"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

March 8, 2018

Gyrinicola batrachiensis

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

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

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

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

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

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

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

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

February 12, 2018

Neocyamus physeteris

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

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

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

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

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

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

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

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

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

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

This post was written by Sean O’Callaghan

February 1, 2018

Glyptapanteles sp.

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

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

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

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

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

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

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

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

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

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

This post was written by Niamh Dalton

January 11, 2018

Riggia puyensis

It is no secret that I am a big fan of parasitic isopods, especially those in the Cymothoidae family - the most well-known of which is the tongue biter parasite, and my love for these adorable crustaceans has even manifest itself in some of my artwork. But while the tongue-biters are no doubt the most (in)famous representatives of that family, to the extent that they even made an appearance on an episode of the Colbert Report, it is their less well-known cousins - the belly-dwellers/burrowers - that turn the horror factor up a notch (or four, or eleven) and as a result, really earned my adoration.

Left: Adult female Riggia puyensis (scale bar = 10 mm), Right: Adult make Riggia puyensis (scale bar = 1 mm)
From Fig. 3 and Fig. 9 of the paper

Imagine if the chest-burster xenomorph from Aliens didn't just explode through your ribcage and leave you for dead - instead, it stays inside your torso for the rest of your life, laying a steady stream of eggs that trickle out through a small(ish) hole in you belly. That's how these belly-dwelling isopod live their lives. So let's kick off the year with a recently described species of these belly-dwellers!

I've previously written a post about a species of belly-dweller call Artysone trysibia which lives in the body cavity of an armoured catfish from the Amazon. This post features Riggia puyensis, which is quite similar to A. trysibia in that it was also found to be parasitising armoured catfish, specifically two species from the Bobonaza River and Puyo River in central Ecuador - Chaetostoma breve and Chaetostoma microps - both of which are better known as suckermouth armoured catfish.

Most of the R. puyensis specimens that the scientists found in this study were females, but the scientists did come across three male specimens which were clinging to the limbs of the female isopods. These male isopods are comparatively tiny reaching only one-tenth the length of the adult female R. puyenesis. The small size and relative rarity of males is par for the course for Riggia. In other studies on this genus of parasite, male isopods are rarely found, if at all. It is possible that this is because the mating strategy of the male isopod is to scoot in, mate with the larger female, then go off and find another infected host.

Riggia puyensis inside its host, from Fig. 2 of the paper
In this study, each infected fish was only parasitised by a single female isopod - which is probably just as well since R. puyensis is quite large in relation to the host. The female R. puyensis reaches over an inch in length and considering one of the host catfish is a species that grows to about four inches long at most, that parasite is a hefty load to be carrying around. It would be like having a corgi living inside you.

So it may seem rather surprising that the survival of these fish does not seem to be compromised by the parasite. In fact, a previous study have shown that the parasite may in fact enhance the infected fish's growth. But this parasite-induced growth spurt comes at a price - after all, there is no free lunch in nature and for the gain in body growth, the parasite incurs a severe penalty on the fish's reproductive functions. A study on bonefish parasitised by Riggia paranensis found that infected fish has reduced level of sex hormones and undeveloped gonads.

So Riggia render its fish host impotent in order to free up more resources for body growth, and a bigger host means more for the parasite to consume. So while a chest-bursting xenomorph invokes a more immediate visceral reaction, the way that R. puyensis and other parasitic castrators modify their hosts' body to fuel their own reproduction presents a more existential form of lingering horror.

Haro, C. R., Montes, M. M., Marcotegui, P., & Martorelli, S. R. (2017). Riggia puyensis n. sp.(Isopoda: Cymothoidae) parasitizing Chaetostoma breve and Chaetostoma microps (Siluriformes: Loricariidae) from Ecuador. Acta Tropica 166: 328-335.