Flashy body-snatchers, parasite services, and intrepid journeys

A lot has certainly happened this year - and I'm not even talking about parasitology. I can barely believe that as of this year, I have been writing for the Parasite of the Day on a regular basis for six years! With that said, what have been some of the highlights from the blog this year?

Well as usual, stories about body-snatching parasites are always popular and this year featured hairworms that parasitise ground beetles living in the frigid cold of the Arctic Circle. There were also posts addressing the true nature of some parasitic barnacles - both how their network of parasitic tendrils actually look like inside their host, and how they might be more diverse than meets to the eye. And you can't talk about life-sucking monsters without mentioning vampires, and this year also brought the tale of vampire mites that literally sucks the life out of developing ant pupae.

Of those body-snatching horrors, there was a new paper about the infamous zombie snail parasite - Leucochloridium paradoxum - which made it just in time for 2016. Leucichloridium is pretty flashy with those colourful broodsacs that cause their host's eyes to bulge out like psychedelic candy canes, but they are not the only parasite that gets into their host's eyes - this year also featured the story of a fluke which lives in the eye of a small freshwater fish and feed on its eye jelly.

But despite what it might seem from those post, having parasites might not be completely bad - there was a post on how tapeworm infection may allow brine shrimps to tolerate higher concentration of heavy metal. In addition, parasite also contribution to the ecosystem in unseen ways. For example, avian blood flukes - the parasites that cause "swimmer's itch" when their larvae blunder under our skin - may seem like a needless irritant to us. But of the millions of larvae that are pumped into the water by infected snails, the majority never reach their bird hosts, and for many other aquatic organisms, all those parasite stages that lost their ways are just another packet of nutritious morsel. And all those larvae can add up to tons of biomass each year.

Speaking of parasites that have lost their ways while trying to reach their hosts, this year featured stories about the lengths parasites go to in order to complete their life-cycles, whether they are
lice hitching rides on louse flies, roundworms that take a temporary detour in hagfish, or flukes that
embedding themselves into jellyfish. In some cases, human activities might incidentally be helping some parasites along in their life's journey.

And simply making it to the host is half of the hassle, the rest involves getting yourself to a specific part of the host, and parasites are remarkably picky about their real estate choices. For example the frog tongue fluke switches where they settle in the host's body depending on what kind of frog it finds itself in. The frog tongue fluke is not as (in)famous as a different tongue parasite - the tongue-biter in fish - but it is not the only parasite that is overshadowed by the tongue-biter's infamy. The tongue-biter belongs to a much larger group of similar parasitic crustaceans call cymothoids. While most are not as famous as the tongue-biter, some are arguably more gruesome - this year featured a species which live in a flesh capsule inside the body cavity of armoured catfishes.

As usual, there were also some guest posts, such as one by Dr Emily Uhrig on a tale about parasite in snake tails, as well as those by students from my parasitology class with posts about picky batflies, monkey botflies, maternal parasitoids, and how noisy frogs gets (bitten by) the midge.

Not directly related to this blog, but not unrelated either, my artwork took a turn for the weird (but in some ways, expected I guess) this year, and 2016 became the year of Parasite Monster Girls - so the work on my scientific side and my artistic side had finally converge...to create something that no one asked for but apparently some people find appealing. Speaking of work from my scientific side, I have a new paper published in Journal of Animal Ecology about the how bird life history and ecology influence the diversity of their roundworm parasite communities.

So that does it for 2016, see you in 2017 for more posts about new research into the world of parasites! In the meantime, you can find me on Twitter @The_Episiarch where I don't always tweet about parasites, but I do try to keep it interesting...

Leucochloridium paradoxum (revisited)

Parasites manipulating their hosts' appearance and behaviour is one aspect of parasitology which seems to have captured the public's imagination. The idea of body-snatching parasitic horrors taking over a host in both body and mind is one that evokes (and exceeds) the scenarios of many horror movies. Among the more well-known example of such parasitic body-snatchers is Leucochloridium paradoxum - the infamous zombie snail parasite, also referred to as the "green brood sacs".

But while L. paradoxum is the most well-known among its kind, it is just one of about a dozen different species in the Leucochloridium genus, all of which infect small land snails (mostly amber snails) and produce the pulsating brood sacs that people recognise. Traditionally, scientists have used the different colours and shapes of the brood sacs to tell apart different Leucochloridium species. More recently, this has been supplemented with genetic analysis, which has confirmed the validity of using brood sac colour and shape for species identification.

Left: A snail infected with two different Leucochloridium Right: Broodsacs of L. paradoxum and L. perturbatum from a double-infected snail
Photos from Fig. 1 of this paper 

In this study, researchers from Russia apply both techniques to examine cases of multiple Leucochloridium infections. Yes, as if being host to a single species of mind-manipulating parasite isn't bad enough, an amber snail can get infected with two (or more)! The researchers examined snails collected from the town of Lyuban in Russia, and upon dissecting them, found that while most of the infected snails were parasitised by the infamous L. paradoxum, a few snails had both L. paradoxum (green brood sacs) and another species call L. perturbatum (brown brood sacs). While simultaneous infections of different flukes species in snails are not uncommon, they also came across the first recorded case of a snail that was infected with three Leucochloridium species - L. paradoxum, L perturbatum, and the third species L. vogtianum which aren't as colourful, but was covered in warty projections

In other trematodes, competition between fluke asexual stages within the snails usually end up with one species overwhelming the other and gaining monopoly on the host. So it is possible that those snails that harboured multiple infection were merely be in the middle of a transitional state before one of the parasite colony is eliminated by the other. Had the snail been examined much later on, it might have revealed only a single parasite colony without any traces of a prior cohabitation with another species.

What most people might not know about Leucochloridium is that the prominent brood sacs are merely a part of an asexually-produced parasite colony inside the host snail. Unlike the asexual stages of many other trematodes which exist as genetically-identical but physically discrete stages call sporocysts or rediae, the asexual stages of Leucochloridium are stitched together into a writhing mass. This living colony is differentiated into different parts in a way that is comparable to the colonies of siphonophores such as the Portuguese Man'O'War. At centre of the parasitic mass, deep inside the snail's body, is where embryonic parasites are produced. As the embryos develop, they move through the colony's branches and into the extremities that form the colourful brood sacs, each packed full of mature parasite larvae that are ready to infect a bird.

This study also revealed another observation which provides insight into how these parasites reach the bird final host. The usual story is that infected snail are manipulated by the parasite into crawling to an exposed location where they can be easily spotted by hungry birds. The bird then mistaken the snail's pulsating, brood sac-engorged eyestalks for caterpillars, and peck them off. This is a classic story of parasite manipulation, told many times in multiple books and documentaries. While the validity of this story was partly demonstrated in 2013 when a study was published showing snails infected with Leucochlordium are indeed attracted to exposed and well-lit locations, it has yet to be demonstrated whether this actually enhance the likelihood of them (or at least their parasite) being eaten by birds

Broodsacs of L. paradoxum leaving the host snail. From Fig. 1 of this paper

But the researchers in this study observed that those pulsating brood sacs are not limited to expressing themselves in the snail's eyestalks. These sacs of parasite larvae can in fact leave the snail - possibly by rupturing through the snail's body wall. If the brood sacs of these parasites can exit the snail on their own and remain viable while still pulsating in the outside world for a brief period of time, then that significantly alter the above oft-repeated narrative of how this parasite is transmitted to its final host.

