Ismaila belciki

Photo of infected Janolus fuscus
used with permission from Jeff Goddard

If you ever find yourself down by the sea, you may come across some very flamboyant sea slugs call nudibranchs. But beneath their colourful exterior, some of them are harbouring a dark secret in the form of a very strange looking parasite. These parasites live hidden inside the main body cavity of their molluscan host, so if you are unfamiliar with this particular critter, you might not even notice it. The main thing that gives away their presence are a pair of egg sacs poking out of the sea slug (see photo on the right). Those egg sacs belong to a parasite call Ismaila belciki - it is a crustacean, though it looks more like one of Cthulhu's lovechild or something out of Men In Black.

Ismaila and other copepods of the Splanchnotrophidae family are specialist parasites of sea slugs and they can get pretty big in comparison with their host, taking up substantial room and resources. Ismaila belciki infects Janolus fuscus, a nudibranch found along the west coast of North America from Alaska to California, as well as the shores of northern Japan. In some areas, such as Coos Bay, Oregon where the study we are featuring today took place, up to 80% of the slugs are infected with this odd creature. Having such a big parasite sitting in the middle of slug's body soaking up nutrient obviously carries some kind of cost - but just how much?

Photo of a female Ismaila belciki with an
embraced dwarf male front and centre.
Photo by and used with permission
from Maya Wolf

A pair of researchers from University of Oregon decided to find out just how costly this parasite is to its host. They compared the growth, survival, and reproductive capacity of infected and uninfected J. fuscus, and measured how much resources the parasite takes up.

While I. belciki did not seem to interfere with sea slug's growth, infected slugs do have a lower survival rate. Additionally, they have shrunken gonads that are only capable of producing about half as many eggs as healthy slugs. But the reproductive capacity of those afflicted sea slug suffers not just in terms of quantity, but in quality as well. In addition to producing fewer eggs, infected slugs also produced eggs that were smaller, and the baby slugs that hatch out of them also have lower survival rates.

So it seems I. belciki can be very harmful indeed, but it cause even greater harm if the parasite itself is breeding. The researchers noted that I. belciki bearing developing egg sacs exert a greater toll on the host than egg-free parasites. A female I. belciki is an egg-laying machine that can churn out over 88000 embryos per month and all the expenses for that are paid for by the host. To fuel the development of its eggs, I. belciki draws from the same pool of resources that the host normally use for its own egg production. Slugs with brooding I. belciki produce even fewer eggs than those that are "just" stuck with an egg-free parasite.

It is as if the sea slug is a factory that has been retooled from solely making slug babies into one which now has to divert some of its attention and raw material to making parasite babies too, via a proxy in the form of a female I. belciki. Given that Janolus fuscus usually only live for five months, by shorten their lives and severely reducing their reproductive capacity, I. belciki might actually be putting a natural check on the population growth of these flamboyant nudibranchs.

Reference:
Wolf, M., & Young, C. M. (2014). Impacts of an endoparasitic copepod, Ismaila belciki, on the reproduction, growth and survivorship of its nudibranch host, Janolus fuscus. International Journal for Parasitology 44: 391-401.

P.S. I will be attending the annual Australian Society for Parasitology annual conference in Canberra, Australia between 30th June to 3rd July. So watch for tweets about highlights from conference at my Twitter @The_Episiarch! Meanwhile, I have written a article for The Conversation about the crab-castrating barnacle Sacculina carcini - you can read it here.

Anilocra nemipteri

Photo from Figure 1 of the paper

The parasite in the study being featured today makes a living riding around on the top of a fish's head and occasionally gnawing on its face. It is in the same family as the infamous tongue-biter, the Cymothoidae, though technically this one is more of a face-hugger.

Anilocra nemipteri is found on the Great Barrier Reef of Australia and it makes a living by hitching a ride (and feeds) from the bridled monocle bream, Scolopsis bilineata. It is a pretty common parasite - in some areas, up to 30 percent of monocle bream carry one of these crustaceans on their head like a nasty blood-sucking beret that stay attached for years.

