodily fluids and being careful not to damage any internal organs in order to keep the host alive and functional. According to the scientists' observations, there is no behavioural modifications of the host during this internal parasitism stage, however, each larva is the size of a rice grain and the density of the larvae in a caterpillar can have morphological alterations. The caterpillar will grow in girth but not in length, looking ready to explode.

It gets worse. Eventually the larvae have to leave the nest, so to speak. To complete the final stage of maturity, all 80 larvae evacuate the host simultaneously by using their newly developed jagged jaws to slice through the caterpillars’ tough skin. Whilst emerging through the tough material, the larvae release a chemical which only paralyses the host, meaning the host is alive throughout this excruciating process. In order for the larvae to keep their host alive, they coincide their last moulting stage with their exit, filling the holes they have excavated with a ‘plug’ made of their sloughed exoskeleton.

Why would the Glyptapanteles larvae have to keep the host alive after emergence? Well, following their exit, the larvae begin to spin silk strings and form cocoons for their last stage of maturity. At this stage, the larvae are vulnerable to predators and other parasitoid wasp species that can inject their eggs into these larvae (ironically). The host develops behavioural modifications during the parasites pupae (cocoon) stage, acting as a bodyguard. As caterpillars are themselves larvae of butterfly and moths, they too construct a cocoon in their life-cycle. As the scientists found, the host caterpillar will use their own silk string to weave a blanket over the Glyptapantele cocoons for further protection.

That’s not all. The host will increase its number of violent head swings in attempt to scare off any form of disturbance. The host is also known to stand on two pairs of back legs in vigilance and spending a substantial amount time bent over the cocoon mound. In the experiments, the research team found an increase in aggression in caterpillars that were infected with the parasitoids compared in caterpillars that were not exposed to parasites.

The main question that remains was: How is there behavioural modifications in the host after the exit of the parasite? After the dissection of previously parasite-stricken caterpillars, there were 1 or 2 active parasitoids found still in the body cavity. The authors of this paper hypothesised that these leftover larvae are responsible for the mind controlling of the host after emergence. In this way, the parasites sacrifice a few individuals for the survival of the majority of the larvae. This is a uniquely evolved survival technique that is obviously very effective and bitter-sweet in a strange way.

Reference
Grosman, A., Janssen, A., de Brito, E., Cordeiro, E., Colares, F., Fonseca, J., Lima, E., Pallini, A. and Sabelis, M. (2008). Parasitoid Increases Survival of Its Pupae by Inducing Hosts to Fight Predators. PLoS ONE, 3(6), p.e2276.

This post was written by Niamh Dalton

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.