Special Report: #ASP2012 (Australia) Part IV: Swimming with the Parasites

This post is part 4 (and final) of my special report on the #ASP2012 (Australia) meeting at Launceston, Tasmania - see part 1 here, part 2 here, and part 3 here.

Photo by Kate Hutson

The last day of the conference was a bumper day for marine parasitology so I will just write as briefly as I can on what I saw to cover some highlights. The day kicked off with a series of plenary lectures on; sea lice on farmed salmonids in British Columbia, the history of using parasites as biological markers to identify stock and age of orange roughy (Hoplostethus atlanticus), and an overview of the various parasitic infections that pose a threat to aquaculture by Prof. Barbara Nowak.

But out of those, the presentation which stood out as being most relevant to the original mission of this blog was a talk by Terry Miller - a research officer from the Queensland Museum. He discussed the outcome (so far) of a project to explore to categorise the diversity and genetics of parasites found in fishes of Lizard Island and Heron Island on the Great Barrier Reef, as well as Ningaloo Reef on Western Australia as a part of the Census of Marine Life project. The sheer biodiversity of parasites was the reason why this blog was started and a subject that we discussed in an essay at the end of 2010 - Terry Miller, with his many collaborators, have certainly been busy finding, describing, and classifying this overlooked wealth of biodiversity. They found all manners of myxozoans, flukes, tapeworms, and roundworms, and have already described 56 new species so far. But there are still many unanswered questions relating to biogeography, life-cycles, phylogenetics of these parasites and their significance for fisheries. With 2000 species of parasitic flukes (not counting other fish parasites) estimated to be in the fishes of Australia alone - that's a lot of species descriptions to come!

Photos and drawings used with permission from Leonie Barnett

Speaking of the weird and wonderful, Leonie Barnett from Central Queensland University presented a poster on the molecular phylogeny of a family of parasitic flukes call acanthocolpids which have very odd-looking and remarkably ornate cercariae (the free-living stage which emerge form the first host in the fluke life-cycle). Most cercariae simply look like microscopic tadpoles, with a leaf-shaped body followed by a tapered tail. Leonie has given those acanthocolpid "funky cercariae" nicknames such as "Ducks" and "Starship Enterprise"(see photo on the right). The question must be asked (which at this point can only be rhetorical) - why produce such remarkably elaborate-looking larvae when the majority of them will die after a day or two? What hosts do these parasites infect which warrant such amazing extravagance?

There were a number of presentations thorough the day which were relevant to the fisheries and aquaculture industry, including talks on the detection and treatment of blood-flukes in ranched tuna, identifying and characterising anisakid nematode larvae (which normally infect marine mammals but can cause disease in human if accidentally ingested) from fishes in Australasian waters, and a presentation by Kate Hutson on assessing risks pose to barramundi and mulloway aquaculture by various parasites.

Different Philophthalmus sp. rediae morphs
(insert: specialised morph attacking
the sporocysts of a rival species)

Ian Whittington started off the afternoon session with some videos of monogeneans and to follow that, I talked about potential caste formation and eusocial-like traits amongst the asexual stage of Philophthalmus sp. and how these specialised morphs may in fact be playing a in interspecific competition (see photo on the right or my alternative rendering here)

Sarah Catalano from the Hutson lab talked about a bizarre and little-known group of parasite called the dicyemids which are found in the kidneys of cephalopods (octopus, squid, cuttlefish). These parasites have a very simple body structure, but a very complicated life-cycle. They are astonishingly diverse and also display high levels of host specificity with each species occurs exclusively in a single host species. Because of their specificity they can also be used as a biological marker to reveal different host species where before they were simply considered as subpopulations.

Also from the Hutson lab was Alex Brazenor who presented a study looking at the effects of different water temperature and salinity levels on Neobenedenia - the little worm mentioned in the previous post which is capable of consecutive bouts of self-fertilisation and kick off an outbreak on its own. Alex found that at higher water temperature, Neobendenia lived a faster life -  whereas it took 18 days to reach sexual maturity at 22°C, it only took 10 days to reached that stage at 30°C. Their eggs are more likely to hatch successfully at the higher temperature and salinity level, although if the temperature reached beyond 32°C they start suffering detrimental effects.

Well, that does it for my reports on the #ASP2012 (Australia) conference. Overall, I had a great time - I got to catch up with some colleagues I haven't seen for a while,we talked about a lot of interesting science, and I saw some great presentations and posters - just about all that you can ask for at a conference really. So for me, it's back to writing up blog posts about new papers being published on all manners of interesting parasites - and I already have quite a few lined up...

Special Report: #ASP2012 (Australia) Part III: Sleepy Lizards, Painted Dogs

This post is part 3 of my special report on the #ASP2012 (Australia) meeting at Launceston, see part 1 here and part 2 here.

