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Scientists have long been puzzled by how fish sometimes show up—and even thrive—in isolated bodies of water where they were never expected. A recent study suggests a startling answer: they might have arrived from above.
More specifically, from a swan’s digestive system.
In a study published in the journal Ecology, researchers from Unisinos University in Brazil discovered that killifish eggs can survive the swan’s digestive journey and hatch once excreted. This strange form of fecal transportation might explain how killifish unexpectedly appear in ponds, floodwaters, and other places where they shouldn’t be found.
After learning that certain plants could survive digestion and thrive in swan droppings, researchers noticed a killifish egg in a frozen fecal sample. They proceeded to mix two species of killifish eggs into the food provided to coscoroba swans in a zoo. After waiting for a day, they collected the droppings and searched for the eggs.
Out of the 650 eggs the researchers estimated the swans ingested, about five remained intact. Of those, three continued developing. Two succumbed to a fungal infection, but one survived, enduring 30 hours in the gut and hatching 49 days after being excreted.
Killifish eggs possess a tough outer membrane, or chorion, which allows them to survive the digestive tract of an animal. Not everything a swan consumes is absorbed; their stomachs quickly extract nutrients and expel the remainder, enabling them to eat again. In rare cases, an egg can make it through intact and continue developing.
However, not all fish eggs are so resilient, and not every fish shares the killifish's traits. Referred to as the "most extreme" fish by the BBC, killifish have evolved to appear in environments where water might eventually dry up. Their eggs can survive in soil, delaying development until conditions—such as not being inside a swan—are ideal. One species, the mangrove killifish, can even breathe through its skin. When water recedes, it survives for over two months on land, waddling on its belly or using its tail to "jump" and catch insects. A fish capable of surviving on land likely doesn't mind living in feces.
The researchers now plan to investigate whether carp eggs can endure similar conditions, undergoing the same journey through a digestive system to see if they, too, can make it through the process.
