"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

April 9, 2014

Bivitellobilharzia nairi

A little over a year ago, I wrote a post about Bivitellobilharzia loxodontae - a species of blood fluke that lives in the African forest elephant. Today I am writing about a study on another species from that genus - Bivitellobilharzia nairi - which parasitises the Indian elephant. However in a newly published study, it turns out the Indian elephant is not the only thick-skinned mammal that harbours this fluke.

Photo of Indian rhino by Krish Dulal
The study we are featuring today took place in southern Nepal at the Chitwan National Park (CNP). Researchers collected fecal samples from both domesticated and wild Indian elephants for examination and as expected, they found B. nairi eggs amongst the samples. But it was when they started looking in the poop of Indian rhinoceros that they found the unexpected. These rhinoceros do not take a dump just anywhere; they are creatures of habit and defecate at specific spots call faecal middens - which is how they mark their territory. When the researchers dug through the contents of those middens, they found blood fluke eggs amidst the rhino dung in half of the fourteen middens they sampled from.

The eggs had the characteristic look of schistosome eggs - an ovoid with a hook at one end (see below). But they were not just any blood fluke eggs, they looked very similar to the eggs of B. nairi - the elephant blood fluke. When the researchers sequenced specific marker section of the fluke eggs' DNA, they found that it matched the known sequences for B. nairi, showing that what is usually thought of as just an elephant parasite can also find a home in the Indian rhinoceros. Furthermore, the B. nairi eggs they recovered from the rhino dung were completely viable, showing that the rhino is a natural and commonly used host for this parasite and that they did not end up there by accident

Image of Bivitellobilharzia nairi egg from here
Evolutionarily speaking, elephants and rhinoceros are fairly far apart on the mammalian tree - the last common ancestor they shared lived about 100 million years ago in the era of non-avian dinosaurs. So what is an elephant schistosome doing in a rhino? Despite their specialised adaptations for living in the circulatory system and evading the immune reactions of their particular hosts, throughout their evolutionary history, schistosome have made a number of leaps across divergent animal taxa. One such jumps had allowed the ancestors of schistosomes to evolve from a sea turtle-infecting parasite into one which live in the blood of warm-blooded animals like birds and mammals. While elephants and rhinoceros have had disparate evolutionary paths for at least a hundred million years, clearly their physiology are similar enough for B. nairi to successfully survive in both. In addition, their shared habitat provided the fluke with plenty of opportunity to encounter and adapt to the rhinoceros.

So there is more than one way for two (or more) different species to end up with the same parasite. You can either share a recent common ancestry, or you can share the same habitat which gives the parasite ample opportunities to cross the evolutionary gulf between different hosts.

Reference:
Devkota, R., Brant, S.V., Thapa, A. & Loker, E.S. (2014) Sharing schistosomes: the elephant schistosome Bivitellobilharzia nairi also infects the greater one-horned rhinoceros (Rhinoceros unicornis) in Chitwan National Park, Nepal. Journal of Helminthology 88: 32–40

March 26, 2014

Octopicola superba

When it comes to reproduction, most living things can be classified along a scale. At one end, you have the r-strategists (many insects and molluscs) that produce a prodigious number of offspring but few survive to adulthood. And on the other end are the K-strategists that produces only a few progeny, but to invest a lot of resources into each to ensure they are more likely to reach maturity (for example, elephants, humans, etc).

SEM photo of female
Octopicola superba from here
There is a cost/benefit trade-off inherent with being on either side of the scale because as a r-strategist, you might be producing a lot of progeny, but most of them will probably die before they get to reproduce themselves. While on the K-strategist end, by investing so much resources into each individual young, you can only afford to produce a few of them. The reproductive strategy of different organisms all fall somewhere along that continuum between low quality mass production or high quality but infrequent output, and different circumstances call for different strategies.

Textbook often use parasites as key examples of r-strategists, as a model of organisms that producing prodigious number of offspring. Indeed some internal parasites are well-known for their reproductive capacity - for example, the female blood fluke Schistosoma mansoni produces 300 to 3000 eggs per day, while tapeworms like Diphyllobothrium dendriticum can produce tens of millions of eggs per day. But not all parasites opt for quantity over quality.