The parasitised snails might not need to have its eyes pecked out by the bird for Leucochloridium to reach its final host after all. Instead of treating the snail as a sacrificial lamb, the parasite could be using it as a unwitting courier that brings itself to an exposed location, drop off a few brood sacs, then those twitching brood sacs would attract the attention of a hungry bird on their own. It is still not a pleasant life for the infected snail - it is still stuck with a constantly regenerating parasite colony which is taking up almost a quarter of its body mass, but at least Leucochloridium would not be adding further injuries to insult by soliciting a avian attack.

Reference:
Ataev, G. L., Zhukova, A. A., Tokmakova, А. S., & Prokhorova, Е. E. (2016). Multiple infection of amber Succinea putris snails with sporocysts of Leucochloridium spp.(Trematoda). Parasitology Research 115:3203–3208.

P.S. Leucochloridium is a very striking-looking parasite and has been subjected to numerous artistic interpretations, so here's one of my own in the form of a Parasite Monster Girl version of a Leucochloridium-infected snail.

Cardiocephaloides longicollis

Human activities are having very significant impacts on the ecosystems of this planet and it is affecting every organisms. Parasites are not exempted from that - indeed parasites with complex life-cycles which involve many different host animals are in prime position to have their usual way of life altered by human intervention. The study being featured here today is on a parasitic fluke - Cardiocephaloides longicollis - which has a life-cycle that involves a carnivorous scavenging whelks, a variety of fish, and gulls. The researchers behind this study set out to investigate how commercial fisheries is affect the transmission dynamics of this parasite.

Left: Stained specimens of C. longicollis under light microscopy from here
Right: SEM of a closely related species Cardiocephaloides physalis from here

The asexual stages of C. longicollis reside in the body of whelks which acts as a kind of clone factory for the parasite, producing a stream of swimming larvae call cercariae. These larvae then go in the water to infect a variety of different fish. While C. longicollis has previously been recorded in 19 fish species, in this study the researchers found a further 12 species which are also viable hosts for C. longicollis, making for a grand total of 31 species of fish. The final host for this parasite are gulls, which acquire the fluke when they eat parasitised fish.

When it comes to C. longicollis infections, fish that hang around near the sea floor or the coast are the most loaded, most likely because they are in close proximity to the whelks which are sources of infection. Furthermore practically all the fish above a certain size (about 14 cm in length) are infected.  Fish in those size range have on average 73 C. longicollis larvae in their brain, with one unlucky fish recorded to have 220. Ironically, while these larger fish are the motherlode when it comes to parasites as they have been accumulating parasites for longer, since they live in deeper waters they are out of the gulls' reach. So regardless of their heavy larval fluke burden, because gulls can't get to them, all those parasites are at a dead end, destined to die or end up in the stomach of another predator which is not a gull - at least not without human intervention.

Many of the 31 species of fish which C. longicollis infects are either targeted by commercial fishing operations, or end up as by-catch. Many of those by-catch fishes - some of which are loaded with parasites - are discarded at the port. This pile of of parasite-laden fish present opportunistic gulls with a rich and accessible feast. It is a similar situation at fish farms, where the researchers found over half the fish there are infected with C. longicollis. At these facilities, organic matter from left-over feedstock, fish poop, and dead fish would also attract hungry gulls. But they're not the only ones who are attending the seafood party - being opportunistic carnivores, the whelks also come along to scavenge - so you end up with a situation where two of the host for C. longicollis are hanging out at the same location.

As the gulls feed on the discarded fish, they also are also getting infected with C. longicollis. Meanwhile, the flukes which have already reached maturity in the gulls' gut from previous feeding bouts are laying eggs which get pooped out into the water, right next to the whelks which have come for the scraps. And as mentioned above, the whelks are next host in the parasite's life-cycle, and some of those attending the feast will end up serving as parasite factories for C. longicollis in the future. For these parasites, this entire arrangement is a blessing - whereas without the activities of commercial fishing many C. longicollis larvae would have been consigned to a dead end in a large, benthic-dwelling fish, never to reach their final host. Indeed, the researchers found the fluke to be more abundant in areas with intensive fishing activity and aquaculture.

Cardiocephaloides longicollis is not the only parasite benefiting from commercial fishing activities, a study published a few years ago showed that overfishing can also benefits the tongue-biter parasite. A more recent study shows that clams living at commercially harvested sites are more heavily infected with parasitic flukes. While this does not apply for all parasites, as many would actually be negatively affected by commercial harvesting as their host population dwindles, for some species like C. longicollis human activities provide them with a rich opportunity for expansion.

Reference:
Born-Torrijos, A., Poulin, R., Pérez-del-Olmo, A., Culurgioni, J., Raga, J. A., & Holzer, A. S. (2016). An optimised multi-host trematode life cycle: fishery discards enhance trophic parasite transmission to scavenging birds. International Journal for Parasitology 46: 745-753.

Macrodinychus multispinosus

There are variety of mites which live with ants, but many of them are not well-studied. Most of them are either phoretic mites which hitch a ride on the ant's body, or detritivores that eat various substances which can be found in ant nests and in those cases, they are relatively harmless commensals. But some mites that live with ants are ectoparasites. The study being featured today is about a mite that lives (and feeds) on ants - Macrodinychus multispinosus. There are variety of other mites that also feed on ant haemolymph (a fluid which is the equivalent of blood in insects), but this vampire takes it to an another level.

Left: Ant pupa host being progressively eaten alive by the parasitoid mite.
Right (top): Adult female and male Macrodinychus multispinosus mites
Right (bottom): A M. multispinosus nymph at the stage when it is attached to the host (note the stumpy legs)
Photos from Figure 1, 3, and 5 of the paper. 

Newly hatched M. multispinosus nymphs are born with fairly long limbs which allows them to move about and find a host, but once they are attached to an ant pupa, their limbs are reduced to stumps. The mite essentially become a tiny biological pump. And whereas other blood-sucking mites that feed on insects are content with imbibing just some of the host's life blood, M. multispinosus does not hold back - it consumes all the developing ant pupa's internal tissue and literally sucks the life out of it.

Macrodinychus multispinosus can be considered as a parasitoid - even though its modus operandi is very different to parasitoid wasp which devour their host alive from the inside and burst out xenomorph-style once they are ready to pupate, the outcome is pretty much the same - a dead, empty host. The researchers behind the paper being featured in this post conducted their study at Quintana Roo, Mexico across a number of field sites where they inspected colonies of the longhorn crazy ant
(Paratrechina longicornis) - the mite's only known host.