Photo by Caroline Wohlfeil

There were a number of interesting talks from the wildlife session, first up was a talk by Caroline Wohlfeil - a student from Michael Bull's lab. She gave a talk on sleepy lizards (see right) and the reptile tick, Bothriocroton hydrosauri. These ticks go through 3 stages in their life-cycle, alternating between feeding on a lizard and dropping off in a sheltered area to develop once they are fully engorged. It is in this latter stage that there ticks are transmitted - when lizards take shelter at refuges that have previously been used by infected lizards, they pick up ticks that were dropped off from the previous lizard. Using GPS loggers which continuously recorded the lizard's activity and location, Caroline was able to use that data to work out how often each of the tracked lizard had opportunities for infection. Her network analysis revealed that lizards that are highly-connected also had higher tick loads.

This was followed with a talk by Luz Botero Gomez, a student at Murdoch University, on trypanosome infections in little marsupial call the Brushed-Tail Bettong or Woylie. We have previously covered trypanosomes in another marsupials on this blog, namely the koala, but as it turns out, there is a great diversity of Trypanosoma in native marsupials - most of it still unknown. Woylie are known to be infected with 3 species - T. cruzi (the species which causes Chagas disease), T. copemani, and an as yet unnamed clade of Trypanosoma. Some of those Trypanosoma species are also found in other Australian marsupials but only the woylie is known to carry all three. Much like the koala-infecting trypanosome, T. copemani seems to only cause problem when it occurs in mixed infection with other Trypanosoma species - such co-infections can leads to inflammations and lesions in the tissue. In addition, these different trypanosomes also seem to have varying degrees of tissue specificity, with some species occurring in the blood, while other in muscle tissues, but overall mixed infections are more likely to occur in organs and muscles. Given the Woylie is currently critically endangered, it is very important to know what kind of diseases are induced by these trypanosomes and how it is affected by whether they are single or mixed infections.

Photo from Wikipedia by Helenabella

For a change of pace from parasites threatening a critically endangered mammal, Amanda Ash (also from Murdoch University) presented a talk call "Parasite: embrace not erase" which praised the important functional roles played by parasites in various ecosystems, and discussed the results of a study she conducted looking at the inestinal parasites of African Painted Dogs. She collected fecal samples from captive and wild Painted Dogs and compared the types of parasite eggs and cysts found in those sample. She found that the intestinal fauna of captive dogs was comparatively depauperated, populated only by Giardia whereas the wild dogs had a more eclectic mix of tapeworms, hookworms, and various protozoan parasites - in addition Giardia. Another stark contrast between the captive and wild dogs is that whereas parasitic infection was ubiquitous in the wild population, with 99% had some sort of parasite, only 15% of the captive dogs carried intestinal parasites of some sort.

This has enormous implications for conservation measures such as captive breeding programs - animals which have not been exposed to a wide range of parasites and pathogens can grow up to become immunologically naive so that when they are release into the wild, they may not be able to cope with the wide range of parasites they encounter. In addition, it is unknown what other physiological side-effects may result from lack of exposure to parasites. According to the hygiene hypothesis, the numerous types of allergies and auto-immune diseases which afflict some of us living in western societies have result from the lack of exposure to parasitic worms which are masters at manipulating and modulating our immune system. By limiting both the prevalence and variety of parasitic infections in those captive African Painted Dogs, are we consigning them to the same fate?

During the poster presentation, we saw some students who have come up with creative ways of presenting a 2 min talk - a student from James Cook University read a poem about whether wild dingoes pose a threat to the health of Indigenous communities in Queensland, while a student from University of Western Australia was literally singing the praises of using volatile chemicals for malaria parasite detection. Some of the most fascinating poster talks may present nightmarish scenarios to some people, but for different reasons.

Photo by Kate Hutson

For non-parasitologists, the tongue-biter seems like a one-off freak of nature. But in fact there are actually many species of tongue-biting isopods and other parasitic crustaceans which inhabit the mouth, gills, and branchial cavity of fish. One genus of tongue-biter isopod - Ceratothoa - encompasses 29 known species worldwide, 6 of which are found in Australia waters. However, a new study by Melissa Martin from University of Tasmania revealed that there at least 12 species of Ceratothoa (from 7 families of fish), and 4 of them are new to science.