The study we are featuring today examined the reproductive capacity of the parasitic copepod Octopicola superba, which, as its name indicates, lives in the common octopus. As far as a parasite goes, this crustacean seems rather innocuous and does not really cause much harm to its host. Octopicola superba can be found all over the body of the octopus but most of them are located on the skin and gills. Even though it is a parasite, it has a reproductive strategy which brings it closer to being a K-strategist.

Each female O. superba produces a clutch of only a few dozen eggs per season; if a female was to produce more than about forty eggs in a clutch, she starts reaching the upper limit of her reproductive capacity and the size of each egg (which reflects how much resources is invested into it) begins to shrunk as the brood imposes too much of a drain. This reproductive capacity varies considerably between individual; the most productive copepods are able to produce over twice as many eggs as the least productive ones, while some produced eggs that were almost twice as big as those produced by others.

Octopicola superba's reproductive strategy also shifts during different seasons; in winter, they produced a larger clutch of smaller eggs, whereas in summer they produce a smaller clutch of bigger eggs. Such season shifts has been observed in other parasitic copepods, though for O. superba, the reason for them doing so remains unknown. Despite these seasonal and individual differences, overall O. superba is certainly low-key when it comes to reproduction - even the most fecund female had just above sixty eggs in a clutch and the rest mostly produced between thirty to forty eggs.

So why has this parasitic copepod evolved to produce so few eggs compared with parasites like tapeworms and blood flukes that pump out thousands or even millions of eggs on a daily basis? It might have something to do with the habits of its host.

Octopus tend to be territorial homebodies that likes to stay in their little corner of the sea. Previous analyses indicate that hosts with such sedentary habits tend to select for parasitic copepods that produce larger eggs. Unlike one infecting more mobile animal (like a fish), parasites of sedentary animals cannot rely upon their host's routine daily movement to bring them into contact with new hosts. Therefore, they must do so under their own steam. By investing more into each egg, the female O.superba ensures each of her babies are better equipped for the long journey to find a new home, even if it means she can only produce just a few dozen of them at a time.

With offspring, you can only invest so much into them - at some point, they are on their own

Cavaleiro, F. I., & Santos, M. J. (2014). Egg number-egg size: an important trade-off in parasite life history strategies. International Journal for Parasitology 44:173-182

March 9, 2014

Cucumispora dikerogammari

Invasive species can be very disruptive - cane toads, rabbits, water hyacinth, and zebra mussels are just a few well-known examples of species that have been introduced to areas outside of their original geographic range and have caused extensive ecological disruption in their new home. One of the hypotheses for why some introduced species become so successful when they arrive at a new region is called the "enemy release hypothesis". In their new home, introduced species run amok as they are no longer hounded by their usual foes that would otherwise keep their population in check.
Top: A heavily infected amphipod
Bottom: Spores of C. dikerogammari
Photo from here

Dikerogammarus villosus is an amphipod (a little, shrimp-like crustacean) from the Ponto-Caspian that has invaded western and central Europe, and is now also found in the United Kingdom. They might only grow up to a little over an inch long, but they are voracious little predators that eat everything smaller than themselves, including each other. Released from their usual predators and parasites, D. villosus rips through the freshwater life of its new neighbourhood. But they have not completely escaped from their past foes; one parasite has managed to come along for the ride, and it is a microsporidian called Cucumispora dikerogammari.

As far as the parasite goes, Cucumispora dikerogammari is a pretty nasty one. It invades the host's muscles, reproduces prolifically and eventually kills the host by overwhelming it with sheer numbers. There is some concern that this parasite can spill over into the native invertebrates and add insult to injury to the local stream life. But on another hand, a new study shows that this parasite might be one of the few things holding back this voracious invasive amphipod from causing even more destruction.

A group of scientists from France conducted a study to looked at how C. dikerogammari affects the activity levels and appetite of D. villosus. They observed the behaviour of both infected and uninfected amhipods in a water-filled glass tube and noticed that amphipods at a late stage of infection that are visibly "filled to the brim" with parasite spores are actually more active than healthy amphipods or those that are not visibly parasitised because they are at a much earlier stage of the infection.

Close-up of a C. dikerogammari spore from here
Furthermore, they also presented amphipods with midge larvae (also known to some as "bloodworms") to see how many they ate. Both infected and uninfected D. villosus pounced on those insect larvae, but the heavily infected amphipods ate far less than the healthy ones. For whatever reason, this parasite seems to cause D. villosus to lose its appetite, and given this crustacean's reputation of eating everything that it can get its claws around, this may have significant ecological ramifications. It could mean that C. dikerogammari may be subtly reducing the impact these amphipods have on the areas where they have been introduced.