They found this vampire mite to be relatively common - of the seventeen colonies they sampled, eight of them were infested with M. multispinosus. Overall, about a quarter (26.2%) of the ant pupae they examined were infected with these mites. In some nests, over three-quarters of all the pupae are parasitised. They noticed that M. multispinosus definitely seems to have a preference for the worker ant pupae and developing queens are usually spared. Even though by doing so, this vampire wouldn't end up killing off potential future colonies by parasitising the reproductive members of the colony, it is still killing off the developing workers and this can be quite harmful at a colony level if the mites are present in high numbers.

It seems that M. multispinosus has settled quite well into its niche as a ectoparasitoid of the longhorn crazy ant, and like other mites in the Macrodinychus genus, it is rather specific about where it attach to the host - in this case the ant pupa's abdomen. But here's the twist - whereas M. multispinosus is native to Quintana Roo, its host is not and is a relatively recent arrival to the region. Even though this vampire mite must have been parasitising ants long before the longhorn crazy ant came along, its original host is still unknown to science - in fact, even though it was described in 1973, it wasn't until now that its ecology and life cycle has been documented.

There's still a lot to learn about this little vampire. Would it be a good biological control for the invasive longhorn crazy ant? What kind of ant did M. multispinosus originally parasitised before it jumped on the invader? How was it able to take to the newly arrived host so quickly?

With so many different kinds of organisms being transported (purposefully or inadvertently) around the world, perhaps is would be useful to consider recruiting parasites are as a mean of controlling invasive species, especially if the parasite is native to the region where biological control is being considered - that way, it'll be fighting on its home turf.

Reference:
Lachaud, J. P., Klompen, H., & Pérez-Lachaud, G. (2016). Macrodinychus mites as parasitoids of invasive ants: an overlooked parasitic association. Scientific Reports 6: 29995

P.S. If you like this post and other posts like it on the blog, then you might been interested in checking out the book "The Wasp that Brainwashed the Caterpillar" by Matt Simon. It is full of funny and informative stories about wonderfully weird and bizarre animals both parasitic and non-parasitic - you should totally check it out!

Alaria spp.

Today we are featuring a guest post by Dr Emily Uhrig, a postdoctoral research fellow currently at Linköping University, Sweden. She has written a post on a study that she and her colleagues conducted on a parasite that congregate in the tail of garter snakes, and the role that these reptiles play in the life cycle of this parasite.

Parasites are found in a tremendous range of hosts spanning the animal kingdom and beyond. However, the consequences of parasites for their hosts have not been thoroughly studied in many cases and this is particularly true for parasites infecting snakes. Even in very common snakes, such as the garter snake which is widespread throughout North America and has been studied extensively with regard to many aspects of their biology, their parasites have received little attention.

Histological cross-section of an infected snake's tail
(m = mesocercariae, v = vertebra)

During my PhD research, I aimed to shed light on snake parasites by focusing on the red-sided garter snake of Manitoba, Canada, and I was especially interested in a trematode of the genus Alaria. Interestingly, Alaria infections in snakes have been noted in the literature for years, but mostly in ecological surveys of parasite communities, and their possible effects on the snakes’ evolutionary fitness were unclear.

Alaria spp. have complex life cycles consisting of a snail host in which Alaria eggs multiplies into asexual stages called sporocysts, which then produce multiple clonal larvae called cercariae. These cercariae emerge from the snail and infect frogs. Within the frog, Alaria develop into mesocercariae, a non-reproductive ‘resting’ stage. A mammalian carnivore, usually a canid (e.g., coyote) or mustelid (e.g., mink), serves as the final host in which the parasite reaches sexual maturity. So where does the snake fit in?

It turns out, the garter snake is a paratenic host, also known as a transport or reservoir host, which ends up accumulating Alaria mesocercariae through eating frogs. Paratenic hosts are not physiologically necessary for the parasite’s development as a part of its life cycle, but they help bridge ecological gaps between hosts. In this instance, the snake, which has quite a penchant for frogs, helps Alaria move from a (mostly) aquatic intermediate host to its terrestrial final host. Interestingly, Alaria spp. can infect many species paratenically - including humans; however, since relatively few humans fall prey to carnivores, Alaria that end up in humans are usually at a dead end.

Tail morphologies observed in red-sided garter snakes. Arrows mark the position of the cloaca.
Photos modified from Figure 1 of the paper.

Once inside the snake, the life of Alaria gets even more interesting. In the field, we commonly observe snakes to have ‘puffy’ tails where the end of the tail is obviously swollen and often discoloured. These puffy tails are fragile and can rupture with the gentlest handling or even by the snake’s own movement along the ground. The ruptured tail oozes a pink-coloured fluid which, on close inspection with the naked eye, clearly contains moving organisms, and microscopic examination reveals multitudes of writhing Alaria mesocercariae. Thus, the parasites apparently make a rather impressive migration through the tissues from the snake’s gut to its tail.

Left: Ruptured tail with ‘ooze’ containing Alaria; Right: Alaria mesocercaria from Figure 2 of the paper.

It is not uncommon for a snake’s tail to harbor several thousand mesocercariae, and the record holder in our studies had over 6000 mesocercariae in its tail. Alaria infections seem to be ubiquitous in our study populations as all snakes examined have been infected to some degree. Parasite mesocercariae are nearly impossible to visually identify to species level because different species are very similar in morphology. Thus, we used genetic analyses to determine that the snakes in our study population are often co-infected with at least three different Alaria species (primarily A. mustelae and A. marcianae, but also another as yet unidentified species).

Having identified the infection, the next obvious question to ask was, what are these parasites doing inside the snake’s tail? To answer this, we collected tails from recently dead snakes and prepared them for histology. Examining those samples, revealed that, in severe infections, the tail essentially becomes a bag of parasites and the tail musculature is destroyed (similar to what another parasite – Curtuteria australis – does to the foot of a New Zealand clam), likely through compressive effects of so many parasites in a relatively small space. The mesocercariae tend to be surrounded by pockets of mucous, the accumulation of which leads to the swollen puffy tails. The source of the mucous (host or parasite) is not entirely clear, but we believe it is the host’s body attempting to ‘wall-off’ the infection. Interestingly, some highly infected snakes do not have puffy tails, which suggests there may be variation in host tolerance of the infection.

As parasites destroy the tail musculature, the connection of the tail to the rest of the body is weakened and the likelihood of tail loss is increased. Loss of the tail is probably beneficial to the parasite because it could help facilitate transfer to the definitive host. In an attempt to catch a fleeing snake, a predator may come away with only the tail, especially if the tail is fragile, so aggregating there could prove a useful strategy for Alaria transmission. As we often observe wild snakes that are missing portions of their tails (stub tails), it may be common for predators to end up snacking on only a tail.

Unlike lizards, snakes cannot regrow their tails so tail loss is permanent, and also costly. Previous work found that males with stub tails have compromised reproductive ability. During the garter snake’s mating season, as many as 100 males compete for a single female. In these “mating balls”, males use their tails to wrestle with one another for access to the female. Males with stub tails are less successful competitors and much less likely to obtain a mating.

For females, tail loss also has reproductive implications because males appear to rely on female tail length to align properly with her cloaca during mating. When attempting to mate with a stub-tailed female, males can misjudge the location of her cloaca, reducing the changes of a successful copulation. Thus, through mechanical impairment, Alaria infections can have a direct effect on the fitness of both male and female snakes.

