Hakai Magazine

Coastal science and societies

killifish egg that was eaten by a coscoroba swan, seven days before hatching
Scientists think a thick protective membrane safeguarded a killifish egg that was eaten by a coscoroba swan. Photo by Vinícius Weber

Killifish Eggs Can Survive Being Eaten

A fish egg hatched after passing through a swan’s digestive tract. The discovery lends credence to the idea that birds help fish spread to far-off lands.

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by Richa Malhotra

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Found in temporary desert pools and mangrove swamps, some species of killifish can go without water for weeks at a time. And their eggs are no different: no water, no problem!

Now, scientists have found that the eggs of two killifish species are so hardy, they can withstand being digested by birds, too. The finding may be the first firm evidence supporting an idea that has long held sway in scientific circles—that birds disperse fish.

In 1809, German naturalist Carl Christian Gmelin posited that ducks spread fish to distant waters by carrying fish eggs stuck to their beaks. Later, in 1876, in The Geographical Distribution of Animals, British naturalist Alfred Russel Wallace wrote that freshwater fish eggs being carried by birds could be one of several means by which the same fish species wind up in distinct rivers.

This idea that birds transport fish eggs became embedded in scientific knowledge. But as recently as 2018, whenever researchers searched the peer-reviewed literature for solid data supporting the hypothesis, they came up short. Except for a few anecdotal accounts, they could not find any evidence of birds carrying fish eggs.

But this long-lived hypothesis, so recently put to bed, has been given a second chance thanks to Giliandro G. Silva, a doctoral student at the University of the Sinos Valley in Brazil. In 2017, Silva inadvertently found a single killifish egg in the droppings of a wild coscoroba swan. “I was not looking for a fish egg. I was looking for other groups, like plants and invertebrates,” Silva says. That serendipitous discovery sparked a deeper investigation.

To definitively determine whether killifish eggs can survive digestion by birds, Silva and his colleagues fed eggs from two killifish species mixed in corn feed to captive-born coscoroba swans. Rummaging through the birds’ poop, they recovered five live killifish eggs. While four succumbed to fungal infections in the lab, one hatched into a tiny fish after 49 days. This egg had survived 30 hours inside a swan.

Killifish eggs can survive, Silva says, because they have a thick membrane that protects them from harsh conditions. This, combined with the inefficient digestive system of the swan, makes it possible for some eggs to pass through intact.

“If you just want a proof of concept, then one egg is enough,” says Benedikt Schmidt, an evolutionary biologist at the University of Zurich in Switzerland who was not involved in the study. Other life forms, including snails and aquatic leaf beetle eggs, are known to survive being eaten by waterfowl. Now, Schmidt says, we have evidence that a fish egg can do the same. “That’s really an interesting result in itself,” he says.

In the wetlands on the coastal plain of southern Brazil where Silva first found an intact egg, the presence of killifish overlaps with the arrival of migratory coscoroba swans. So, when swans fly between wetlands for feeding or roosting, or when they migrate, they might be carrying killifish eggs inside them.

Gmelin and Wallace had the right idea after all. It just took an egg to hatch new evidence for their age-old theory.