A Swan Swallowed This Fish Egg, Pooped It Out—and Then 49 Days Later, It Hatched

Killifish eggs are known for their resilience: They’ve been found thriving in polluted Superfund sites and the short-lived ephemeral pools of Mozambique. To really put their survival skills to the test, researchers sent some unlucky eggs on a not-so-glamorous journey: through a swan’s digestive tract, emerging in the bird’s feces.

As Veronique Greenwood reports for The New York Times, at least one egg in the study survived the hours-long journey as an intact egg and then later hatched successfully, “apparently none the worse for wear."

The surprising survival story suggests that birds can act as carriers for fish eggs, transporting the casings far from their original locations, researchers suggest in a new study published in the journal Ecology. When fish appear in unexpected places, study co-author Andrew Green of Spain’s Estación Biológica de Doñana tells Greenwood, they “may have literally fallen from the sky.”

Lead author Giliandro Silva, a graduate student at Brazil’s Unisinos University, first raised the idea of bird feces-facilitated fish egg transport while conducting research for a 2018 study on duckweeds, which are flowering water plants that similarly thrive even after spending time in a bird’s digestive system. To their surprise, Silva and his colleagues found an intact killifish egg in a frozen fecal sample collected from a wild coscoroba swan.

Speaking with Tiago Marconi of Brazilian blog Ciência na Rua, or Science on the Street, Silva explains that birds, particularly waterfowl traveling between “isolated and distant environments,” often disperse organisms such as plant seeds and invertebrate eggs. Avian creatures have also been known to transport fish eggs with the help of their feathers, legs or beaks, but the new study is one of the first to demonstrate egg dispersal via fecal matter.

The researchers tested their hypothesis by mixing 650 killifish eggs into a group of coscoroba swans’ corn-based feed. At various intervals over the following 48 hours, the team collected 55 fecal samples, then tested this excrement for intact eggs. Overall, they found five viable specimens in four droppings, or roughly one percent of the original 650 eggs. Of these five, three exhibited embryonic development; two later died of fungal infections unrelated to their ingestion by waterfowl, while one successfully hatched 49 days after removal from the dropping.

As Greenwood of The New York Times notes, killifish are renowned for their ability to thrive in diverse environments including isolated desert pools, flood water lakes and seasonal ponds no bigger than puddles. Silva further tells Marconi that killifish eggs deposited in dry soil can survive until rain replenishes their habitat.

The killifish eggs featured in the study likely withstood the swans’ digestive systems because the birds’ guts are somewhat inefficient, excreting still-undigested food in order to prepare the stomach for the next meal.

Moving forward, Silva and his colleagues plan on conducting a similar round of experiments, this time with carp instead of killifish eggs. Both fish species are invasive outside of their normal range, Greenwood concludes, making it essential for scientists to better understand how they spread and, subsequently, how to contain them.

Meilan Solly | | READ MORE

Meilan Solly is Smithsonian magazine's senior associate digital editor, history.