Scientists Accidentally Bred a School of ‘Sturddlefish’

Researchers mixed paddlefish sperm with sturgeon eggs because the fish seemed too dissimilar to form a hybrid

The top fish is a Russian sturgeon and the bottom is an American paddlefish. In between, two varieties of hybrid 'sturddlefish' created by accident. Kaldy et al., Genes 2020

Sometimes, if unrelated sperm float near sturgeon eggs, the eggs will begin to develop baby fish without actually being fertilized. To see if this was possible with Russian sturgeon eggs, which are both endangered and the source of much of the world’s caviar, scientists decided to introduce paddlefish sperm.

In the experiment, some of the sperm were irradiated to damage the genetic material inside. But in another group, the scientists mixed sturgeon eggs with untreated, healthy paddlefish sperm. Several past attempts to mix the two species “failed to result in viable offspring,” and the laundry list of differences between the fish “suggests an inability to hybridize,” the scientists wrote in their paper, published on July 6 in the journal Genes.

All that to say, the resulting school of hybrid offspring came as a surprise. About two thirds of the fish survived the first month, and about 100 are still alive today, Annie Roth reports for the New York Times.

"This hybrid should die," University of Pannonia aquaculture geneticist Miklós Bercsényi, a co-author on the study, tells USA Today’s N'dea Yancey-Bragg. "The embryonic development should not happen."

Bercesényi adds that such a hybrid would be impossible in the wild. For one thing, Russian sturgeon and American paddlefish live on opposite sides of the globe. They have been evolving separately for over 184 million years, and have vastly different survival strategies. Papa paddlefish is a filter feeder with a long, sensitive nose that enjoys large, slow-moving rivers. Meanwhile, the Russian sturgeon mother is carnivorous and highly valued for its eggs.

Both fish species are exceptionally rare. The American paddlefish may be the last paddlefish species left, as its nearest cousin, the Chinese paddlefish, probably went extinct between 2005 and 2010, according to a study published in March. And sturgeon as a group of species are critically endangered, with Russian sturgeon even more rare among them.

That’s why the scientists were initially attempting gynogenesis—to induce the sturgeon eggs into development without fertilization—as a way to possibly replenish the species. Gynogenesis was successful in 2014 on the eggs of ship sturgeon, which are critically endangered, using Siberian sturgeon sperm.

When mixing Russian sturgeon eggs and paddlefish sperm, “we never wanted to play around with hybridization,” co-author Attila Mozsár of Hungary’s National Agricultural Research and Innovation Centre tells the New York Times. “It was absolutely unintentional.”

But it turns out that the two species might be more similar than they initially seemed. They are both ‘fossil fish,’ which grow slowly and can live for decades. Like sharks, they have skeletons made of cartilage instead of bone. They also have skin without scales and similar intestines, according to the Times.

“These living fossil fishes have extremely slow evolutionary rates, so what might seem like a long time to us isn’t quite as long of a time to them,” Nicholls State University aquatic ecologist Solomon David tells the New York Times. So even though the fish have been evolving independently for twice as long as humans and mice, they still had enough in common for their genes to mingle.

The hybrids, dubbed sturddlefish, have their mother’s appetite and their father’s long snout. Genetic analysis showed that the fish that more closely resembled sturgeons also had twice as much of their mother’s DNA than the more paddlefish-like offspring.

“I did a double-take when I saw it,” David adds to the Times. “I just didn’t believe it. I thought, hybridization between sturgeon and paddlefish? There’s no way.”

The scientists suspect that the sturddlefish are sterile, like other man-made hybrids like mules and ligers. That means they wouldn’t be useful for caviar production. And while the fish are doing well in captivity, the researchers don’t plan to make more.

"They grow well, they eat well," Mozsár tells USA Today. "We keep them in a very safe place."

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