Dinh Hoai Truong, a student from the Hutson lab at James Cook University presented a horror of a different kind - less visceral than having a parasite in your mouth, but more of a biosecurity nightmare to aquaculturists. He presented a poster on Neobenedenia - a hermaphroditic monogenean which infects the skin of Barramundi. His experiment showed that a single Neobenedenia is able produce eggs through self-fertilisation for consecutive generations without suffering any deleterious effects of inbreeding - each consecutive generation of inbred Neobenedenia are just as infective as the last. This means that even a single worm can start an entire sustained infestation at a fish farm. Unlike the widespread monogenean Gyrodactylus - a notorious aquacuture pest which has the viviparous "Russian Doll"-style "worm-within-a-worm" reproductive set-up (which allows them to swarm a fish like aphids on a rose bush) - Neobenedenia does what most monogeneans do and simply produce eggs. However because they are able to self-fertilise and have very short generation time, they can still become a serious pest to aquaculture.

In the next and final post on #ASP2012 (Australia), we will talk about how environmental factors can affect the generation time of Neobenedenia, and meet many other weird and wonderful marine parasites.

Next post: Swimming with the Parasites

Special Report: #ASP2012 (Australia) Part II: Parasites Gone Wild!

 This is Part 2 of my special report on #ASP2012 (Australia) - for part 1 see here.

The presentation on DFTD as the "perfect parasite" was followed with a talk by Andrew Thompson who holds a Chair in Parasitology at Murdoch University. He talked about how the presence of humans and our activities have often exposed wildlife to various infectious diseases. Wildlife are usually seen as a source of potentially harmful infectious diseases, and are often treated as the "bad guys" when it comes to pathogens. But in fact, sometimes wildlife have more to fear from us, and they act as sentinels, sinks, and sufferers of emerging infectious diseases which had been brought about through human action.

Tapeworm cysts in wallaby lung (photo from here)

For example, since the introduction of dogs and domestic livestock, macropods such as kangaroos and wallaby have become host to cysts of hydatid tapeworms (which Carl Zimmer wrote a post on about a month ago and was among the first batch of parasites to be featured on this blog). Hydatid infections in macropods come from eggs which are deposited in the environment by farm dogs which have the adult tapeworms living in their intestine. The dogs themselves acquire the worm from feeding on offal of infected livestock. Thus our canine companions are acting as the conduit for hydatid to jump from livestocks (one of their original hosts) to the likes of Skippy.

Bandicoot photo by JJ Harrison
from the Wikipedia

But hydatids are not the only introduced parasites which is afflicting Australia's marsupial fauna. Everyone's favourite cat parasite - Toxoplasma gondii - can also end up infecting the brain of bandicoots. However, this did not result from bandicoots coming into in close contact with cats (or rather, their feces). Instead, being such cute little mammals, people often leave food out for bandicoots in their backyard, and encourage them to enter into urban environments where they are more likely to be exposed to infective cysts.

In addition to introduced parasites, Australia's marsupials are home to all manner of little known vector-borne infections. There are multiple species of trypanosomes found in native marsupials (which I will discuss in more details in the next post), but there is very little information on their ecology and vectors. The vector for Trypanosoma cruzi (which causes Chagas disease) is a triatomine bug, but next to nothing is known about the Australian species and their potential role in vectoring those parasites. Thompson also discussed what appears to be a unique species of Leishmania in red kangaroos - which is transmitted via a midge (photo below right) instead of sandflies like other Leishmania.

Photo from this paper

Speaking of little known parasite fauna of Australian animals, Prof. Ian Beveridge - who is an absolute goldmine of knowledge on parasite biodiversity - gave a talk on that very topic. A fact he presented during his talk, which got retweeted a few times during the livestream, was that the average kangaroo is carrying 60000 nematode worms inside them. What I did not tweet at the time was that some individual kangaroos can be infected with up to half a million nematodes. Beveridge estimated that there are about 300 species of nematodes found in macropods. Traditionally horses and other equine are considered as particular "wormy" hosts (Sorry about that Bronies, but your Ponies be loaded with Wormies), harbouring a great diversity of nematode parasites - something which was remarked upon by Hippocrates. However, Beveridge estimated that macropods may in fact be equally "wormy" if not more so - there are so many nematode species which are yet to be found and described, and many host species which have not been properly examined for parasites (Beveridge mentioned road kills as an opportunistic sources of parasite samples - something which I have done on occasions.)

Even with the worms that are already known, it could be that they are even more diverse than we initially expected. Beveridge talked about a case where nematodes from rock wallabies which have previously been classified as 3 species (based on their morphological features) were later revealed by DNA analyses to be composed of 15 distinct genetic lineages (we have previously posted about cryptic species complex on this blog here and here).

