Zombifying fungi, Hitch-hiking parasites, and making the most out of your hosts

It's been another year and as usual there were many interesting studies on various parasites that were published in peer-reviewed journals - far more than what I ended up writing about for the blog. While papers about parasites are usually published in Parasitology journals - as one would expect - because parasitism is a life style rather than a taxonomic group, there were also many studies that were published in various evolutionary, ecological, and multi-disciplinary journals.

So I've tried to browse widely to find papers which would make for an interesting story and can be written up in a reasonable timeframe. So what are some of the highlights from 2017?

Of the papers that I did manage to write up, some of them were on fungi that infect and zombify insects and other terrestrial arthropod, there are ants, beetles, even millipedes that have fallen under their spells - admittedly, I do have a soft spot for those fungi, so in a way I have fallen for them too.

And the fungi did not have a monopoly on the insect killing business - this year, the blog featured two separate studies on parasitic nematodes that turn an insect's innards into a soupy baby food for the next generation of killer worms. They have many ways of doing so - the main way is with help of a bacterial ally, but some species also have an arsenal of toxins.

But microbes are not the only allies that are enlisted by parasites, one post featured a flea that hitch a ride on earwigs to reach bats. Being able to arrive at a new host is a vital part of any parasite's life-cycle, and while that bat flea uses an earwig to get there, there are many other ways to accomplish that end. This year there were blog posts on two turtle parasites - a copepod and a blood fluke - which have evolved very different ways of reaching their marine reptile hosts amidst the oceanic expanse.

While the size of those parasites are minuscule compared to their rather large host, other parasites can reach alarmingly large sizes in proportion to their host. Some parasite take up so much space that they represent a major drain on their host's resources, and become parasitic castrators. The rhizocephalans is one such example and when it comes to body-snatching, these parasitic barnacles give the insect-zombifying fungi a run for their conidia. There's a good reason for being so imposing upon their host, as the more space they take up, the more eggs they can produce.

Those barnacles have a network of tendrils that can squeeze through the nooks and crannies of the host's body, but if the host doesn't provide you with a space, then you have to make your own, as with the case of a parasitic snail that lives in the spines of a sea urchin. Often, getting through life as a parasite is all about making the most out of the living condition that you've been dealt with. Whether you happen to be parasitic plant that spends your whole life underground except when it comes to flowering, or flukes living in the brain of some endangered fishes, or a seal parasite that has found itself living in the gut of a penguin.

Amidst the zombifying fungi and body-snatching barnacles, it is important to remember that not all parasites are nearly so deadly or harmful to their host. In fact, one of the post featured a downright benign parasite - a fungus that live as an external hyperparasite on bat flies, which are themselves parasites of bats. There was even a post featuring a parasite that live in the gut of cat fleas and helps it reach maturity more quickly to start producing more baby fleas - after all, more fleas means more hosts for that parasite.

Both of those parasites happen to be parasitic on ectoparasitic blood-sucking insect - so it looks like those hyperparasite are showing those insect killers mentioned earlier in this post that there is more than one way to make the most of your host

So that does it for 2017! As I hinted at the start of the post, there is only so many papers I can possibly cover in one year - let's hope there's more to come next year so I can continue to bring you more parasite stories! Meanwhile, I often tweet about the parasitology (and other) papers that I didn't get to write up as a full post at @The_Episiarch - so you can go there to see more.

In August, I was also interviewed for the In Situ Science podcast where I talk about parasites (for a bit anyway, we ended up talking about sciart, social media, and many other things), and of course, those who follow my work online for long enough (especially on Twitter) would also know that in addition to science, I also do art, and sometimes my science intersects with my art to create... Parasite Monster Girls? Since I do plan on continuing to draw Parasite Monster Girls in 2018, I guess in addition to blog posts about parasites, that's another form of parasitological content that you can look forward to seeing from me in the new year...

See you in 2018!

