A Parasite, Clickbait and Honesty
Virtue from a Flatworm Living Inside Bird Guts and Snail Eyes
Note from Bryan: Greetings to the folks who’ve arrived here by way of my conversation with Erica Heilman for her Rumble Strip podcast. So here below is my journey in strange ecology and due respect for a parasite, a snail, and perhaps the human capacity for discovery and wonder in nature. (For the Vermonters among us, I’ve included a note in postscripts to this essay.) Onward.
IN THE battle waged online for your attention, it would have been easy for me in this essay to go with “zombie snails.” Or maybe “brain-hacked suicide snails.” After all, those are common tropes in internet posts about the flatworm unceremoniously named Green-banded Broodsac (Leucochloridium paradoxum), which makes a snail’s eyes resemble an insect (or in internet parlance a “bizarre pulsing disco show”).
For clicks and likes, this kind of baiting might have been expected of me, perhaps even excusable. But it would have been a disservice to the snail, to the parasite, and certainly to you. We’re better than that. (We are, right?)
So instead I bring you a genuine story about how I spent my summer vacation with a parasite that lives inside a bird’s gut and a snail’s body. It's a tale of how I discovered the flatworm in my own community—Montpelier, Vermont—and soon became obsessed with learning about it. It is sincerity from the earth. Because if the world needs anything right now it is rectitude—and not exaggeration, lies, and clickbait.
A Most Vivid and Pulsing Parasite
The first thing to know about this flattened, tiny worm—a trematode—is that it does not live free on its own in the outdoors. The flatworm spends the entirety of its adult life inside a bird’s intestine or cloaca, which is where waste and either sperm or eggs are passed out of a bird’s body.
The only way and form in which the trematode gets outside is when it produces eggs, which are then delivered to the free world by the bird in its droppings. Yep, the adult flatworm lives in bird guts, its eggs live in shit, and it matures inside a snail. (Think about that next time you’re stuck in the office.)
The vast majority of those eggs, I suspect, dry up and perish wherever bird droppings land—end of the line for a flatworm. Except when an amber snail in the family Succineidae crawls along and … well … eats shit. It’s a way of life for this parasite: only within a snail can the trematode continue its lifecycle. The snail is the parasite’s essential intermediate host. And this is when things get interesting.
Once inside a snail’s gut, the flatworm’s eggs hatch to release elongated, hairy larvae (called miracidia), which are now free to move about the snail’s body. There the hatchlings continue to develop, but now within a new accessory: a colorful, sausage-shaped chamber called a broodsac. The broodsac acts like an ornate, pulsing, living entity in its own right (as in the video above and the images below). Its destiny is to get itself and its trematode larvae back into a bird’s gut.
The snail is the ride. Except that songbirds generally don’t eat snails. They prefer fruit, seeds, and insects. But the broodsac, banded and flecked with pigments, resembles a moth or butterfly caterpillar (and to some extent a sawfly larva)—good eating for a bird. So the broodsac duly makes its way to the snail’s head and then into the eyestalks, wherein it pulses in an odd and wonderful spectacle that presumably attracts the attention of birds (and the internet). It’s quite a show.
Once a bird takes the bait, the broodsac’s larvae go on to mature into adult flatworms in its intestine or cloaca, ultimately producing trematode eggs destined for bird droppings—a lifecycle complete. All good … except for how it plays online.
The Flatworm, the Internet and Us
The broodsac certainly got my attention, pulsing in my direction as I botanized during a walk with my pup Odin in a park in my home city of Montpelier, Vermont. It was at 7:58 AM on 9 July 2024. My life hasn’t been the same since.
In the days after my discovery I searched my city for more occupied snails—without success. Until my initial encounter, the parasite had only been reported (to GBIF and iNaturalist) and positively identified 11 times in all of North America. (It’s more common and probably native in Europe and Asia.) I nonetheless kept searching. Four days later came my triumph: a population of amber snails only a short distance from my home. I now had a study site (although I admit this was more entertainment than study for me).
I visited with the snails regularly, brought some home for observation, took lots of pictures and video. I felt kinship with Carl Jørgen Wesenberg-Lund (1867-1955), a Danish zoologist who studied the parasite and the snail, publishing an exhaustive 69-page treatise (Wesenberg-Lund 1931). (Like most of us, Carl could have used an editor.) All the while, I have to say, I was dazzled by an organism in my city that only days earlier I had never known existed. Nature’s like that—full of opportunity, even among parasites.
Rather than recounting the details of my investigations, I’ll take issue with internet distortions of the parasite. First, it’s easy (and somewhat gratuitous on my part) to nitpick the zombie metaphor. In fact, these snails are not soulless, walking crawling corpses whose bodies have been overtaken by some sinister spirit or force (flesh-eating or otherwise). Admittedly, anything zombie plays well in the zeitgeist these days. (I myself even deployed it leading up to this post and elsewhere—shame on me.)
Next, the parasite supposedly manipulates the snail’s behavior, forcing it to crawl up and out into the open on plants leaves where it can be more easily spotted and preyed upon by songbirds. It’s often described online as the parasite’s exerting “mind control” over the snail—or that the poor snail is “hypnotized” and “doomed to follow the parasite’s will.”
Be wary of this kind of language online relating to nature. I’m not sure how to read a snail’s mind—or that anyone in fact has. (I wish I could.) Nor does anyone, I presume, know that a parasite larvae is capable of exercising such willful mind-control. We need not confer sinister intent on the part of the parasite or victimization on the part of the snail. That’s anthropomorphizing. To be sure, there’s a role for metaphor in explanatory science and nature writing. But the pitched battle for attention online too often sends science writing into speculation and excessive drama.