The association of Alaria and garter snakes was first mentioned in the literature nearly a century ago, but has received little attention until very recently. Thus, one need not visit exotic locations to learn new things about host-parasite associations as there is still much to learn about the consequences of parasites even in common species.

Reference:
Uhrig, E. J., Spagnoli, S. T., Tkach, V. V., Kent, M. L., & Mason, R. T. (2015). Alaria mesocercariae in the tails of red-sided garter snakes: evidence for parasite-mediated caudectomy. Parasitology Research 114: 4451-4461.

This post was written by Dr Emily Uhrig.

Peltogaster sp.

Most people are familiar with how barnacles look like - sedentary creatures which filter the surrounding water for food while being stuck attached to rocks or other hard surfaces. Parasitic barnacles on the other hand looks nothing like those creatures. In fact, they don't look anything like what most people would expect an animal to look like. The most well-know example of a parasitic barnacle is Sacculina carcini, but that infamous species is only one of an entire order of such body-snatching parasites that infect crustaceans like crabs and crayfish.

Left: Peltogaster externa attached to their hermit crab host.  
Right: The externa (orange) and interna (green) of Peltogaster in its hermit crab host
Photos from Fig. 1 and Fig. 2 of the paper

These parasitic barnacles belong to a group call Rhizocephala and the body of the adult parasite can broadly be divided into two parts: The "externa" which is the bulbous reproductive organ that sticks out of the host's abdomen, and the "interna" which is found inside their host's body. The interna is a network of root-like tendrils which wrap themselves around the host's organs (hence the name "Rhizocephala" which roughly translates into "root-head").

Most depiction of rhizocephalans have those parasitic roots running throughout the entire body of the host - this is based on an illustration of S. carncini drawn by the famous artist and biologist Ernst Haeckel. Haeckel's original drawing has been copied by many others since it was first published in the book Kunstformen der Natur, and has been treated as the definitive depiction of the rhizocephalan interna. But the thing is, Haeckel has never actual seen a Sacculina in person - he simply based his illustration upon descriptions of the parasite in a monography published in 1884. So while Haeckel's original drawing is iconic and has been replicated countless time in many books, that depiction of these parasitic barnacle is not entirely accurate. Much like tropes in other areas of scientific illustration (such as depictions of extinct animals), Haeckel's depiction of Sacculina has been faithfully and unquestioningly used and copied ever since.

It is understandable that not much is known about the true three-dimensional structure of the rhizocephalan interna - because of its complex and delicate nature, it would really difficult to tease apart all those roots which are tightly intertwined with host tissue to get an accurate picture of the parasite's extensive root network. But now there is technology available which can resolve this question. In the study featured in this post, a group of researchers used X-ray microtomography to obtain a 3D image of these parasites' root network inside their hosts. They performed this procedure on five species of rhizocephalan barnacles collected from the coast of Norway and the United States; four of the species were hermit crab parasites belonging to the genus Peltogaster, and one - Briarosaccus tennellus - was from the hairy crab.

From the microCT scans, they found that the barnacle's "roots" are not spread evenly throughout the body, but were wrapped around certain organs, with most concentrated near the hepatopancreas  - an organ found in crustaceans which is also known as the digestive gland, which would be prime place to suck up nutrients. And in contrast to Haeckel oft-cited and copied drawing, none of the roots actually penetrate into the muscles. While the roots of the four Peltogaster species were mostly wrapped around the hepatopancreas, the roots of Briarosaccus also extended to the host's brain and central nervous system, which may explain how some of these parasites can manipulate the behaviour of their crustacean host.

Parasite can often manipulate their host's behaviour and physiology to an amazing degree. While many of those interactions are very complex, with the use of techniques such as micro CT, we can begin to unravel the intricacies of how these body-snatchers interact with and manipulate their hosts.

Reference:
Noever, C., Keiler, J., & Glenner, H. (2016). First 3D reconstruction of the rhizocephalan root system using MicroCT. Journal of Sea Research 113:51-57

Plectocarpon lichenum

Lichens can be found all over the world, even in the most barren and inhospitable environments (even near active volcanoes). They grow on exposed surface like moss, but they are very different to those plants. Lichens are the outcome of a highly successful conglomerate resulting from the fusion of a pair of very different lineages of fungi combined with a photosynthetic alga. Together they form a beneficial tripartite that has allowed lichen to colonise environments all over the globe

Photo of parasitised lichen from Fig 2. of this paper

However, lichen are also involved in other forms of symbiosis which are more deleterious - to the lichen anyway. There are parasitic fungi call lichenicolous fungi that have evolved to parasitised lichens. There are about 1500 known species of such fungi and they vary in their specificity and their harmfulness, and some of them form galls on their host. In addition to such parasites, lichen also has to contend with a range of animals that feed on them, including (but not limited to) reindeers, caterpillars, and snails.

Since lichens are constantly being attacked from multiple fronts, some species have evolved various forms of chemical defences that make themselves less appetising to animals that try to eat them. But no counter-measures ever survive intact in evolution's battlefield, and lichen-infecting fungi like Plectocarpon lichenum can mess with their host's attempt at avoiding being eaten.

The scientists in this study looked at how these parasitic fungi affect their lichen host, specifically how tasty they might be to other animals. Plectocarpon lichenum infects a species of lichen call Lobaria pulmonaria. Generally, snails prefer eating those parasitised lichens over unparasitised lichens, but they avoid eating the parasitic galls themselves (see the photo above where the snail has neatly grazed the lichen around the parasitic galls).

So on top of already drawing nutrient away from its host, this parasitic fungus also make the lichen more delicious to the lichen's predators - a double whammy. Now this isn't like other cases featured on this blog where the parasite alters the host to make it more edible because it would help get the parasite transmitted. It doesn't benefit P. lichenum to have the snail munching on its host, but this is simply a side effect (a tasty one for the snail) of the infection. But how is how is P. lichenum causing this?

The scientists measured the level of carbon and nitrogen in both parasitised and unparasitised lichen, and found those with the parasitic galls had lower concentration of carbon. In addition to altering the nutritional content of L. pulmonaria, parasitised lichen also had lower level of defensive chemicals. So is this reduction in defensive chemicals the reason why the snails preferred parasitised lichen? While it seems to make intuitive sense, that turned out not to be the case. When they remove the influence of those defensive compounds in both parasitised and unparasitised lichen by rinsing them with acetone, the snails still preferred the parasitised lichen.

So lichen-infecting fungi makes their lichen host more tasty to snails - but that's not the full story. When they investigated a related host-parasite pairing - in this case Lobaria scrobiculata infected by Plectocarpon scrobiculatae, they found that the presence of L. scrobiculata did not make any difference to their palatability to snails

Despite being in the same genus, these two parasitic fungi affected their lichen host differently. In any case, the effects that P. lichenum have on its lichen host was what the scientists had predicted, but not for the reasons they had thought. This shows that ecological interactions are often messy and complicated, and the dynamics found in one particular relationship or species may not be applicable to another - even if they are closely related.