And just bring it full circle and refer back to the previous post - the Tassie Devil is host to some unique parasites itself. Out of the two species of flukes, two species of tapeworms, and three species of nematode that it hosts, one of the flukes and two of the tapeworms are unique to the Tassie Devil and found in no other animals. If we lose the devils thanks to DFTD, we will lose those one-of-a-kind parasites too. Sometimes parasite extinctions can be brought about through the best of intentions (see the case of the Californian Condor) - when the devils are brought in for captive breeding or as an "insurance population", the vets treat them for parasites - so good-bye special worms! However, Dasyurotaenia robusta - a species of tapeworm unique to the Tassie Devil - is actually covered by the Threatened Species Protection Act in Tasmania.

Save the devil, save the D. robusta!

Coming up in the next part: Sleepy Lizards, Painted Dogs.

Special Report: #ASP2012 (Australia) Part I: Better the Devil you (get to) know

This post is the first in a series of special reports that I will be writing based on my recent experience at the Australian Society of Parasitology conference in Launceston, Tasmania (some of you might have noticed my tweets from the event tagged with #ASP2012). As a disclaimer, I will only be writing about the talks and posters which I personally saw, thus they will be those which reflect my interest in natural history, evolutionary biology, ecology, and wildlife diseases. Those also happens to be the type of post which appears on this blog anyway, so if you are a long-time reader of Parasite of the Day, it might as well be business as usual.

The conference started off with a series of free-to-public talk. The first was a presentation by Greg Woods, an associate professor from University of Tasmania, on the Devil Facial Transmissible Tumour as the "perfect" parasites - keep in mind I have emphasised many times on this blog (directly or indirectly) that there is no such thing as "perfect" in evolution. I first encountered the concept of DFTD as a parasite in an abstract I read from the program of the eleventh ICOPA (International Conference of Parasitology) held at Glasgow in 2006. Ever since I have been intrigued by the idea of cancerous cell lines becoming infectious and transmissible between different individual hosts/carriers (DFTD is not a lone anomaly - see also the Canine Transmissible Venereal Tumour and reticulum cell sarcoma of the Syrian hamster). According to Woods, based on current evidence, the DFTD cell line currently being circulating in the devil population had originated in 1996 from the mutated Schwann cells of a female devil (the "DFTD Eve" I guess). The original function of Schwann cells is to protect and maintain the peripheral nerve cells. The cells of DFTD are heavily modified from their ancestral form both in appearance and function, which made it difficult to identify their origin.

During the audience Q&A session, Woods also mentioned that during the course of research on DFTD, scientists were able to co-opt the background knowledge and pre-existing tools we already have for the study of human cancer cells to understand the biology of the DFTD cell line. In turn, molecular tools which were subsequently developed specifically for identifying and elucidating DFTD cells have also become useful for studying human cancer cells. Those parallel cancer research programs have proved to be a far more mutualistic relationship than that between the Tassie Devil and the DFTD cells.

Woods discussed some of the criteria which qualifies DFTD a parasite including their ability to produce immunological suppression factor, allowing them to escape the host's immune system. While some may object to calling the facial tumour cell line a "parasite" because it eventually kills their host - many parasites eventually kill their host, whether by accident or as a part of their life-cycle (as we have written about many times on this blog) - there is no universal parasite creed that goes "thou shalt not kill your host" - if parasites did have a creed it would read more like "thou shalt use the host in whatever way you see fit and get away with".

In a review on DFTD published in Trends in Ecology and Evolution, Prof Hamish McCallum wrote that the cell causing DFTD is "...essentially a clonally reproducing mammal that is an obligate parasite." which may seen radical, but once you get over the preconceived notions you may have about what a "parasite" ought to be (or what a mammal, or a "species" or "life" ought to be), it makes biological sense. After all, there are stranger things which exists in nature and biology. Those who want to read more about DFTD should see this article here. Apart from learning about DFTD, thanks to the appearance of a special guest (see right) we also found out that the infamous Tasmanian Devil is nowhere near as aggressive as most people might think...

Coming up in the next part: Parasites Gone Wild!

Xenopsylla ramesis

There is no parasite that is universally infective, even generalist parasites that can infect many different host species are usually limited to a particular taxonomic group - such as fish, insects, or mammals. Some parasites may infect a broad spectrum of hosts during one stage of their life-cycle, but are very specific in another. For parasites that are host specialists, this can be taken to an extreme level where they are found exclusively on just one particular host species. Just how parasites evolve from generalist to become so specialised is one of the enduring questions in studies of the evolutionary ecology of parasites.

Image from of the related and more well-known
Xenopsylla cheopis (also known as the plague flea)
from the NHM

To investigate this question, a group of scientists from Israel carried out an experiment on Xenopsylla ramesis - a species of flea that infects a number of different desert rodents. For their experiment, the scientists raised separate populations of fleas on two species of desert rodents - Wagner's Gerbil and Sundeval's Jird - both of which are commonly infested with X. ramesis in the wild. Each of the experimental flea populations were assigned to either gerbils or jirds, and raised for nine consecutive generations on their specifically assigned rodent species. Out of every generation, the scientists also took a subset of 30-50 fleas from each of the experimental populations, and transferred them onto the other host species to see how those fleas performed compared with their counterparts that got to stay with the specific rodent host that they have been assigned with.