As you can see from the photo, A. nemipteri is a fairly big parasites comparing with the size of the fish (in some case they can reach as almost one-third the length of the host fish!), and having a parasite of that size hanging off your face is going to be quite a drag - literally. That is bad news for a little fish like the monocle bream that needs to make a quick getaway from any hungry predators on the reef. So just how much of a drag is A. nemipteri? A related species - Anilocra apogonae - which clings to the cardinal fish (Cheilodipterus quinquelineatus) is known to cause their host to swim slower and have lower endurance. Does the same apply for A. nemipteri and the monocle bream?

To find out, scientists compared how quickly the fish can respond to an attack and their Flight Initiation Distance (FID) in both a laboratory setting and in the field. The FID is the distance from a predator at which an animal decides to flee - risk-takers have a shorter FID. They divided the monocle bream into three different groups: parasite-free fish, fish carrying an A. nemipteri, infected fish which just had their parasite removed.

Photo from Figure 1 of the paper

The research team simulated an attack by a bird (with a weighted PVC pipe) on fish in specially-designed experimental tanks and filmed the response to measure the fish's reaction time to the attack. Even though one would think all that face-gnawing from A. nemipteri would have weakened their host, and not to mention the body of the parasite itself causing significant drag, the escape performance of parasitised fish was not all that different from unparasitised - they reacted and got away from the attack just as quickly as their unburdened buddies. In the field experiment, the scientists donned snorkelling gear and tried to approach any monocle breams they spotted and measured how close they could get to the fish before they fled. There, they found parasitised fish have a slightly shorter FID than parasite-free fish, but not significantly so.

Fish that are infected by A. nemipteri are smaller than uninfected ones, and it just so happen that smaller fish tend to allow predators to get closer to them before fleeing. But whether this is due to the parasite is another matter. Are parasitised fish smaller because their growth have been stunt by A. nemipteri? Or does this face-hugger simply prefer smaller fish because larger and older fish might have built up an immunity to it?

Though it may seem less exciting when we find a parasite doesn't cause much behavioural changes in its host, it is vital to our understanding of host-parasite relationships. Perhaps it means the host is able to compensate for the presence of the parasite. Also it is not clear what the long term cost of having A. nemipteri might be over the life time of the fish. It is also important to treat such a case in its context. Unlike other parasite which have a complex life-cycle and depend upon its host getting eaten by a predator to reach maturity, A. nemipteri is an external parasite that simply sticks to a host and stay for life - if the parasitised fish is eaten by a predator, it'll go down with the host like a bit of garnish and be digested too.

So it is probably just as well that A. nemipteri is not too much of a drag to have around.

Reference:
Binning, S. A., Barnes, J. I., Davies, J. N., Backwell, P. R., Keogh, J. S., & Roche, D. G. (2014). Ectoparasites modify escape behaviour, but not performance, in a coral reef fish. Animal Behaviour 93: 1-7.

Loxothylacus panopei

Photo by Inken Kruse via the Hare Lab

Some parasites can manipulate their host's behaviour in very spectacular ways, but there are also other parasites that change their host's habits in more subtle manners. While such alteration to the host can seem fairly minor, they can still result in some very profound impact on the rest of the ecosystem.

There is a group of parasitic barnacles call Rhizocephala (the most well-known species is Sacculina carcini) that are capable of castrating their host, turning them into unwitting babysitters that nurture the parasites' brood. The infected crab display some very obvious changes to their behaviour, and in some cases, their appearance. But the study we are featuring today shows that apart from turning them into doting mothers for the parasite's babies, these barnacles can also alter the crab's behaviour in less obvious ways that have ramifications for other marine inhabitants.

The flatback mud crab (Eurypanopeus depressus) lives in estuaries on the coast of South Carolina and it is infected by a species of rhizocephalan call Loxothylacus panopei. In addition to doing the usual host castrating and commandeering trick, L. panopei also changes how this crab responds to potential prey. Usually, the mud crab has an omnivorous diet, dining on algae as well as worms, smaller crustaceans, and sponges. Sometimes they may also have a crack at more armoured prey like mussels. But crabs that are infected with L. panopei lose their appetite for such shell-covered fares.