But why would heavily-infected D. villosus, which would have much of their muscle mass already converted to parasite spores by C. dikerogammari, be more active? Well, it could just be an odd manifestation of the disease, but if it is, it is certainly a useful one for this parasite - as it depends upon cannibalism for transmission to new hosts. Dikerogammarus villosus are rather homely creatures and usually prefer to stay under a shelter and wait for potential prey to wander by. By getting their host out and about, C. dikerogammari might increase the chances that its host will either run into one of its cannibalistic buddies, or die out in the open where it can be scavenged by other D. villosus.

It seems that for this little invasive amphipod, no matter how far you go, you can never really run away from your past (foes).

Reference:
Bacela-Spychalska, K., Rigaud, T., & Wattier, R. A. (2013). A co-invasive microsporidian parasite that reduces the predatory behaviour of its host Dikerogammarus villosus (Crustacea, Amphipoda). Parasitology 141: 254-258.

February 14, 2014

Gordionus chinensis

Hairworms are known for their ability to make their host go for an impromptu (and terminal) swim in a stream or a pond, but by doing that they are not just sending ripples through the water, but also into the surrounding ecosystem. The paper we are looking at today features a species of hairworm from Japan call Gordionus chinensis - this parasite infects three different species of forest-dwelling camel crickets from the genus Diestrammena.

Photo by Danue Sachiko from here
The scientists who conducted the study that this paper is based on wanted to find out what happens to the the cricket population and their hairworm parasites after their home forest has been cut down. They conducted an observational field study at an experimental forest in the upper parts of the Totsu River at Nara Prefecture, Japan. The forest was originally clear-cut in 1912 and 1916 and since then, parts of it have been replanted and cut down at different point in time over the last century. Each study site corresponds with a different replanted forests of Japanese cypress ranging from 3 to 48 years old.

These scientists found that the camel crickets began returning a few years after a forest has been replanted, their abundance steadily increasing and eventually reaching a peak after the forest has been standing for at least 30 years. But their hairworm parasites did not return with similar gusto. In fact, they estimated that only second-growth forests that are more than 50 years old have hairworm populations that are as abundance as those found at undisturbed sites.

One possible reason for the hairworms' slow recovery is their complex life cycle which requires infection of more than one host. The replanted forest might be lacking some of the other host G. chinensis needs to complete its life cycle. Because parasites has such a negative public image, a forest which is free of parasites (or at least a specific parasite) might sound good to most people. But these hairworms actually play a very vital role in the ecosystem.

By causing their cricket host to jump into a stream, they actually serve as a kind of fast food delivery service for the fish living in those streams. A cricket infected with a hair worm is 20 times more likely to stumble into a stream and become fish food than an uninfected cricket - so fish which would not usually get to feed on such large land-loving insects on a regular basis can now do so thanks to the hairworm, and it has calculated that this straight-to-your-stream food delivery service accounts for 60% of the trout population's energy intake in some watersheds.

For hairworms, new forests just do not have the same creature comforts of old forests. And if you are a keen angler or simply appreciate a fish-rich stream - you have a parasite to thank for all the fishes.

Reference:
Sato, T., Watanabe, K., Fukushima, K., & Tokuchi, N. (2014). Parasites and forest chronosequence: Long-term recovery of nematomorph parasites after clear-cut logging. Forest Ecology and Management, 314: 166-171.

February 2, 2014

Daubaylia potomaca

Photo is of a related species,
Daubaylia malayanum from here
For a parasite, the host provides provides food, shelter, and a site for reproduction - in short, a complete habitat. While for some parasites, host death is a necessary condition for the parasite to complete its life-cycle, for others, the death of a host amounts to the end of the world (or a sinking ship at the very least).

Meet Daubaylia potomaca, a roundworm which infects the freshwater snail Helisoma anceps. Unlike other roundworms that use snails as vehicles to reach the next host in their life-cycle, the snail is the only hosts for D. potomaca. But seeing as snails do not live forever, any parasite it harbours would need an exit strategy or risk perishing with their host when the end finally comes. For a parasite like D. potomaca which completes its entire life-cycle in the snail, it would be useful for it to recognise when they should abandon their host.