For the first three generations, there were no noticeable differences when the fleas were switched from either gerbils or jirds onto the alternative host. But by the sixth generation, the fleas have become so attenuated to the specifically assigned rodent species that when they were transferred to a host that was different to the one that their parents were raised on, they suffered drastically. Female fleas which had been transferred to the alternative host produced far fewer eggs (only about a quarter of the number produced by fleas that got to stay with their assigned host), and out of those eggs which were laid, fewer of them actually hatched, and out of the larvae that hatched, very only one-quarter to one-fifth reached full maturity.

The scientists who conducted this study suggested that this came about through what is known as "relaxed selection". When the fleas had been infecting multiple host species, there was selection pressure on them to maintain whatever full suite of adaptations that had allowed them to feed off a broader spectrum of hosts. But when the population is restricted to a single host species, there is no longer any selective advantage in maintaining the full suite of traits. Thus the adaptation(s) associated with infecting those other hosts (which they are no longer exposed to) were discarded, leaving only the specific adaptation(s) that are relevant to exploiting the available host species.

Another thing to note is that the natural ability of X. ramesis to live off multiple rodent hosts deteriorated very rapidly - within just a few generations - and the effects were drastic. The authors suggested it might have occurred through epigenetic modifications - inheritable changes in gene expressions which do not involve any changes in organism's DNA sequences (instead of mutations, which alter the underlying structure of the DNA). Another possibility, which the scientists who conducted this study did not raise, is whether the gut microbes of the fleas played a role in their ability to exploit different host, as it has been shown to be the case for some plant pests. However, little is known about the microbes that inhabit flea guts, apart from pathogens that are known to be vectored by fleas such as the bacteria that causes the plague.

Reference:
Arbiv, A., Khokhlova, I.S., Ovadia, O., Novoplansky, A. and Krasnov, B.R. (2012) Use it or lose it: reproductive implications of ecological specialization in a haematophagous ectoparasite. Journal of Evolutionary Biology 25: 1140-1148.

P.S. Don't forget, both Susan and I will be attending parasitology conferences happening on our respective continents in July and we will be tweeting about them - you can find me on Twitter @The_Episiarch and Susan @NYCuratrix. I will be tweeting the Australian Society for Parasitology conference 2-5 July, while Susan will be tweeting the American Society of Parasitologists conference 13-16 July. Follow the hashtag #ASP2012 for relevant tweets. On the 2 July, there will also be a livestream public talk call "Parasite Encounters in the Wild" - Twitter participation is encouraged so feel free to tweet your question with the hashtag #ParasWild during the talk.

Corynosoma cetaceum

image from here

In the last post we met Acanthocephalus rhinensis - an acanthocephalan which lives a pretty normal life (for a thorny-headed worm) - it spends its adult life anchored to the intestinal wall of its eel host, absorbing the nutrient-rich slurry of the intestinal content through its body surface. Today, meet Corynosoma cetaceum - it is yet another acanthocephalan, but that's about where its similarity with A. rhinesis ends. Corynosoma cetaceum lives inside the stomach of dolphins, and it is one prickly customer. As well as having the signature thorny proboscis (see the lower right picture), its entire body is covered with a spiky coat of wickedly-sharp spines (see picture on the upper left showing spines extending well pass the proboscis) which would put a hedgehog to shame.

Whereas in other acanthocephalans the proboscis plays the main attachment role, in C. cetaceum uses its entire body to cling on. The study which forms the basis of today's post looked at differences in the spines of male and female C. cetaceum, and found a high degree of divergence between the sexes. While female worms are smaller, overall they have much longer spines than males. In fact only in females do the spines grow significantly during maturation from larva (known as a cystacanth) to adult. In contrast, the body spines of adult male C. cetaceum remains more or less the same length as they were as cystacanths.

image composed from here and here

This seems odd, because being smaller, the females are actually at less risk of being dislodged (less surface area exposed to the dragging flow of the stomach content) - so why the longer spines? One possibility raised by the researchers is that perhaps the males simply depend upon attachment mechanisms other than body spines - but compared with females, the male worms have smaller proboscis and hooks too. Alternatively (and more likely), perhaps female worms need to stay in the host for longer than the males in order to produce and release eggs. There are indirect data which indicates female C. cetaceum live longer than their male counterpart - this is inferred from what is known for other acanthocephalans, and the sex ratio of C. cetaceum populations found in the stomach of dolphins which is skewed towards having more females.