When researchers offered uninfected crabs with piles of mussels, the crabs acted like they were at an all-you-can-eat seafood buffet and ate as much as they can - the more mussels the researchers presented them with, the more they ate. But no matter how many mussels they offered to crabs that were infected with L. panopei, they simply eat one and call it a day. The parasitised crabs also took longer to get their act together and this seems to be related to the size of the crab's parasite - the larger the parasite has grown, the longer the crab takes to start digging into a mussel.

Based on a field survey of the estuary where the study took place, the researcher concluded that about a fifth of the crab at that location were infected with L. panopei. Given the effects that L. panopei has on their crab's appetite for shellfish, it seems that the mussels might have an unlikely ally in the form a parasitic barnacle. The finding of this study share some parallel to another paper that we featured on this blog earlier this year, on the muscle-wasting parasite that infects a predatory shrimp and curb its otherwise ravenous appetite.

Ecosystems are made up of complicated networks of biological interactions and parasites can mediate predator-prey interactions in different, and sometimes conflicting ways. While some parasites can make prey animals more vulnerable or accessible to predators, there are other like L. panopei that may be reducing the appetite of the said predators. The subtle interplay of such parasite-mediated interactions are often overlooked or ignored, but their effects on the ecosystem are certainly there if you know what to look for.

Reference:
Toscano, B. J., Newsome, B., & Griffen, B. D. (2014). Parasite modification of predator functional response. Oecologia 175: 345-352.

Nematocenator marisprofundi

Parasitism is the most common mode of life on Earth and it can found everywhere, in all kinds of environments. Even in extreme places such deep sea hydrothermal vents, amidst hellish geysers pouring out hot sulfide or seeping methane, parasitism carries on as usual - the players may change, but the game stays the same. While on this blog most of the nematodes we have featured are the parasites, in this particular case, they play the role of the host.

SEM photo of D. marci from the paper

Laying about 85 kilometres off the coast of Oregon, under about half a mile (800 metres) of water is the Hydrate Ridge methane seeps. These vents are covered in mats of sulfide oxidizing bacteria which are crawling with worms - mostly nematodes from the genus Desmodora. One of these species happens to be a host to the parasite we are featuring today - Nematocenator marsiprofundi - which translates into "nematode eater of the deep sea". It is a microsporidian - a group of single-cell parasite somewhat related to fungi, and taxonomically speaking, N. marsiprofundi lies right near the base of the split between these two groups.

Microsporidians are found in a wide range of animals including vertebrates such as fish and reptiles, as well as invertebrates such as insect, crustaceans, and nematode worms. The host of N. marsiprofundi, a nematode named Desmodora marci (see above), is one of the more abundant animal at methane seeps. There can be as many as twenty worms for each millilitre of carbonate rocks from such locales, and over half of those worms would be infected with N. marsiprofundi. This parasite seems to be common at such vents and were found at sites which are 15 kilometres, so N. marsiprofundi is not localised to just a particular location and/or worm population.

Spores of N. marisprofundi
(image from the paper)

The spores of this parasite (see left) are mostly found in the worm's reproductive tract; in female worms, the spores sit in the uterus next to the eggs, and in the male, the spores lined the worm's sperm duct and cloaca. This led the researchers who found this parasite to suggest that N. marisprofundi is sexually transmitted between its host. However, they also noticed some stages of the parasite were situated in the body wall, where they seem to degrade and digest the worm's muscle tissue, not unlike the microsporidian we featured a two months ago which infects an amphipod that has become invasive in Central Europe.

While their presence in the body wall and the effects they have on their host's muscle indicates they can be quite harmful and may transmit through means other than the worms' sexual activities, that is not to say that this parasite might not exploit multiple mode of transmission. Some parasite change their shape and infect different host tissue at different stages of their lives, and it is possible that N. marisprofundi can both be sexually transmitted and also eventually kill their host to allow their spores to disperse from a rotting cadaver

Studies like this shows parasites might be more common in the deep sea that we might have previously suspected, and that even in seemingly extreme environments like hydrothermal vents, there is good living to had as a parasite. Parasitism is everywhere on this planet, and while many people may think parasites are odd freaks of nature, in reality they are just a normal part of life on Earth.

