As It Happens

A fish egg hatched after getting eaten and pooped out by a swan

They say the journey is more important than the destination. And if you're a killifish egg, that journey may include being eaten by a swan and travelling through its guts.

Researchers discover that killifish eggs can survive a trip through a waterfowl's digestive system

A coscoroba swan swims behind a glass panel in its enclosure at the zoo in Berlin. (Paul Zinken/EPA)


A fish in Brazil travelled through the entire digestive system of a coscoroba swan — all before it even hatched. 

The killifish egg's strange journey of being eaten and defecated was the work of a team of international scientists trying to figure out how the small creatures manage to travel between unconnected bodies of water. 

"A lot of people who work with fish often ... find them in places where they can't explain how they got there," study co-author Andy Green, an ecologist at Spain's Estación Biológica de Doñana, told As It Happens host Carol Off. 

"For that reason, there has been a lot of speculation for decades about whether or not eggs can get stuck on the outside of birds — but we're the first people to actually find that they can actually move on the inside."

The study, led by Giliandro G. Silva at Unisinos University in Brazil, was published last week in the journal Ecology.

The surviving killifish egg, about 1 millimetre in diameter, is pictured here 7 days before it hatched. (Lecea-Unisinos)

Green — who says he's "been interested for a long time in understanding what kind of organisms actually can move around inside waterfowl" — says the idea for the experiment came when scientists were searching frozen swan droppings for plant matter, and found a number of intact fish eggs.

"That really gave us evidence that there might really be something going on, which then led us to do an experiment under control conditions to actually see if the eggs could survive being swallowed by a swan and then coming out in the poop," he said.

The researchers mixed about 600 eggs from two different species of Brazilian killifish into swan feed at a zoo.

Five of those eggs survived the digestive process. Of those, four died from an unrelated fungal infection.

But one little egg remained unscathed and hatched into a healthy killifish 49 days after emerging from the swan.

"It was a risky thing to do because certainly most people probably would've thought they wouldn't have a chance to survive," Green said. "It's a bit of amazing, really."

This adult male Austrolebias minuano killifish was used to fertilize the eggs for the study. (Lecea-Unisinos)

Green says the findings have implications for understanding how fish are able to travel between distant bodies of water — but they could also shed light on how certain species will adapt to climate change.

"Biodiversity is under threat and with the climate crisis it is becoming increasingly important for species to move quickly from one place to another, especially sort of towards the poles," he said.

"Most of these organisms won't be able to move fast enough unless they can get help from the birds. So we really need to know what kind of organisms can move with the birds."

But, he noted, it's possible killifish eggs are particularly well-suited to make the journey through a bird's guts. They are already known for being able to survive in the mud when the water dries up.

"We do hope to do more experiments with other fish just to try and find out how widespread this process is," he said.

And while sorting through frozen swan poop for fish eggs may sound like an unpleasant job, Green and his colleagues are up to the task.

"It doesn't smell that bad actually," he said. "Nowhere near as bad as dog poop."

Written by Sheena Goodyear. Produced by Morgan Passi. 


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