Reference:

Asplund, J., Gauslaa, Y., & Merinero, S. (2016). The role of fungal parasites in tri‐trophic interactions involving lichens and lichen‐feeding snails. New Phytologist 211: 1352–1357

Tylodelphys sp.

There are many examples in nature where parasites are able to alter their host's behaviour in some way. More recently, some scientists have been investigating just how the parasite are altering or controlling host behaviour. Most of them had looked at the chemicals secreted by the parasites to lull the host into compliance, but the study we're featuring today looked at something different - how the behaviour of the parasite itself can affect the behaviour of the host.

Left: Histology section of an infected bully's eye from Fig. 1. of the paper (r = retina, l = lens, m = metacercariae [flukes])
Right: Tylodelphys in the eye of a common bully from this video

The star of today's post is Tylodelphys - a parasitic fluke which infects a small freshwater fish in New Zealand call the common bully. In order for this parasite to complete its life cycle, Tylodelphys must enter the gut of a fish-eating bird, which would naturally involve the unfortunate fish being eaten by the said bird. While it is in the common bully, Tylodelphys dwells in its host's eyes in the vitreous liquid between the lens and the retina (see video here).

Unlike other species of flukes which turn into dormant cysts at a similar stage of development, Tylodelphys stays active and free to roam around inside the fish's eye - which provides it with plenty of opportunity to get up to all kinds of parasitic hi-jinx. When Tylodelphys larvae are crawling around inside a fish's eye and happen to get in between the retina and the lens, this can partially blind the fish and prevent it from being able to notice incoming predators such as birds.

To examine what Tylodelphys gets up to during the day, researchers at the Otago Parasitology Lab collected some common bullies and gave them eye examinations using an opthalmoscope (yes, like the one used for your eye exam). Using the opthalmoscope, they captured a series of short videos of the infected fish's eyes at different time of day, and watched what eye flukes got up to. They also performed histology to examine if the flukes are damaging the fish's eyes.

The bullies they examined varied in how heavily infested their eyes were - this range from just having a single fluke in the eye, or it can be up to seventeen flukes, with the average being about seven. Living in a crowded eye is not good for the parasites either, as the researchers found that flukes from heavily infected fish are comparatively smaller. But despite being found in a vital and sensitive part of the host body, Tylodelphys was otherwise a relatively benign tenant - they didn't mess up any eye tissue.

Compare this with other species of eye flukes which can cause cataracts in the eye of their fish host, Tylodelphys seems rather well behaved. However, that does not mean Tylodelphys isn't bad news for the bully - just that its modus operandi is more subtle. Instead of impairing the fish's sight by damaging the eye, as mentioned the above, when the flukes position themselves in front of the retina, they act like internal blinkers. Surprisingly, fish that are more heavily infected didn't have their retina more covered up by the flukes than less heavily infected fish, which means it's not simply the sheer number of flukes that blinds the fish - it's something else the flukes are doing.

Tylodelphys  has a daily routine and shifts its position in the eye throughout the day. During day time, the flukes sit between the lens and the retina, blocking the host's line of sight. But at night, they settle down to the bottom of the eye, allowing the fish to see properly again. But if Tylodelphys is trying to get its host eaten by a predator, why doesn't it just stay in front of the retina all the time? That is probably because not all predators are the same for Tylodelphys.

During the day, the main predators of bullies are fish-eating birds (which are Tylodelphys' final host), whereas at night, the main predators are longfin eels (which are not suitable as host), so it'll be good for to the fluke if their host fish can still see and avoid the incoming predators at night. So the flukes keep this fish's eyes covered during the day, but move aside to not get in the way at night, and this seems to follow a circadian rhythm.

While this helps the fluke reach its final bird host, the reason why this behaviour manifests probably has nothing to do with trying to change the host's behaviour. As mentioned above, unlike other flukes that become a dormant cyst at this stage of development, Tylodelphys keeps growing so it needs to feed - and the only thing around to eat in the eye of a fish is the fluid in the eye's vitreous body. The partial blinding of the fish host during daytime might simply be a side-effect of the parasite's feeding routine.

So while the fluke moves around in the fish's eye to get its daily dose of eye jelly, this also produce a useful side-effect by making the host more vulnerable to fish-eating birds. Such "useful side-effects" could be how many parasite host manipulation tactics have evolved. Indeed, that is often how evolution often work; co-opting preexisting features and behaviours into new roles. To understand how a parasite affect the behaviour of its host, sometimes perhaps it is best to start with studying the behaviour of the parasite itself.

Reference:
Stumbo, A.D., and R. Poulin. 2016. Possible mechanism of host manipulation resulting from a diel behaviour pattern of eye-dwelling parasites? Parasitology 143: 1261-1267

Trypanosoma tungara

This is the fourth and final posts in a series of posts written by students from my third year Evolutionary Parasitology unit (ZOOL329/529) class of 2016. This particular post was written by Sierra Weston and it is about how male Tungara frog end up receiving a parasitic present while trying to call out to female frogs.(see also the previous post about picky bat flies, monkey-infesting botflies, and the caring maternal side of a parasitoid wasp).

Like many species of frogs, during breeding season the males of the túngara frog produce a mating call to attract female frogs. But instead of just serenading his own species, the male frog may inadvertently be announcing his location to nearby frog-biting midges.

TFW you're trying to serenade the ladies and end up with a face full of 

blood-sucking midges. Photo from Fig. 1 of this paper

All adult frogs are at the mercy of a range of opportunistic and specialised insects, some of which are potential vectors for all kinds of blood parasite. There is a species of frog-biting midge (Corethrella spp.) which preys predominantly on the túngara frog (Engystomops pustulosus) - a species of small frog found in the region between the south of Mexico to northern South America. The female midge takes advantage of the male frog’s mating call during their breeding season as a host detection system. The midge follows the sound, finds the frog and voila, it gets a blood meal. A male can attract up to 511 midges in half an hour. Unfortunately, again for the poor frog, midges are the perfect vector for a wide variety of diseases and parasites including trypanosomes.

Trypanosomes are single-celled protozoan parasites that infects hosts from all vertebrate classes; birds, mammals, reptiles, fish and amphibians. Some of these protozoans can cause diseases, including sleeping sickness (Trypanosoma brucei gambiense), in humans, as well as making their hosts more susceptible to sickness. Although frog trypanosomes are a less studied group, there are some parasite-vector-host relationships that have been documented.

The study featured in this post investigated trypanosome infection in túngara frogs. The aim of the study was both to determine that trypanosomes affected the túngara frog and identify the species of parasite if present, and whether there is a difference in trypanosome prevalence between male and female frogs. Since it is the males that produce the mating call, it was predicted that any midge transmitted trypanosomes would only occur in male frogs.

The researchers confirmed the presence of trypanosomes in the blood of the frogs, but also observed that the parasites possessed a some unique characteristics that set them apart from previously described species. However, frog trypanosomes are also known to be able to significantly change their shape when infecting different hosts. This presents the possibility that the trypanosomes infecting the túngara frogs could be a previously identified species with a slightly altered form which make them more suited to life as a parasite in the túngara frog.

