Hybrid Fish Stuns Scientists As Two Species That Last Shared An Ancestor In The Jurassic Have Babies

The sturddlefish seemed more surprised than anyone to find itself alive. Flórián Tóth, NARIC, Research Institute for Fisheries and Aquaculture

A surprise fertilization in a fish nursery has led to a new hybrid fish bred from two species that are further apart on the evolutionary tree than humans and mice.

Published in the journal Genes, a new study reveals how sperm from an American paddlefish was able to fertilize eggs from a Russian sturgeon to create a hybrid group of animals, dubbed "sturddlefish," never before seen. Made up from the gametes of seven individuals, the proceeding group exhibited different ratios of features from the two species, and survival rates beyond 30 days that ranged from 62 percent to 74 percent.

If you’re someone of refined taste, Russian sturgeon eggs may be familiar to you as they’re a sought-after ingredient for fancy caviar. These enormous carnivores shuffle along the beds of rivers, lakes, and coastal waters hoovering up crustaceans and fish.

The hybrids exhibited different ratios of sturgeon-to-paddlefish. Flórián Tóth, NARIC, Research Institute for Fisheries and Aquaculture

American paddlefish on the other hand are filter feeders sucking up zooplankton from the water in 22 US states. They have shark-like bodies with a comically long and highly sensitive snoot attached to their face that constitutes one-third of their total body length.

Both animals are some of the largest and longest-living fish species in the world, with an ancient divergence from the evolutionary tree that constitutes them as living fossils. They exist in different parts of the planet and last shared a common ancestor when dinosaurs roamed the Earth, leading researchers to think they were far too removed on the evolutionary tree to be hybridized. But apparently not.

“I did a double-take when I saw it,” said Solomon David, an aquatic ecologist at Nicholls State University in Louisiana in an interview with the New York Times. “I just didn’t believe it. I thought, hybridization between sturgeon and paddlefish? There’s no way.”

Top: typical young Russian sturgeon. Middle two: hybrids of sturgeon and paddlefish, better known to the Internet as "sturddlefish". Bottom: typical young paddlefishKaldy et al., Genes 2020

The accident came about as scientists at the Research Institute for Fisheries and Aquaculture in Hungary were trying to ascertain if introducing asexual reproduction into paddlefish and sturgeon could protect their population numbers, which are plummeting in the face of overfishing, pollution, and habitat loss. Instead of ending up with lots of paddlefish and sturgeon clones, they found themselves with a new animal altogether, and since – despite their evolutionary distance – these two species share some physical traits, the resulting offspring appear to be surviving.

Hybridization is not encouraged in science, with previous attempts to hybridize tiger species having catastrophic results, and the researchers stress that it was never their intention that the two species should combine. They currently have over 100 offspring still alive from the hybridizing event, which was first reported in May, with some individuals getting a double dose of maternal DNA and appearing more sturgeon, while others got a near 50/50 split and are a mashup of the two species.

Like many other hybrids, the offspring are sterile and so the lab has no intention of creating any further sturddlefish. The unexpected result of housing these two very, very distantly related species goes to show that sometimes life really does, uh, find a way.

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