Meet the 'sturddlefish': The hybrid fish created by accident

The photos show two examples of the hybrid "sturddlefish" species created from American paddlefish and Russian sturgeon. (Kaldy et al., Genes, 2020)

TORONTO — American paddlefish and Russian sturgeon were never meant to reproduce with one another.

Living on opposite ends of the planet, the Russian sturgeon feeds on the floors of seas, lakes, and rivers in eastern Europe, Serbia, and the Middle East while the American paddlefish is a filter feeder that is only found in the United States, particularly in the Mississippi River and its tributaries.

It was highly unlikely the two “fossil fish,” as they are called because of their ancient lineage, would ever cross paths and even more unlikely they would ever mate.

Russian sturgeon are distinguishable by their relatively short, rounded snouts and spiky dorsal fins while the American paddlefish have distinctive long, pointy snouts.

However, researchers in Hungary surprised the scientific community, and even themselves, when they accidentally bred Russian sturgeon and American paddlefish in captivity and created a new hybrid species – now unofficially known as the “sturddlefish.”

According to findings published in the journal Genes earlier this month, the team of researchers were attempting to induce asexual reproduction in the endangered Russian sturgeon last year.

To do this, they tried to initiate a process called gynogenesis in the sturgeon by placing sperm from the American paddlefish near the eggs from the Russian sturgeon. Gynogenesis requires the presence of sperm without the actual introduction of any DNA.

The scientists were hoping the Russian sturgeon would reproduce on their own because of the proximity of the sperm from the American paddlefish.

Russian sturgeon are a valuable economic commodity as their unfertilized eggs are the source of much of the world’s high-end caviar. They are listed by the International Union for the Conservation of Nature (IUCN) as one of the most critically endangered species, however, due to slow rate of growth, threatened habitat, and overfishing.

American paddlefish are also considered endangered because of declining populations and habitat, according to the IUCN.

While the point of the experiment was to induce asexual reproduction, something unexpected happened instead.

The American paddlefish sperm was able to successfully fertilize the Russian sturgeon eggs.

“We never wanted to play around with hybridization. It was absolutely unintentional,” Attila Mozsar, the study’s co-author and a senior research fellow at the Research Institute for Fisheries and Aquaculture in Hungary, recently told The New York Times.

As a result, hundreds of hybrid fish were created with nearly three quarters of them surviving a month after hatching.

In their study, the researchers describe the event as the “first successful hybridization between these two species.”

The scientists also noted there were two distinct groups of hybrids – one that received more than twice as much DNA from their mother and looked more like Russian sturgeon and another group that received the same amount of maternal and paternal DNA and appeared to be a blend of the two species.

Mozsar told The Times he and his fellow researchers suspect the sturddlefish are sterile like other man-made hybrids, such as ligers (a mix of lions and tigers) and mules (a mix of horses and donkeys). He said, while they’re still caring for the approximately 100 sturddlefish that have survived, they don’t plan to breed any more of them out of concern they could endanger existing wild fish populations.

The study’s authors said their findings suggest the species may not have evolved to be that different after all.

“The reproductive success of these two distant species could be the combined consequence of the ancient genome duplication [that occurred in a shared ancient ancestor] and the slow evolution rate of acipenserids [sturgeons],” the study said.

In conclusion, the research team remarked on the potential for future hybridizations among American paddlefish and sturgeon.

“Based on our findings, successful interspecific hybridization is also supposed in other species combinations in the order Acipenseriformes [an order of freshwater fish that includes sturgeon and paddlefish],” the study said.