There are further, as yet unsolved mysteries relating to C. cetaceum. As mentioned at the start of this post, the stomach is a very different habitat to the intestine. The life of parasites living in the intestine is fairly leisurely, being bathed a steady flow of nutrient-rich slush composed of finely-digested food infused with a cocktail of the host's bodily secretions. In stark contrast, the stomach is an extremely harsh environment. It is where early stages of digestion takes place - where chunks of food are mashed up and soaked in harsh digestive juices. The content of the stomach is composed largely of chyme - an acidic mixture of partially digested food and acid which is not all that nutritious for parasites like acanthocephalans which absorb nutrients through their body surface. In addition, carnivorous marine mammals consume huge quantity of food whenever the opportunity arises; this results in unpredictable and heavy flows of food through the stomach which makes for an extremely turbulent environment that can easily dislodge any parasitic worms (see this paper).

Of all the places in the digestive tract that C. cetaceum can occupy, why has this species evolved to live in such an inhospital environment?

Reference:
Hernández-Orts, J.S., Timi, J.T., Raga, J.A., García-Varela, M., Crespo, E.A. and Aznar, F.J. (2012) Patterns of trunk spine growth in two congeneric species of acanthocephalan: investment in attachment may differ between sexes and species. Parasitology 139:945-955.

P.S. Attention parasite appreciators! Both Susan and I will be attending parasitology conferences happening on our respective continents in July and we will be tweeting about them. So as if this blog isn't already enough, you can your 140 characters or less fix of parasitology goodness on Twitter - you can find me on Twitter @The_Episiarch and Susan @NYCuratrix. I will be tweeting the Australian Society for Parasitology conference 2-5 July, while Susan will be tweeting the American Society of Parasitologists conference 13-16 July. 

Acanthocephalus rhinensis

image from figure 1 of the paper

The study which forms the basis of today's post features an acanthocephalan - also known as a thorny-headed worm - which lives in the intestine of European eels in Lake Piediluco in central Italy. Acanthocephalans spend their adult lives like tapeworms, clinging to the wall of their host's intestine, and absorbing nutrients from the pre-digested gut content. But unlike tapeworms, which mostly use suckers and small hooks to cling to the intestinal wall, an acanthocephalan has a formidable bit of armament which puts the tapeworms to shame. As its name indicates, at the front of the acanthocephalan is a hook-laden proboscis (see the picture on the right) to stab into the intestinal wall and firmly anchor themselves in place.

In Lake Piediluco, some eels were found to be infected with up to 350 Acanthocephalus rhinensis, though most eels had fewer than 50 worms. The eels become infected through eating little shrimp-like crustaceans called amphipods. The amphipods live mostly amongst the aquatic vegetation at the edge of the lake, and they are parasitised by the larval stage of A. rhinensis. If you thought the idea of having dozens of prickly-headed worms clinging to your intestinal wall with their nightmarish probosces is bad, A. rhinensis is downright brutal to the amphipod host.

image from figure 3 of the paper

The larval worm (called a cystacanth) occupies a large part of the little crustacean's body (see picture on the left), displacing many of its internal organs. About one in ten amphipods at Lake Piediluco are infected with A. rhinensis, and each amphipod had one or two worms inside them (probably because there wouldn't be much room for more). Acanthocephalus rhinensis imposes a massive burden on the little crustaceans - infected females can only successfully produce half as many eggs as uninfected females.

Armed with that formidable anchor, you would think that A. rhinensis would be able to establish itself in the gut of just about any fish it finds itself in. But it appears to be remarkably faithful to eels, which are the only fish found to have A. rhinensis in their intestines. Perhaps there are other immunological or ecological reasons that prevent this species from successfully infecting other fish.

In addition to establishing the life-cycle of A. rhinesis, another discovery made by the researchers actually served to amend an existing error in the scientific literature. In the original description of A. rhinensis, which was made based on nine specimens, this species is supposed to have a distinctive band of orange-brown (think spray-on tan) pigment just behind their proboscis, a feature that apparently distinguishes it from all the other Acanthocephalus species. However, the researchers who wrote this paper examined a total of over a thousand worms and not a single one had the supposed distinguishing band. But what gave those worms that orange-brown collar? The researchers suggested that this was caused by discolouration from being jammed so deeply into the intestinal wall that the worms inadvertently absorbed pigment from host's intestinal vessel which gave them a distinctive tinge just behind their proboscis.

So in addition to working out the life-cycle of A. rhinensis, this study also served to clarify old mistakes, which will help out any future researchers who work on this species.

Reference:
Dezfuli, B.S., Lui, A., Squerzanti, S., Lorenzoni, M. and Shinn, A.P. (2012) Confirmation of the hosts involved in the life cycle of an acanthocephalan parasite of Anguilla anguilla (L.) from Lake Piediluco and its effect on the reproductive potential of its amphipod intermediate host. Parasitology Research 11: 2137-2143.