Through further analysis and DNA sequencing, researchers were able to confirmed the discovery of a new species of trypanosome: Trypanosoma tungara. In terms of prevalence in male and female hosts, results showed a much greater percentage of males infected with trypanosomes showing that the mating call results in the male frog being the ‘easiest’ and most predominant target for the frog-biting, trypanosome vectoring midges. There were also female frogs infected with trypanosomes, which was surprising because female frogs do not vocalise. A potential transmission path is presumed to be the close proximity of the frogs when they are in amplexus, (the mating ‘embrace’) which allows the midge to move directly from the male to the female frog.

Reference:
X.E. Bernal, C. M. Pinto (2015) Sexual differences in prevalence of a new species of trypanosome infecting túngara frogs. Internations Journal for Parasitology: Parasites and Wildlife 5: 40-47

This post was written by Sierra Weston.

That wraps it up for ZOOL329 class of 2016 - I would like to thank all the students for their posts! Next month, it's back to writing my usual posts about newly published parasite-related papers which you might not have noticed, and/or papers that were not as widely covered by the press - so stay tuned for more!

Sclerodermus harmandi

This is the third post in a series of blog posts written by students from my third year Evolutionary Parasitology unit (ZOOL329/529) class of 2016. This particular post was written by Jarrod Mesken the more maternal side of a parasitoid wasp (see also the previous post about picky bat flies and monkey-infesting botflies).

When most people think of parasite behaviour, horrific tales of behavioural and physiological manipulation are what come to mind. This is not without cause; many parasites are definitely scary to think about. However, many pursuers of the parasitic lifestyle also display behaviour that would be thought of as normal, perhaps even charming in an anthropomorphic kind of way. An example of this is seen in the parasitoid wasp Sclerodermus harmandi, in the form of maternal care.

Photo of multiple female Sclerodermus harmandi engaging in brood care from Figure 1 of this paper

This stories begins when the female wasp finds a suitable host for her eggs. She injects the host with paralysing venom, and cleans an area of the body to lay eggs on. Once laid, she routinely inspects the eggs with her antennae and mouthparts. If an egg is found to have detached from the host, she would gently reattach it. Maternal behaviour continues when the eggs hatch, when the larvae must be fed. To do so, the mother wasp bites a hole in the host, which is still alive at this point and injected with paralysing venom periodically to prevent it from moving. The hole fills with haemolymph, the insect equivalent of blood, which is consumed by the larvae.

During this stage the mother S. harmandi also moves the larvae around to prevent them from overlapping each other as they grow. If a larva dies, the mother moves the body far from the other larvae to prevent their habitat becoming unsanitary. Even during the cocoon stage the mother continues to rub the offspring, despite them being encased. Eventually, the males of the clutch hatch out as adults. These few males (there is considerable female bias in the ratio of this species) chew holes in the female wasps’ cocoons to assist them in emerging, after which they mate with them. While it has negative affects in many taxa, this kind of inbreeding is less likely to have negative effects in hymenopteran insects, where haploid males act as a purge of deleterious alleles.

So why does S. harmandi provide such comprehensive maternal care? Because it increases the likelihood of offspring surviving. Experiments in which the wasp mothers were removed at varying stages of offspring development showed that not only were offspring that received maternal care more likely to survive to adulthood, but that this was proportional to how much maternal care they received. Experiments also showed that when a mother was taken away and replaced with another female who has previously laid eggs, the ‘stepmother’ will exhibit the same behaviour as the mother would, with the same rise in offspring survival.

Why the stepmother expend her own energy to raise another wasp’s offspring is just as interesting; it is because of the high levels of inbreeding in the population. Since most of the reproduction in this species is done through inbreeding, there isn’t much genetic variation going around. This means that there is a good chance of two wasps being related, so the stepmother may increase the chance of her genes being passed on by raising another wasp’s offspring. Female S. harmandi that haven’t laid eggs yet do not exhibit this behaviour, preferring to leave the offspring alone; this would indicate that the maternal behaviour is initiated by laying eggs.

So for a parasitoid wasp, it turns out that the females of S. harmandi make for very responsible parents or stepparents. That is, if you consider letting your children live on the body of something you paralysed, feeding off its blood until they grow up, and then mate with each other to be “responsible”.

Reference;
Hu, Z., Zhao, X., Li, Y., Liu, X., & Zhang, Q. (2012). Maternal care in the parasitoid Sclerodermus harmandi (Hymenoptera: Bethylidae). PloS One 7: e51246.

This post was written by Jarrod Mesken

Alouattamyia baeri

This is the second post in a series of blog posts written by students from my third year Evolutionary Parasitology unit (ZOOL329/529) class of 2016. This particular post was written by Gabrielle Keaton and it is about a nasty botfly that lives in the neck of howler monkeys (you can read the previous post about picky bat flies that live on bats here).

Photo of botfly pores on howler monkey neck from Plate 1 of this paper

You know how itchy a mosquito bite can be - you scratch it then a lump forms. Imagine that lump forming but not going away. Instead, it grows and grows inside of you until finally a black grub plops out of a hole in your neck onto the ground. Well that’s what it’s like for the howler monkeys of Panama!

This nasty parasite in this case is the larvae of Alouattamyia baeri, a botfly that lives on free-ranging howler monkeys (Alouatta palliate). In a study conducted over seven years from 1987 to 1993, researcher Dr Katherine Milton investigated a variety of factors relating to this parasite's life cycle including its infection prevalence and intensity. She found that 60% of the howler monkeys on Barro Colorado Island (BCI) were infected by this botfly.

Alouattamyia baeri are large (18 to 20mm in length) black flies. The adult fly sounds and look like neotropical bees. When flies that were collected from the howler populations on BCI were reared in captivity, it was found that female flies produced an average 1400 eggs each, laid in discrete rows. These eggs required the appropriate stimuli (carbon dioxide and heat) to hatch into parasitic larvae that then invade their host through the nose and mouth where it migrates to the neck and opens a up larval pore. The larva reside in the howler’s neck for approximately 6 weeks, passing through 3 instars (developmental stages). After this, the larva drops out of the monkey's neck warble and burrows into the soil where it finishes the last developmental stage underground.  The study found that the entire life cycle takes approximately 13 weeks.

Dr Milton discovered that most of larval growth (86%) occurs during the 3rd instar when its food consumption increase by about 20%. This means the larva was trying to extract the most resources at the last possible moment of its stay, so if it ends up killing the host, it wouldn't matter to them because they are out of there.

Infestations were the highest during the wetter seasons and these periods also strongly correlated with peaks in the monkey’s mortality. Monkeys carrying the botfly larvae lack subcutaneous fat reserves. As if having a 2.4 centimetre long and 1.5 centimetre wide maggot in your neck wasn’t bad enough, even after the botfly has made its exit, the hole they made in their host remains open for several days. That’s pretty like much waving a neon ‘vacancy’ sign in front of the primary screw worm fly (Cochliomyia hominivorax) - an even nastier parasite that lays its eggs in open flesh wounds. When the screwfly larvae hatch, they feast on anything and everything surrounding that wound. Some monkey cadavers were even found with hands eaten down to the bone from these nasty little maggot and at least half of the C. hominivorax infestations found on howler monkeys were the result of prior A. baeri infections

Now, I’m sure you might have a bit of a panic next time you feel a little raised bump hanging around your neck, but remember - even if it is a botfly, at least you know it will be gone in 6 weeks' time.