Megadenus atrae

A few months ago, I wrote about a snail that forms galls in the spines of sea urchins, and while most people might not think of snails as parasites - let alone parasites that live on animals like sea urchins, sea stars, and sea cucumbers - the parasite-host relationship of snails and echinoderms actually goes back hundreds of millions of years. There are fossils of snail boreholes and galls on ancient echinoderms. In fact, they are probably one of the few examples of parasitism that leaves a clear trace in the fossil record. If a sea cucumber is to write a parasitology textbook, most of it would be devoted to snails.

(1) A pair of Megadenus atrae - female on the left, male on the right; (2) Drawing of a M, atrae showing the proboscis (pr) and the pseudopallium (pp) cut away to show the shell (sh); (3) The shell of M. atrae - the larger ones are the female snail
Photos from Fig. 1 of the paper

Most of these parasites are from a family of snails call Eulimidae and the study that this blog post is covering was focused on a species call Megadenus atrae.  This parasitic snail has a few peculiar features when compared with the kind of snails that most people would be more familiar with. The shell is mostly wrapped up in a fleshy hood call the pseudopallium with only the tip visible, and it also has a giant sucker-like proboscis which it uses to cling to its host.

While other parasitic snails may simply attach to the skin or reside in the spines of their echinoderm hosts, this snails hangs out at a very specific spot - M. atrae lives in the cloaca of Holothuria atra - the black sea cucumber.

As strange as it may seem to us land-lubbers, the sea cucumber's butt is a popular hangout or gateway for many animals. There's the pearlfish which inserts its slim body into the sea cucumber through the echinoderm's cloaca and uses it as a kind of living shelter (some species also nibble on the sea cucumber's gonads while it is in there). There are also various crustaceans that are perfectly at home in a sea cucumber's butt. It is at this prime piece of real estate that M. atrae spends its adult life

In this study researchers collected black sea cucumbers from the chain of islands known as the Nansei Islands which stretches from the southern tip of Japan to the north eastern part of Taiwan, and recorded the presence of this parasitic snail. The snail is not particularly abundant, it was only found at two of the seven island sites they sampled from, and even on a reef flat at Kuroshima where they were most common, it was only found in one out of every ten sea cucumbers. Megadenus atrae has also been reported from other parts of the world including New Caledonia, India, and Australia. And in those other studies, the prevalence of this snail range from one in ten sea cucumbers to as few as one in a thousand.

Given that this parasitic snail is sparsely distributed in the sea cucumber population, this presents some challenges when it comes to reproduction - the likelihood of a larval snail encountering a host which is already occupied by another M. atrae is low enough, but the chance of that snail being of the compatible sex is even lower. Unlike other symbionts like pea crabs which can leave their host for a booty call, the only mobile stage of M. atrae is when it is a free-drifting immature larva. Once they are in a sea cucumber's butt, they are there for life

While it is possible that the snail can send out some kind of pheromone to recruit other M. atrae to settle in their host, how can they guarantee the new arrival would be of the suitable sex? After all there's no dating apps for snails living in a sea cucumber's butt.

Despite such obstacles, the researchers noticed that these snails were always found in pairs, and always as a female-male pair. They suggested that that this parasitic snail might have a sex determination system which is similar to that of the tongue-biter parasite and a range of other animals call protandry. With a protandric system, the larva starts out life as an immature male. If it settles down alone, it grows into a mature female snail. But if the snail larva happens to settle in a sea cucumber which is already occupied by a mature female, it will grow into a mature male. That way, M. atrae ensures that it will end up with a suitable reproductive partner no matter the circumstance.

So life finds a way, even for a parasitic snail trying to find a life partner amidst a sea of unlikely butts

Reference:
Takano, T., Warén, A., & Kano, Y. (2017). Megadenus atrae n. sp., an endoparasitic eulimid gastropod (Mollusca) from the black sea cucumber Holothuria atra Jaeger (Aspidochirotida: Holothuriidae) in the Indo-West Pacific. Systematic Parasitology 94: 699-709.