Macrodasyceras hirsutum

On this blog, we have featured many parasites that drastically alter the appearance and/or behaviour of their host, usually to make them more likely to be eaten by the next host in the parasite's life-cycle. But today, we are featuring a parasite that makes their hosts appear less appetising - a seed parasitoid that has other plans for its host - none of which involves being eaten.

From the perspective of the plants that produce them, fruits are a way to turn animals into willing seed couriers. By wrapping seeds up in a tasty package, plants can deposit their seeds temporarily inside the body of an animal that will carry them off to a new location. We have even featured a (parasitic of course) plant on this blog that uses beetles for such a purpose.

photo from Figure 1 of the paper

Unlike the rest of the plant, which is often indigestible and laden with defensive toxins, the fruit is supposed to be attractive and appetising to would-be animal dispersers. However seed parasitoids such as Macrodasyceras hirsute have other plans for the fruits - they do not care for the fruit's flesh - they are only after the nutritious seed. Unlike the parasite we featured in the last post, the gullet of a bird is a death sentence for the larvae of this parasitoid (though as always in nature, there are some exceptions), which is a bit of an inconvenience as the fruits it parasitises are meant to be eaten by birds.

Macrodasyceras hirsutum parasitises the fruit of the mochi tree Ilex integra and all it wants to do is to live out its larval stage munching on seeds and grow up to be a wasp. It would rather not have its life suddenly interrupted by a hungry bird feasting on the mochi tree's bright red ripe berries.

So to ensure that its home will not end up tumbling down the throat of a bird, M. hirsutum larvae counteract the berry's usual ripening process, and ensure that it stays green (and unappetising to birds, which disdain unripe berries). A team of Japanese scientists found that if they shielded the fruits from wasp attack, almost all the mochi berries ripened to red. But, if they are exposed to M. hirsutum, some of them stayed green, and all the berries that stayed green had M. hirsutum larvae living inside them. Furthermore, they found that the more larvae there are in the berry, the more intensely green the fruit becomes - M. hirsutum did not merely stop the berries turning from green to red, they actually turned the dial on the green tone all the way up.

This little wasp is not the only insect to do this. Holly berries infected with a species of midge also stay green. It is unknown how this wasp interferes with the berry's pigment production/development, though for the holly berry midge it has been suggested that a symbiotic fungi is responsible for maintaining the host fruit's green colour. The relationship between fruit-bearing plants and fruit-eating animals has evolved to be a mutually beneficial interaction whereby one party provides food (fruits) while the other returns with a service (seed dispersal). But, the actions of M. hirsutum and other such seed parasitoids tinkering away in the background can certainly undermine the effectiveness of this mutualistic partnership if they cause otherwise ripened fruits to go uneaten. The extent of the impact such seed parasitoids have on the ecology and evolution of such plant-animal interaction is currently unknown.

Reference:
Takagi, E., Iguchi, K., Suzuki, M. and Togashi, K. (2012) A seed parasitoid wasp prevents berries from changing their colour, reducing their attractiveness to frugivorous birds. Ecological Entomology 37: 99-107.

Philophthalmus sp.

We have featured many parasitic flukes on this blog, and a part of their life-cycle consists of the free-living stage (called a cercaria) being expelled from its mollusc host (where they are produced asexually), and proceeding to infect the next host. There, they form a cyst and wait to be eaten by the final host. But today's parasite, also a fluke, does something slightly different. Unlike most other trematodes that penetrate and embed themselves inside the host's body, Philophthalmus attaches itself to hard surfaces, which just so happen to be objects that are likely to be swallowed by shorebirds - such as the surface of a shellfish. From my personal experience, this tendency also made them a bit of a nuisance in the lab as they tended to stick to pipette tips and the bottom of plastic petri dishes and had to be scraped off (see the accompanying photo).

Not all of the objects that are available for attachment by Philophthalmus would necessarily end up going down the gullet of a bird, however. There is an abundance of small rocks and seaweed in the intertidal environment where Philophthalmus is found - but those are not the usual fare of shorebirds that prefer a more appetising diet of seafood like snails and crabs. A pair of scientists from University of Otago set out to see if Philophthalmus shows preferences for certain types of substrates. First, they surveyed and calculated the exposed surface area of various objects on the mudflats (such as snails, crabs, seaweed, rocks, etc), then estimated the number of Philophthalmus cysts that would be expected to be attached to each of them if the cercariae just distributed themselves evenly across the environment and stuck to whatever they come across first. They also did a series of laboratory experiments to test if Philophthalmus cercariae show any preference for specific types of objects.