References:
Milton, K. (1996), Effects of bot fly (Alouattamyia baeri) parasitism on a free-ranging howler monkey (Alouatta palliata) population in Panama. Journal of Zoology 239: 39–63.

This post was written by Gabrielle Keaton

Trichobius sp.

Those who have been reading this blog for a while will know that August is student guest post month! All this month this blog will be featuring posts written by students from my Evolutionary Parasitology  (ZOOL329/529) class. One of the assessment I set for the students is for them to summarise a paper that they have read, and write it in the manner of a blog post. The best blog posts from the class are selected for re-posting (with their permission) here on the Parasite of the Day blog. I am pleased to be presenting these posts from the ZOOL329/529 class of 2016. To kick things off, here's a post by Melissa Chenery about some picky bat flies.

Photo of Trichobius johnsonae from Figure 2 of this paper

The public often view bats as repulsive, disease-carrying animals and are subsequently disliked. “Argh! They’re repulsive!” is just one of the many lines I have heard from people walking by while I observed a local colony. But do you know what is even more horrifying than a bat? A bat fly! These ectoparasites belong to two families of flies know as Streblidae and Nycteribiidae. But these hematophagous (blood-feeding) parasites don’t always fly like the name suggests - most species actually have no wings at all, and some look more like spiders than flies.

Disgusting, right? But not to worry, these external parasites have evolved to feed exclusively on bats. The bat flies are quite specific towards their hosts and tend to stay on a particular bat host. They are even picky about where they live on the host, whether on the bat’s fur or hiding within folded wing membranes. Occasionally they can be found in the fur and these individuals possessed comb-like structures (called ctenidia) for attaching to fur. It is assumed that long-legged species move quickly to avoid being scratched by the bat during grooming, whereas the short-legged species hide within the membrane folds to avoid getting licked. Bats use grooming as a behavioural defence against bat flies and other external parasites, and bats with a high number of flies groom more often than those with only a few. For the parasites, action can result in their removal and often their death.

In a study which took place in Belize, Central America, a team of researchers demonstrated just how host-specific the bat flies can be. They examined over thirty two species of bat flies, and in the twenty species for which they were able to collect more than five individuals, they found that eighteen of those species showed strong site preferences. The majority of the bat flies were constrained to a single host-species, and amazingly, bat flies with functional wings (which would allow them to be more mobile) weren't any more or less picky than those without. The study also found that only two species (Trichobius yunkeri and Trichobius dugesioides) weren’t too fussy in respect to host-site preference.

For bat flies that were the dominant species of their respective hosts, six out of those seven species were fur-specific, suggesting that in most cases, bat flies are highly host site-specific. They also discovered an interesting correlation between leg length and host-site preference. Bat flies with longer legs are able to push up over the surface of the fur, and are more likely to be found dwelling in fur. Conversely, short-legged individuals moved much more slowly and were mostly membrane-dwelling.

The team also conducted a study where three bats were restrained and three left unrestrained, with six bat flies placed on each. All unrestrained bats had only one bat fly remaining after five days, whereas all bat flies remained on the restrained bats. This suggests that the elimination of the flies is due to grooming behaviour. This may also be the cause of host-site specificity in bat flies, although further studies are needed Despite their nightmarish appearance, bat flies can still be very fussy eaters, and they have adaptations which allows them to specialise on particular bat species and host-sites.

Reference:
Hofstede, H., Fenton, M., & Whitaker. J. (2004). Host and host-site specificity of bat flies (Diptera: Streblidae and Nycteribiidae) on neotropical bats (Chiroptera). Canadian Journal of Zoology 82: 616-626.

This post was written by Melissa Chenery

Gordionus kimberleyae

We have featured hairworms (Nematomorpha) quite a few times before on this blog, but for those who are new to them, they are parasitic body snatchers of insects and other terrestrial arthropods. The hairworm larvae are parasitic and must develop inside an arthropod host, but the adult worm is free-living and has to enter a water body in order to reproduce. To achieve that end, once they reach maturity, they make their host seek out a body of water and jump in, at which point the adult worm make their exit. This can look quite dramatic / horrifying to onlookers as the length of a fully grown hairworm can be several times longer than the host itself.

Specimens of Gordionus kimberlyae emerging from beetles, from Figure 2 of this paper

Hairworms are found all over the world, and the study we are featuring today shows that they can thrive even at the Arctic Circle, in the most northern parts of Canada. Despite the harsh conditions there, Arctic Canada is home to over 2000 species of terrestrial arthropods, including many species of beetles - some of which are host to parasitic hairworms.

In this study, a team of researchers sampled insects from twelve different locations in northern parts of Canada, and at five of those locations, they found seven species of beetles which are host to hairworms. All the hairworms they found belonged to a single species - a newly discovered one that has never been described before. They named it Gordionus kimberleyae, and these parasites happened to be most common in beetles on Banks Island in the Canadian Arctic Archipelago.

At Banks Island, 13.4% of the beetles that the research team collected were found with G. kimberleyae emerging from them. Most of the hairworms were found in beetles that were hanging around near water, and it's possible that those infected beetles weren't there by accident - as mentioned above, hairworms are known to modify their host's behaviour so that they would seek out water when the parasite is ready to exit their host. Most beetles were only infected with a single worm, which is bad enough considering how big they are and what they do to the host, but there were a few beetles that had two or even three inside them.

So how do these parasite manage to survive in the frigid cold of Arctic Canada? While adult hairworms don't do particular well in cold conditions, they are very short lived and die shortly after they reproduce anyway during the warmer months. However, the larvae have adaptations for surviving in freezing conditions.

In addition to the seven species found in this study, it's not clear how many other ground beetles that G. kimberleyae infects. The ground beetles which these hairworms infect are carnivorous insects, which means they probably acquire their hairworms from eating flying insects that have aquatic larval stage (such as mosquitoes) which can become host to these parasites' larvae. The beetles occupy an important part of the food web as the link between tiny invertebrates and larger insect-eating animals. But the hairworms may also be keeping their population in check.

Additionally, by causing their host to jump in the water, they are transferring where nutrients are flowing in the environment. While these beetles would usually be eaten by land-dwelling animals, by dunking their host in water, the hairworms may also be feeding a range of aquatic animals that depend on their hairworm's "donation" of dying insects into their realm. Despite their gruesome methods, parasites also play important roles in many ecosystems.

Reference:
Ernst, C. M., Hanelt, B., & Buddle, C. M. (2016). Parasitism of ground beetles (Coleoptera: Carabidae) by a new species of hairworm (Nematomorpha: Gordiida) in Arctic Canada. Journal of Parasitology 102: 327-335

Cuscata campestris

One of the key characteristic of plants is their ability to produce food using sunlight via the process of photosynthesis. But there are many plants that do not photosynthesise - holoparasitic plants obtain all the nutrient they need from their host. One of the most well-known group of holoparasites are the dodders. There plants have no roots, their leaves have been reduced to tiny scales, and are composed entirely of elongated tendril-like vines. Because of their appearance and what they do to the host, the dodder has also acquired many common names including "Angel Hair", "Witch's Hair" and "Devil's Hair". When the dodder latches on to an appropriate host, it extends structure call haustoria which penetrate into the host plant's tissue to suck out its nutrients. This has led to the dodder also being called "vampire plants".