A dying host is not the only reason to leave - finding a new host is integral to most parasites' life-cycle, but you would not want to leave too early either - the outside world is a hostile place and as a parasite, you would want to get as much out of the host as possible before you make a run for it. Unlike most other parasites that usually infect a new host as larval stages, D. potomaca actually leave their hosts as fully-matured females laden with eggs, all tangled in mucus-coated bundles composed of 10-50 worms. Therefore the female worms would not want to depart too early as it needs to gather as much resources as possible from its host to nurture the developing eggs. So ideally, they leave it to the last possible moment before they emerge from the snail.

So how well does D. potomaca time its escape? In the paper we are featuring today, a team of researchers studying this host-parasite system observed some worms leaving as early as 52 days before their host snail died, but the majority (almost a quarter) of the worms came out in the last five days of the snail's life. The percentage of worms that emerged increased as the snail's life draws closer to its end - it seems almost as if the parasite can sense when the host is near death's door and took that as a cue for when to bail.

As an additional factor, the researchers also found that infection intensity of D. potomaca affected the snail's lifespan - the more heavily infected it is, the sooner the snail dies. So perhaps D. potomaca can also gauge how crowded the inside of the host is, and schedule their departure accordingly. This is some what reminiscent of a parasite previously featured on this blog; Coitocaecum parvum.

Unlike D. potomacaC. parvum is a fluke with a complex life-cycle and infects multiple hosts through out its life. However, it does face the potential problem of its amphipod host dying before it is eaten by the parasite's next host, in this case a freshwater fish where it can mate with other flukes and produce eggs. But if become increasingly unlikely that its amphipod host would be eaten by a fish before it expires, C. parvum would alters its usual life history schedule to start producing eggs in the amphipod instead of waiting until it end up in the gut of a fish (which might not happen).

While it is a different kind of response to imminent host death compared with D. potomaca, it is another example of how parasites can assess the status of its host and the surrounding environment, and adjust their own life schedule accordingly. Throughout the course of co-evolving with their hosts, in addition to adapting to their host's defences, parasites have also developed many strategies to ensure their survival even as their environment (i.e. the host) faces imminent collapse.

Reference:
Zimmermann, M. R., Luth, K. E., & Esch, G. W. (2013). Shedding Patterns of Daubaylia potomaca (Nematoda: Rhabditida). Journal of Parasitology 99: 966-969.

January 20, 2014

Phronima sp.

Today's guest post is by Katie O'Dwyer, a PhD student currently at University of Otago in the Evolutionary and Ecological Parasitology research group. In one of my conference reports last year, I mentioned some of the research that she is currently conducting on parasitic flukes that live in periwinkles. She has provided us with a post about a parasite that she came across while walking along a beach in New Zealand.

Phronima and its salp barrel.
Photo by Katie O'Dwyer, used here with permission
After recently finding some salps containing the amphipod Phronima, washed up on a beach in New Zealand, I decided this was a worthy group to compose a blog about. It helped too that I was already interested in this group of crustaceans, having assisted with some work on them in Ireland. Read on for some interesting information on this little studied group of parasitic organisms…

Imagine a parasite which can create its own mobile nursery for its young, a parasite which is thought to be the inspiration behind the chestbusting xenomorph in the popular movie Alien. Well imagine no more! Introducing Phronima, the pram bug. These amphipods are members of the Phronimidae, a group of ten species of hyperiid amphipod, which occur in the water column throughout the open ocean. This sets them apart from their close relatives, which typically inhabit the benthic environment of the seafloor. So what has allowed this particular family to adapt to the pelagic or open water environment?

Those adorable little babies!
Photo by Katie O'Dwyer, used here with permission
Enter salps. What is a salp? Salps are gelatinous zooplankton which drift throughout our oceans. They may occur singly or in huge chains composed of individual salps linked together. Phronima is equipped with impressive front claws and with these they attach to an individual salp and carve away its insides until it forms a barrel. Phronima then climbs inside and sails the sea from inside a gelatinous barrel, collecting food from the water column. A number of questions may now come to mind regarding this symbiosis; has Phronima killed its host, which suggests that it is a parasitoid rather than a parasite, and why does it carry this barrel around as it must be pretty energetically expensive, right?

