Fish eggs can hatch after being eaten and pooped out by ducks

For fish eggs, getting gobbled by a duck kicks off a harrowing journey that includes a pummeling in the gizzard and an attack by stomach acids. But a few eggs can exit unscathed in a duck’s excrement, possibly helping to spread those fish, including invasive species, to different places, a new study finds.

It’s been an “open question for centuries how these isolated water bodies can be populated by fish,” says fish biologist Patricia Burkhardt-Holm of the University of Basel in Switzerland, who was not involved with the work. This study shows one way that water birds may disperse fish, she says.

Birds’ feathers, feet and feces can spread hardy plant seeds and invertebrates (SN: 1/14/16). But since many fish eggs are soft, researchers didn’t expect that they could survive a bird’s gut, says Orsolya Vincze, an evolutionary biologist at the Centre for Ecological Research in Debrecen, Hungary.

In the lab, Vincze and her colleagues fed thousands of eggs from two invasive carp species to eight mallard ducks. About 0.2 percent of ingested eggs, 18 of 8,000, were intact after defecation, the team found. Some of those eggs contained wriggling embryos and a few eggs hatched, the team reports June 22 in the Proceedings of the National Academy of Sciences. It’s not clear yet whether eggs survive in this way in the wild.

Most of the viable eggs were pooped out within an hour of being eaten, while one took at least four hours to pass. Migratory ducks could travel dozens or possibly hundreds of kilometers before excreting those eggs, the scientists suggest.

Though the surviving egg count is low, their numbers may add up, making bird poop a possibly important vehicle for spreading fish. A single carp can release hundreds of thousands of eggs at a time, Vincze says. And there are huge numbers of mallards and other water birds throughout the world that may gorge themselves on those eggs.