Dodder seedling in the process of coiling around its host (image from Fig. 1. of this paper)

There are 200 known Cuscuta species and of those, a small handful of them (10-15 species) are considered as serious agricultural weeds. These parasitic plants wrap their vines around their hosts and literally suck the life out of them. Infestation of this plant have been known to cause massive losses in alfalfa, tomatoes, carrots, and cranberry crops, and these "vampire plants" are very difficult to get rid off; since dodder vines often completely smother their hosts, it requires a lot of manual and mechanical labour to remove them. Additionally, dodders can also produce large numbers of resilient seeds that can linger in the soil for a long time, waiting for the next year to erupt in another outbreak.

For dodders, as with most other organism, the first moments of its life are the most critical - the newly germinated dodder seedling must secure a grip on a host plant within two to three weeks of germinating, otherwise the seedling would use up its energy reserves and expire. From the moment they germinate, these parasitic plants have various ways of finding their host. Dodder vines are able to "sniff out" host plants through the chemical they give off (essentially plant BO), but they can also use other senses to find their host. A newly germinated dodder seedling can also detect the specific wavelength of light which are reflected off the surface of plants, and use it to reach a host.

The scientists in this study investigated whether exposing dodder seedling to different spectrum of light can disrupt their ability to find their hosts. They conducted their experiments on the seedlings of two dodder species - Cuscuta campestris (which parasitises tomatoes) and Cuscata gronovii (which parasitises jewelweeds), and exposed them to three light source with different spectrums - the spectrum similar to unfiltered sunlight, mostly red light, and mostly far red light. Far red is a wavelength of light which is barely visible to our eyes, but it is the wavelength which is most reflected by the surface of plants. It is also the wavelength which dodders use to home in on their host.

From the experiment, the scientists found that almost all the dodder seedlings that were exposed to unfiltered light and mostly far red light were able to attach to a host after a week or two, in fact those bathed in far red light grew faster than the other groups. However, most of those that were bathed in red light lost their ways. Only 15% of the C. campestris and 27.2% of the C. gronovii seedling that were exposed to high levels of red light had managed to wrap themselves around a host.

So will it possible to control dodder infestation simply by bathing crops in red light? No, not quite since some dodder still managed to wrap themselves around a host plant and the red light treatment is only effective during the earliest stage of the dodder's life. But at least the findings of this experiments have shown that perhaps light manipulation can be combined with other control methods to control dodder infestation.  Additionally, we have gained an insight about how these parasitic plants sense and find their way through the world.

For this particular vampire, its weakness is not against sunlight, but rather, red light.

Reference:
Johnson, B. I., De Moraes, C. M., & Mescher, M. C. (2016). Manipulation of light spectral quality disrupts host location and attachment by parasitic plants in the genus Cuscuta. Journal of Applied Ecology 53: 794-803.

P.S. I recently drew some dodder-inspired art, yes, it is more Parasite Monster Girls - meet Cassandra the Dodder...

Halipegus occidualis

The tongue-biter parasite is infamous for living inside the mouths of fish and replacing their tongue. But that parasitic crustacean is not the only parasite with a predilection for that part of host - the tongue is also the preferred site for some species of Halipegus, a genus of digenean trematode fluke that lives in frogs and toads. While Halipegus doesn't replace the tongue the way the tongue-biter does, but they are very particular about where they hangout. Specifically, the adult fluke attaches itself to the lingual vein under the frog's tongue.

Halipegus occidualis in the gray tree frog (left), Southern leopard frog (centre), American bullfrog (right)
Photos from Figure 1 of the paper 

While that may seem oddly specific on Halipegus' part, in fact many parasites are like that - not only are they particular about what host they infect, but they can also be very picky about which part of the host they live in. To a parasite, the host is a collection of very different habitats and only certain parts might meet the parasite's very specific needs. But not all hosts are equal, and a parasite that infect different species of host might also behave differently in each of them.

A pair of researchers from Oklahoma State University conducted a series of experiments to find out more about Halipegus' specific preferences. They collected seven different species of frogs and toads from various locations in Oklahoma, and infected them with a species of fluke call Halipegus occidualis to observed how the parasite behaved and developed in those different hosts. To control for natural variations and to ensure that the parasites they are using is the right species and not some other similar-looking cryptic species (see this for example), they used parasites from a colony of H. occidualis which they have been maintaining in their lab.

They exposed the frogs to larval H. occidualis by feeding them with seed shrimps which they have previously infected with H. occidualis. This procedure through which the frogs are exposed imitates the process of how frogs in the wild become infected with this parasite. After exposure, they inspected the frogs' mouths everyday for the parasite's presence. When H. occidualis is initially swallowed by a frog, it does so as a tiny larva encased in the body of an arthropod. The digestive action of the frog's stomach free the fluke from the arthropod host, and it then migrate to the frog's mouth over the course of a few weeks to develop into a sexually-mature adult. Or at least that's what happens in most frog species.

For six of the frog species in the experiments, H. occidualis showed up as expected under their tongue as mature, egg-laying flukes about 6-8 weeks after they have been fed with infected seed shrimps. The parasite was most successful at establishing in the American toads (93%) and had comparatively lowest success with the southern leopard frogs (67%), but aside from that, there were no major differences among those six species in terms of how H. occidualis performed. But things were a bit different in the American bullfrog. In that host, H. occidualis never show up under the tongue - instead, they simply stayed in the stomach and developed to full maturity there.

It seems that not only is H. occidualis very specific are where it settles, it will also adjust accordingly if the host is different, to the degree that it would do so even if it has already developed into a fully-fledged adult fluke. When the researchers conducted further experiments where they transplanted adult flukes from under the tongue of gray tree frogs to other species of frogs, the flukes were quick to adjust. When the flukes were transplanted from a gray tree frog to yet another gray tree frog or a green frog, the fluke will move to its usual spot under the host's tongue, even though it is now in a new host. But if those flukes were transplanted to an American bullfrog, the flukes would migrate to the bullfrog's stomach. Furthermore, when those parasites were then extracted from the bullfrog and transplanted back to the tree frog, they went back to living underneath the host's tongue.

So what so special about the bullfrog, or specifically its stomach? At this point, it is not entirely clear. Perhaps the bullfrog stomach has some kind of chemical that encourage the fluke to stay instead of migrating to the host's tongue. While a parasite might be very specific about where it exactly it lives in the host, it might not always behave the same way when it finds itself in different host species. For a parasite like H. occidualis, not all frogs are equal.

Reference:
Stigge, H. A., & Bolek, M. G. (2016). Anuran Host Species Influences Site Fidelity of Halipegus occidualis. Journal of Parasitology 102: 47-53