Well, as mentioned, these organisms live in the open ocean which presents several challenges to collecting samples for answering these questions. However, some dedicated researchers have indeed managed to study these fascinating creatures on the rare occasion that such an opportunity arises. From their research they have found that the salp in fact still contains live cells, although it hardly resembles a salp anymore with just a barrel of tissue remaining. The presence of live cells means that the barrel maintains its structure and that is important for Phronima to have a sturdy home. As the barrel barely resembles a live salp any longer, Phronima should really be considered as parasitoids rather than parasites.
Do a barrel roll!
Photo by Katie O'Dwyer, used here with permission

As for the energy involved in carrying around this barrel, the barrel provides a larger structure than the amphipod itself and this enables the Phronima to be more buoyant in the water column. However, some energy is still required to carry around this jelly barrel. Overall energy usage by Phronima is higher than that of benthic amphipods but on the lower spectrum compared with other pelagic or open water amphipods. This suggests that Phronima have indeed adapted to a unique niche which enables them to travel in the water column with their young and access new food resources without this behaviour being too energetically costly.

One unusual finding in the research thus far is that male Phronima are also found in barrels. If Phronima is known as the pram bug, which suggests the barrel is important for carrying offspring, then why should males carry a barrel too? Could they use it as part of some mating strategy, where they pass the barrel on to the female they mate with? Due to the difficulties associated with studying organisms that dwell in the open ocean many questions remain unanswered and this leaves us ever more curious and fascinated by creatures such as Phronima.

References:
Hirose, E., Aoki, M. N., & Nishikawa, J. (2005). Still alive? Fine structure of the barrels made by Phronima (Crustacea: Amphipoda). Journal of the Marine Biological Association of the United Kingdom 85: 1435-1439.

Bishop, R. E., & Geiger, S. P. (2006). Phronima energetics: is there a bonus to the barrel? Crustaceana 79: 1059-1070.

This post was written by Katie O'Dwyer.

January 12, 2014

Choniomyzon inflatus

Photo of C. inflatus from the paper
I guess you could say that the parasite we are featuring today is a "balloon animal" and indeed its name refers to that property. According to the paper that described and named this copepod - Choniomyzon inflatus - "The specific name of the new species is a reference to its swollen prosome, which resembles a balloon."

But you won't be finding this odd little crustacean at any kid's party, instead it is usually attached to the egg masses of smooth fan lobsters (Ibacus novemdentatus) on the coast of western Japan. It is the third species from the genus Choniomyzon to have ever been described. The other two known species are C. panuliri, which are found on spiny lobsters from India, the British Solomon Islands and the Great Barrier Reef, and C. libiniae, which live on spider crabs from São Sebastião Island, Brazil. All three species attach themselves to the external eggs masses of their respective hosts.

SEM photo of C.inflatus
from the paper
So why do they look like a miniature hopper ball toy? Well, that relates to where they live and what they feed on. Chioniomyzon inflatus belongs to a family of copepods called the Nicothoidae and the reason they do this Humpty Dumpty impersonation is so that they can insinuate themselves amidst the eggs masses of larger crustaceans.

Normally the host crustaceans would remove any foreign particles or organisms that get caught up in their brood pouch or egg mass, but by disguising themselves as an egg, C. inflatus and their relatives can stay there undisturbed. And while the appearance seems comical to us, it is seriously bad news for its host because nicothoid copepods are egg-eaters - they have a syringe-like mouthpart with which they puncture their host's eggs and suck out their contents.

So C. inflatus masquerades as just another egg in the brood to avoid being expelled meanwhile munching on the actual eggs around it. This strategy is rather reminiscent of another creature that we featured during the first year of the Parasite of the Day blog - the cuckoo catfish which hides its eggs amongst that of mouth-brooding cichlids. You can read more about the cuckoo catfish here.

Reference:
Wakabayashi, K., Otake, S., Tanaka, Y., & Nagasawa, K. (2013). Choniomyzon inflatus n. sp.(Crustacea: Copepoda: Nicothoidae) associated with Ibacus novemdentatus (Crustacea: Decapoda: Scyllaridae) from Japanese waters. Systematic parasitology 84: 157-165.