Do the occupied snails move higher and to more exposed spots? Absolutely. At dawn on dewy summer mornings at my study site, lots of amber snails would routinely ascend from the soil—usually less than a meter or so—to sit on plant stems and leaves. But the parasitized snails among them climbed higher than others and were plainly more visible to me (presumably to birds as well). I had noticed this even before reading about it in published scientific literature (or online for that matter). And it’s been well documented by researchers working in Poland (Wesołowska 2013).
Mind control? Maybe. Or maybe those snails, their eyestalks occupied and blocked by pulsing broodsacs, simply went higher in order to better see or sense the dawn light. Nothing sinister about it. We could even speculate that it’s the snails running the show here, climbing higher and exposing themselves so that birds will come along and pluck those broodsacs from their eyestalks, which the snails reportedly can then regenerate. To be sure, there are well-known examples of parasites manipulating animal behavior. We just need to be cautious on the mechanisms.
I’m no parasitologist. No matter. Whatever the case, to my mind this is a parasite being a parasite, a snail being a snail, a bird being a bird—intrinsically worthy without dramatization on our part. In the intersecting lives of the flatworm, the snail, and the bird, I need no ominous music or manipulative prose. In nature, all I need is exuberance, curiosity, and most of all an open mind.
Postscript: A Note For Vermonters
Yet another reason Montpelier, Vermont, is the best city on Planet Earth: We seem to be the North American hotspot for these flatworms. And that is an entirely unscientific assertion owing to sampling bias on my part. I alone skew the number of sightings of these flatworms in favor of Montpelier (my home) and Vermont (or disfavor depending on your point of view).
But I will point that out that even before I put more of these pulsing broodsacs on the iNaturalist map for North America, folks across Vermont had been seeing and reporting them as well. After all, we’re tuned to nature, even its oddities, here in the Green Mountain State, perhaps more so than any other group of citizen scientists in the nation. (One reason is the Vermont Center for Ecostudies.)
So let’s keep it up, Vermont. As we walk our trails and streets by morning, especially on dewy mornings, search the shrubby vegetation for amber snails. They’re now out in force before the sunshine hits the leaves (then they’re gone into hiding for the day). I tend to find them in bunches—lots of snails sitting around within a few feet of the ground. Search among those for any snails with pulsing broodsacs. Then post your images to iNaturalist or notify me of your find in comments below. Who knows? Maybe Vermont will become an international destination for flatworm watching (you know, like birdwatching or brewery tours). Uh, well, okay … maybe not.
A New Essay from Vermont and Beyond
Floods and Smoke
The first of three floods laid waste to my city. From my canoe on Montpelier’s downtown streets, I could see that nearly every business, our fire station, our library, and city hall offices were wrecked.
Post-Postscripts, References and Clickbait
Summertime is the season for these snails and parasites. Here’s an iNaturalist map of Research Grade records of Green-banded Broodsac around the world, mostly in Europe.
The broodsac’s behavior goes by the term “aggressive mimicry”—an organism resembling another either to lure in prey or, in this case, to itself become the prey.
I’m not sure which snail species is involved here. The trematode is often reported in the species Succinea putris (Common European Amber Snail). The abundant North American amber snail species is Novisuccinea ovalis (Oval Amber Snail).
Casey, S., Bakke, T., Harris, P. et al. 2003. Use of ITS rDNA for discrimination of European green- and brown-banded sporocysts within the genus Leucochloridium Carus, 1835 (Digenea: Leucochloriidae). Syst Parasitol 56, 163–168 (2003). doi.org/10.1023/B:SYPA.0000003809.15982.ca
Doherty JF. 2020. When fiction becomes fact: exaggerating host manipulation by parasites. Proc Biol Sci. 2020 Oct 14;287(1936):20201081. doi: 10.1098/rspb.2020.1081. Epub 2020 Oct 14. PMID: 33049168; PMCID: PMC7657867.
Nandy, G., et al. 2022. Infestation of sporocysts of parasite Leucochloridium in the snails Succinea daucina and Indosuccinea semiserica. J. Environ. Biol., 43, 722-726 (2022).
Robinson, EJ Jr. 1947. Notes on the Life History of Leucochloridium fuscostriatum n. sp. provis. (Trematoda: Brachylaemidae). The Journal of Parasitology, Dec., 1947, Vol. 33, No. 6, Section 1 (Dec., 1947), pp. 467-475 Downloaded from 132.198.50.13 on Sun, 25 Aug 2024 14:15:32 UTC
Usmanova, RR, et al. 2023. Genotypic and morphological diversity of trematodes Leucochloridium paradoxum. Parasitol Res. 2023 Apr;122(4):997-1007. doi: 10.1007/s00436-023-07805-7. Epub 2023 Mar 6. PMID: 36872373.
Wesenberg Lund, C. 1931. Contributions to the development of the trematoda Digenea. Part I. The biology of Leucochloridium paradoxum. K. danske Vidensk. Selsk. Skr., nat. mat. Afd. 9: 89–142.
Wesołowska, W. and Wesołowski, T. 2013. Do Leucochloridium sporocysts manipulate the behaviour of their snail hosts? Journal of Zoology. 292. 10.1111/jzo.12094.
(All of this work informed this post, although not all of it is cited above.)
If I subscribe, does that make you my host?
This was so fascinating! Not something I ever would have thought to investigate, but I'm so glad I know a writer who does.
And something I thought of regarding the clickbait. When you've built enough trust with your readers, you don't need clickbait titles. We're going to read your essay because we know we'll like it.