Snails Hitch a Ride in Duck Guts

A mallard and ducklings. Photographed at Cape May, New Jersey by the author.

I hate flying. From the moment I arrive at the airport to when I finally step off the plane, the experience ultimately makes me feel like a zombie – shuffling, incoherent, and grimy. If there were a King Kong travel package – in which I could be sedated, placed in a crate, and revived at my destination without having to be awake for the discomfort in between – I’d take it. But I suppose there are slightly worse ways to travel. At least I don’t have to spend hours nestled in the guts of a duck.

How do species disperse from one place to another? Naturalists have been mulling over the question for over a century and a half. Charles Darwin even devoted a significant amount of time to the subject in his famously-controversial 1859 book On the Origin of Species. How large creatures moved from place to place – or didn’t – was easy enough to understand. The dynamic nature of the planet made sea levels rise and drop, corridors open and close, and otherwise acted to allow the travels of some organisms while restricting others.

But not all dispersals are intentional. Organisms can be transported from place to place by accident, Darwin pointed out, and he demonstrated this with some seeds and saltwater. Even though naturalists assumed that plants could not disperse very far, Darwin wondered if some seeds might be able to survive short trips at sea between one landmass and another. To investigate this notion, Darwin tracked the progress of many seeds he placed in saltwater over several weeks. Not all the seeds Darwin tried floated, but some did, and some even germinated after their seawater bath. After his homegrown experiments, Darwin wrote “[W]e may conclude that the seeds of 14/100 plants of any country might be floated by sea-currents during 28 days, and would retain their power of germination.”

And this wasn’t the only method by which seeds might be transported. “Living birds can hardly fail to be highly effective agents in the transportation of seeds,” Darwin noted, and avians even had more than one method of doing so. The dirt and mud which cake bird feet undoubtedly contain small seeds which can be carried to distant locations, and Darwin also suspected that some seeds might remain fertile after passing through the digestive systems of birds. Another backyard experiment provided the necessary evidence. “In the course of two months,” Darwin explained, “I picked up in my garden 12 kinds of seeds, out of the excrement of small birds, and these seemed perfect, and some of them, which I tried, germinated.”

Their relatively gentle digestive processes, combined with their ability to fly, made birds excellent unwitting plant couriers. But it’s not just plants that have been transported this way. In a paper published this week in PLoS One, Netherlands Institute of Ecology biologist Casper van Leeuwen and co-authors demonstrated that small snails might catch a lift to new locales inside mallards.

Though not as flashy as rainforests or coral reefs, wetlands are centers of high biomass and biodiversity. In the case of isolated and relatively short-lived wetlands, however, naturalists have wondered how diverse assemblages of animals have come together in such ephemeral settings. One possibility is that wetlands are colonized by far-ranging species from existing pools of biodiversity elsewhere – wetland species can either travel to new habitats themselves, or be transported by wind, water, and even animals. Aquatic birds, in particular, would seem to be creatures which can unintentionally bring some hitchhikers to new digs.

Organisms might hang on to an outside of a bird’s body, and dormant life stages of some aquatic invertebrates have been found in waterbird droppings, but, according to van Leeuwen and co-authors, we don’t know very much about whether “fully functional” organisms can survive similar trips. To investigate this mystery, the researchers fed snails to mallards.

The scientists selected four snail species found in the Netherlands. There was a common native species (Bithynia leachii), a marine species (Hydrobia ulvae), a flat-shelled species (Bathyomphalus contortus), and a widespread invasive species (Potamopyrgus antipodarum). After they collected and measured the snails, the scientists surrounded the gastropods with dough to make little snail pellets which ensured the experimental ducks would swallow the invertebrates.

After feeding, the scientists just had to wait. A removable tray had been placed beneath each captive duck to collect the snail-filled droppings without bothering the birds. And once the excreted snails were eventually recovered, van Leeuwen and collaborators looked to see if the snails were still moving or reacted to touch to determine whether the creatures were alive or dead.

Three of the snail species didn’t fare so well. The flat-shelled Bathyomphalus contortus and the native Bithynia leachii snails were entirely broken by the duck digestive system. And while the shells of the invasive snail Potamopyrgus antipodarum came out in better shape, the snails were dead. Only the marine snail species – Hydrobia ulvae – seemed capable of coming out alive. Based upon the recorded amount of time the snails spent inside the ducks, a Hydrobia ulvae snail can survive a five hour trip in a duck’s digestive system, but no more.

Exactly how far a bird might transport an insider snail depends on the behavior of the bird. If the bird hangs around the same wetland, the snail won’t go far. But if the bird takes flight and keeps aloft for all five hours, a snail could be transported as far as 186 miles away. Should the snails take hold in their new homes, and should ducks sample the new population of gastropods, the snails could be dispersed even further still.

Why the one species of snail was able to survive while the other three perished is not immediately clear. But van Leeuwen and co-authors suspect that the pre-existing adaptations of Hydrobia ulvae were key. The strong shell and other traits possessed by the snail species – thought to be defenses against predation and drying out when exposed – may better protect these snails from being crushed or killed by enzymes in duck digestive tracts. The snails just happened to have a suite of traits which allowed them to survive the internal journey.

Regardless of why the snails survived, though, the fact that they passed through ducks and lived to forage another day might have important implications for the ecology of species spread by birds. In the specific case of Hydrobia ulvae, van Leeuwen and colleagues point out, the snail reproduces by producing free-swimming aquatic larvae which can float for long distances. This makes the snail species doubly effective at dispersing over wide areas, and so even relatively distant populations of the snails might be genetically connected by way of hitchhikers on birds and larvae floating through the sea.

And even though the invasive snail species used in the study did not make it out alive, the fact that at least one species of snail survived might highlight an important pathway for how other invasives and parasites can spread to new habitats. Provided that they can survive, alien species might be introduced to new habitats by far-ranging birds, and parasites which are themselves hidden within the snails – such as trematode worms – might spread to new locations as snails do. For snails, and whatever hangers-on they may have, the fastest way to get from one place to another may be as the duck flies.


Darwin, C. 1859. On the Origin of Species by Means of Natural Selection. London: John Murray. pp. 358-364

van Leeuwen, C., van der Velde, G., van Lith, B., & Klaassen, M. (2012). Experimental Quantification of Long Distance Dispersal Potential of Aquatic Snails in the Gut of Migratory Birds PLoS ONE, 7 (3) DOI: 10.1371/journal.pone.0032292

One thought on “Snails Hitch a Ride in Duck Guts

  1. To the same end, raptorial birds that eat frugivorous lizards can also help disperse seeds.

    Significance and extent of secondary seed dispersal by predatory birds on oceanic islands: the case of the Canary archipelago. Padilla et al., 2012. Journal of Ecology.

    DOI: 10.1111/j.1365-2745.2011.01924.x

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