December 30, 2013

Another year of parasites in insects, in shellfish and in extreme environments

It is hard to believe that it's already been another year again, and it was a particularly exciting year too, with a lot happening with and around this blog. In terms of the parasites we featured on here, there were some which can be considered to be pretty extreme; like the only external parasite found on guppies that live in noxious tar pits, and some tapeworms with an special affinity for heavy metal. There are those that might make your squirm; like the sexually-transmitted roundworm in anole lizards, and a crustacean that lives in a fish's bladder.

We gave seafood fans some food for thought with some parasites that plague catfish and flounder, and checked in on bunch of clam parasites (tapeworms and flukes) and mussel parasites too (Himasthla elongata). And while fish and shellfish might provide some fodder for parasites, on land, insects provide plenty more opportunities for parasitism, after all, insects are the most diverse group of animals on Earth and they make abundant hosts; from crickets to hornets to ants, and amongst these parasite of insects (some of which are insects themselves) there are some rather sinister ones - like the parasitoid wasp that takes its host to the edge of death so it can be a more compliant host, or the mosquito-killing round worms which sit like mines to be activated upon detecting the presence of its mosquito larva host.

Of course, this year we also had some guest bloggers in the form of students from the University of New England ZOOL329/529 class of 2013 who wrote about how toxic birds makes for sad lice, self-medicating in bees, avian malaria parasites that make their host more attractive to mosquitoes, and how an intertidal fluke might respond to a rise in global temperature. Also, as with last year, we brought you some conference coverage too (part 1, part 2).

We will be back next year with plenty more posts about the newest research in fields relating to parasitology which you might not have heard or read about elsewhere, and as usual, I have already lined up a few which I am going to be writing about... See you all next year!

P.S. If you can't wait until next year, you can find some of my other parasite-related writing on The Conversation about freeze-tolerant parasites, a worm that usurp hornet queens, and fungi that plague the zombie ant fungus. And alongside writing this blog, I've doing a regular radio segment call "Creepy but Curious" where I sometimes talk about parasitic (among other things), like the zombie ants, the infamous crab-castrating Sacculina, the tongue-biter parasite, and the virus that melts caterpillars.

December 13, 2013

Lethacotyle vera

Images from the paper 
While "many sucker-cups at the rear" sounds like the description for a Lovecraftian monstrosity, that is the name of a group of monogenean parasites called the Polyopisthocotylea. Let's just refer to them as "Poly-Opees" from this point to avoid that tongue-twister. They are ectoparasitic flatworms usually found on the gills of marine fish. Seeing as fish use their gills to extract oxygen from their aquatic environment, there is a constant flow of water washing over these parasites, which means these flatworms are essentially living in a high-flow environment. To secure themselves to the gill filaments, they have a sucker structure on their rear - this sucker anchors the worm in place, allowing it to flex the rest of its body and browse on gill tissue and blood.

The rear suckers of monogeneans are not just a simple suction cup, but are composed of an array of intricate anchors, hooks, and clamps that vary considerably between different groups. In the case of the Poly-Opees, this sucker is armed with a series of clamps that gives that entire group its name. But today we are featuring a species that completely bucks that trend. Like most other Poly-Opees, it is also found on the gills of fish, but stands out due to the complete lack of clamps on its rear sucker.

Lethacotyle vera is closely related to a monogenean that was originally described over sixty years ago. The first species described from the genus Lethacotyle was Lethacotyle fijiensis - which was described from a unspecified carangid fish from Fiji (note to fellow scientists - please take detailed notes!), but there are only four specimens of this parasite in existence and only one of them is stored in a museum available for researchers to examine.

A group of researchers revisiting this species' description noted the unusual absence of clamps on its rear sucker and decided to follow up the lead to look for this mysterious monogenean (or at least a related species - which was what they found). As L. fijiensis was originally described from a carangid fish (the group which include jacks, pompanos, trevally and scad), they decided that's where they should start looking. They obtained some Brassy trevally (Caranax papuensis) from some amateur fishermen and fish markets at New Caledonia and looked through the fish's gills for monogenean parasites.

In was on the gills of those trevally that they came across the new species we are featuring today. They were able to confirm that monogeneans in the Lethacotyle genus do indeed lack clamps compeltely on their rear end. Poly-Opees vary in the number of clamps they have - some species have dozens of well-developed clamps while others have clamps that are rather small and may even be considered as vestigial. In the case of Lethocotyle, they are completely gone.

