One place scientists didn’t expect to find them was in swan poop. But an international team of researchers reported last week in the journal Ecology that whole killifish eggs make it through the digestive tract of water birds intact, with one egg in the study even hatching more than a month after its transit through a swan. The findings suggest that bird feces may be capable of carrying fish eggs far from their original locations.
Giliandro Silva, a graduate student at Unisinos University in Brazil, and colleagues found last year that small flowering water plants in bird feces were still alive and able to grow. While they were completing that study, they found a killifish egg in a frozen fecal sample from a wild coscoroba swan. They realized that what was true for plants might also be true for fish eggs.
To test this hypothesis, they mixed eggs of two killifish species found in Brazil into the feed of swans living in a zoo. Over the next two days, they collected what the swans excreted and looked for intact eggs. They found five, about one percent of the 650 eggs they’d mixed in.
Then, they kept the eggs in the lab to see if they would continue developing. Of the three that did, two eventually died from an unrelated fungal infection. But one hatched into a young killifish 49 days after its emergence from a swan’s gut, apparently none the worse for wear.
This killifish egg passed through a swan’s gut, then hatched and developed for 49 days.CreditCreditBy G. G. Silva, V. Weber, And P. Hoffmann
When the water they live in dries up, killifish eggs drop into a hibernation-like state, able to revive and hatch months later if water returns. This special ability is often why the fish sometimes seem to appear out of nowhere when a seasonal pool forms.
“They’re famous because of their amazing ability to survive in the mud,” said Andrew Green, a researcher at Estación Biológica de Doñana in Seville, Spain, and a co-author of the new paper.
But the fact that a small fraction of all killifish eggs consumed can make it through a bird unharmed may explain the appearance of fish in places where no one can imagine a plausible arrival story, he continued. In some cases, the fish may have literally fallen from the sky.
That any survive at all, without the protective casing of a seed or nut, could be because the guts of swans, like the guts of most creatures, are not 100 percent efficient. An animal’s digestion extracts the nutrients that are readily available fairly quickly. To be able to eat another meal, the animal must excrete whatever else is left. That includes, in this case, as-yet-undigested killifish eggs.
The researchers are planning a similar experiment now that uses eggs from carp, which hatch much faster than killifish. As killifish and carp can be invasive species outside of their normal range, understanding how they spread can help in containment.
What happened to the sole survivor of the experiment, though, the lone killifish that hatched? Is it swimming in a tank at the lab?
“It’s been preserved for scientific posterity,” Dr. Green said.
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