They found that instead of attaching themselves to whatever that just happened to be laying around, there were a higher number of Philophthalmus cysts on snails than expected given their comparatively small surface area. This makes sense in terms of the parasite's life-cycle as the shell of a live snail is more likely to deliver Philophthalmus to the mouth of a bird than say, bits of rock and seaweed. Additionally, in laboratory experiments, when Philophthalmus cercariae were presented with the type of substrate usually present on the mudflats, they showed a very high tendency to stick themselves to snails, but shunned rocks and the shells of cockles.

So why the preference for snails but not cockle shells? Some birds do eat cockles and for some parasites that is how they complete their life-cycle, but when birds such as the oystercatcher eats a cockle, they usually crack open the valves to swallow the flesh (where the parasites are embedded), leaving the shell behind. Snails, however, are more likely to be eaten whole (with the attached Philophthalmus cysts). Oddly enough, the Otago scientists found very few cysts attached to crabs - which is another staple food for birds. However, a crab tends to go through several molts during its life, and every time it does so, it would leave behind any Philophthalmus cysts which are attached to its carapace, and birds don't usually go around picking up empty crab shells to eat.

It is worth pointing out that it would be equally useless for Philophthalmus to attach itself to the shell of a dead snail. Therefore, it is possible that these cercariae are able to detect chemical cues that allow them to distinguish between dead and live snails. Other cercariae are known to be able to respond to a wide range of stimuli, and there no reason to think that Philophthalmus would be all that different - perhaps that would be the next stage of research into this species...so watch this space (or species as the case may be)...

Reference:
Neal, A.T., and R. Poulin. 2012. Substratum preference of Philophthalmus sp. cercariae for cyst formation under natural and experimental conditions. Journal of Parasitology 98: 293-298

Cuscuta chinensis

Today's post has some plant-on-plant action, featuring a species of dodder. Dodders are a group of holoparasitic plants made up of about 100-170 species. They are plants that consistent entirely of stem, with leaves that have been reduced to tiny scales. They smother the host plant in a tangled mess, and dig deep into the host tissue using modified roots called haustoria to draw out water and nutrients. We have previously featured the European Dodder on this blog, but today, we will be looking at the Chinese Dodder Cuscuta chinensis, and how it interacts with plants that are not native to its home range.

Image Credit: Jayesh Patil

There are many studies that look at characteristics of successful invasive species (an introduced species that has subsequently become a pest). Generally, plants that have become invasive after their introduction have faster growth rates and are able to utilise nutrients more efficiently, allowing them to outcompete the native flora. In addition, according to the "enemy release" hypothesis, one of the reasons why newly introduced plants and animals become so successful in their new homes is because they are freed from the burden of their natural predators and pestilence, thus allowing them to propagate unchecked across the new land. While this seems to indicate that the best way to control invasive species is to introduce their natural enemies as well, the main problem is that you are introducing yet another new species. Remember that folklore about the old lady who swallowed a fly and subsequently introduced a sequential menagerie into her body? You don't really know what cascading effects the new biological control species will have on the local ecosystem - after all, the cane toad (Bufo/Rhinella marinus) was introduced to Australia to control beetles in sugar cane plantations, but have since become a huge ecological problem.

The higher growth rate and resource-usage efficiency of these invasive plant does have a drawback though - it makes them more attractive targets to parasites. So what if a native parasite can turn the table on the invaders? What if a native parasite acquires a taste for an exotic new host?

The Chinese Dodder is a parasite with eclectic tastes, as it is capable of infecting more than 100 species of wild and cultivated plant species. To find out how well C. chinensis grows on native flora compared to their introduced counterparts, a team of researchers in China evaluated the performance of C. chinesis on 3 invasive plant species and the native equivalent from the same genus. They found that not only did C. chinensis grow much more prolifically on the introduced plants,but it also caused more damage. In fact, C chinensis is more damaging to plants that are more efficient in using their resources - the very trait which makes them so good at being invasive in the first place.

There is also another possibility - one which the researchers did not mention in the paper: Unlike the native plants which have had a long co-evolution history with the dodder and have thus evolved various means to counter the parasite's tricks blow-by-blow, the naive introduced species have never encountered C. chinesis before, which leaves them more vulnerable to attacks by the parasitic dodder. For those exotic introduced plants, it seems that the very thing which had brought them so much success in their new home may end up causing their downfall when confronted with a certain holoparasite.

Reference:
Li J, Jin Z, Song W (2012) Do Native Parasitic Plants Cause More Damage to Exotic Invasive Hosts Than Native Non-Invasive Hosts? An Implication for Biocontrol. PLoS ONE 7(4): e34577. doi:10.1371/journal.pone.0034577