But if they have no clamps, how do they hang on? They have four tiny hooks on their rear, but they are so small that they probably contribute little to securing the worm in place. The researchers noted that instead, the rear sucker has turned into a flap covered in "tegumental striations" in the place of clamps. These are microscopic wrinkles that increase friction and provide traction against a substrate - these microscopic structures might be somewhat comparable to those found on the foot pads of some insects. In this case, it provides enough traction to keep L. vera securely fastened to the gills of its host.

What the story of the Lethocotyle genus and their rear suckers shows us is that parasites are far from being "simplified" evolutionary dead ends, but that they continue to evolve new structures even as they shed others. As with free-living species, certain features often become lost or vestigial over the course of evolution, but then new structures evolve in their place. Lethacotyle might have lost its clamps, but it has also gained a new attachment feature (striation-covered flap) that makes it unique among all the known monogeneans.

Reference:
Justine, J. L., Rahmouni, C., Gey, D., Schoelinck, C., & Hoberg, E. P. (2013). The Monogenean Which Lost Its Clamps. PloS one, 8(11): e79155.

November 24, 2013

Tracheliastes polycolpus

Photo of adult T. polycolpus from here
Tracheliastes polycolpus is a parasitic copepod that lives on freshwater fish and does so by attaching to the fins of its host, grazing on mucus and epithelial cells. While T. polycolpus can infect a handful of different freshwater fishes, it is primarily found on the beaked dace (Leuciscus burdigalensis). When they occur in large numbers, their feeding activities can severely erode the fins of their hosts, so much that in some fish the fins are gnawed down to mere nubs (see the photo below of a heavily parasitised dace, with outlines showing the missing fin tissue).

So when it gets crowded on this parasite's usual, preferred host, some T. polycolpus find a home elsewhere and start parasitising other species of fish living in the same area. Even though T. polycolpus is considered to be a host generalist and can infect multiple species of fish, not all fish are considered equally habitable for this parasite and it does have a predilection for certain species over others. So what determines which other fish end up acquiring these parasitic copepods?

A group of scientists from France conducted a study looking at T. polycolpus population on freshwater fish in two French rivers, focusing on the 10 most abundant fish species in those rivers. Of the fish that they examined, eight of them were cyprinids (the family of fish that include dace, roach, and carp) while the two remaining species were the stone loach and brown trout.
Photo of parasitised dace with missing fin tissue from this paper

Only cyrpinids were found to be infected with T. polycolpus and of those only four species (dace, nase, gudgeon, minnows) were found to be consistently infected across both study sites. It turns out that next to the beaked dace, the second most preferred host for T. polycolpus is Parachondrostoma toxostoma, also known as South-west European Nase. After the beaked dace, it was the most commonly infected fish, especially in the Viaur river where there was generally higher abundance of the parasite.

It just so happens that out of all the fishes in those rivers, the nase is most similar to the dace in terms of its general body size, feeding style and habitat, making it the ideal second choice for T. polycolpus. On the flip side, it seems that minnow is the worst host for T. polycolpus - it hosted the least parasites out of the four fish species that were found with T. polycolpus and the parasites that were found on minnows were smaller and produced less eggs than those found on the other fish species. This is probably due to the minnow being a smaller fish than the beaked dace or the nase, so it does not produce as much mucus for T. polycolpus to graze on.

So even when generalist parasites do infect other hosts, they prefer some familiarity. The more similar you are (physiologically and/or ecologically) to the parasite's preferred host, the more likely that you will be next in line to get infected should the parasite's preferred host become too heavily parasitised.

But here's an added to layer to this story which you might want to consider - the South-west European nase is actually listed as a vulnerable species - its population has declined by at least 30% in the past 10 years due to habitat destruction and hybridisation with introduced species, so if the number of nase continues to decline, what does this mean for T. polycolpus? Would this result in increased parasite pressure on other fish species as they find themselves soaking up the "excess" T. polycolpus? Or will the the beaked dace experience even more exacerbated pathology as T. polycolpus are left with less alternative hosts to infect?

Reference:
Lootvoet, A., Blanchet, S., Gevrey, M., Buisson, L., Tudesque, L., & Loot, G. (2013). Patterns and processes of alternative host use in a generalist parasite: insights from a natural host–parasite interaction. Functional Ecology 27: 1403-1414