Reference:
Sapir, A., Dillman, A. R., Connon, S. A., Grupe, B. M., Ingels, J., Mundo-Ocampo, M., Levin, L. A., Baldwin, J. G., Orphan, V. J. & Sternberg, P. W. (2014). Microsporidia-nematode associations in methane seeps reveal basal fungal parasitism in the deep sea. Frontiers in Microbiology 5: 43.

Controrchis sp.

Extreme weather events can cause significant changes to ecosystems and their inhabitants. When Hurricane Iris made landfall at Belize, it caused widespread devastation in its wake. The study we are featuring today was a part of a larger project to look at how Hurricane Iris affected a population of black howler monkeys (Alouatta pigra) which has been monitored since 1998. In the aftermath of the hurricane, the number of monkeys in the forest decreased by 78 percent until their population began to stablise and increase three and a half years later. But aside from such outwardly visible impacts, there were also other changes afoot within the monkeys themselves.

Photo of black howler monkey by Ian Morton

A team of scientists interested in monitoring the recovery carried out a study to see how this has affected the monkeys' parasites. It is possible that these primates are harbouring higher parasite loads than they did before the hurricane due to the stress of living in a disturbed habitat. The scientists collected samples of monkey fece over the course of 3 years and look for parasite eggs. They also measured the level of cortisol, a hormone associated with stress, present in the fecal sample, and collected data on other aspects of the monkey's behaviour to see if they were associated with their parasite burden.

Photo of Controrchis eggs
from here

The black howler monkeys were found to have five species of roundworms and a species of digenean fluke (based on the presence of their eggs in the monkey poop), but the prevalence and abundance of those parasites were not associated with the level of stress hormones. Instead, nematode (roundworms) prevalence was heavily dependent on population density and the size of the groups in which the monkeys gathered. This is to be expected as these worms are transmitted via accidental ingestion of eggs or larvae from the host's feces. The more monkeys there are around in a given area, the more opportunities for these particular parasites to be passed on. This is similar to what has previously found in other studies on primate parasites. But the only factor that successfully predicted the occurrence of the digenean trematode fluke Controrchis was the amount of leaves the monkeys consumed.

While black howler monkeys usually prefer a diet filled with fruit, in the aftermath of Hurricane Iris there were no fruit-bearing plants in the forest for 18 months. So the monkeys were forced to go on a leaf-based diet instead of the fruit-based one they enjoyed before the hurricane, and the plant most readily available and palatable to the monkeys was Cecropia. These fast-growing leafy plants usually happens to be the first on the scene in the wake of such habitat disturbances. They do not contain as much fibre as other plants and have little in the way of noxious defensive chemicals - which makes Cecropia excellent fodder for the black howler monkeys. Cecropia also contains a lot of what these monkeys need in a balanced diet, so in the absence of fruits, the howler monkeys munched readily on these nutritious greens

But why is the consumption of Cecropia associated with the prevalence of Controrchis? The fluke does not use leaves and vegetation as a mean of transmission (unlike Fasciola the liver fluke), instead, Controrchis uses ants as a go-between to get in their vertebrate host. But these monkeys don't really have a taste for ants, so why is Controrchis prevalence linked to the amount of leaves they have consumed? That is because Cecropia also happens to be myrmecophtyes, or ant-plants. Monkeys that are chowing down those leafy greens are also inadvertently swallowing a lot of ants, which means taking onboard a lot of Controrchis waiting to make a connection with a suitable monkey host.

For another more detailed take on this paper, from the lead author herself, see this post here. 

Reference:
Behie, A. M., Kutz, S., & Pavelka, M. S. (2014). Cascading Effects of Climate Change: Do Hurricane‐damaged Forests Increase Risk of Exposure to Parasites?. Biotropica 46: 25-31.

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

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

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.

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.

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. potomaca, C. 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.