Gulf killifish
Photo: Andrew Whitehead

A small fish somehow evolved resistance to the heavily polluted water of the Houston Ship Channel by mysteriously acquiring genes from another fish from thousands of miles away, according to a new paper.

The Houston Ship Channel’s filthy water is the result of 60 years of industrial dumping, contaminating the water out into Galveston Bay. But despite the grime, a population of Gulf killifish have managed to evolve resistance to the otherwise lethal pollution. Genetic sequences revealed that the fish have integrated a small bit of DNA from another species of fish, the Atlantic killifish. These results show the extreme luck and difficulty required for animals to adapt to environments changed quickly by humans.

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In order to survive humans, “you need to be a superstar in terms of genetic diversity and need to be super lucky,” study author Andrew Whitehead, associate professor at the University of California, Davis, told Gizmodo. “That doesn’t bode well for most species we care about conserving.”

Atlantic killifish
Photo: Andrew Whitehead

The researchers captured killifish from 12 increasingly polluted sites along the Houston Shop Channel and in Galveston Bay. They had the fish spawn in a laboratory and tested how the embryos fared against a pollutant called polychlorinated biphenyl. Fish from the most polluted sites could withstand concentrations 1,000 times higher than normally harmful levels, according to the paper published in Science.

But the incredible realization of how this fish evolved its pollution resistance came from genetic testing. A pair genes from the pollutant-resistant Gulf killifish, including a segment with deleted DNA that seemed to account for the resistance, appeared to come directly from another species, the Atlantic killifish. The researchers estimated that this little bit of genetic material entered the Gulf killifish’s gene pool no more than 34 generations ago, after a Gulf killifish hybridized with an Atlantic killifish.

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You might be thinking, oh, that’s not so surprising; they’re both killifish in the same ocean. But that’s not really what’s happening. “These fish are the size of your thumb and can’t swim very far,” Whitehead explained. “They’ve got a small home range and they’re not a migratory fish. A fish you catch in a marsh was probably born within a hundred meters or so.”

The mystery, now, is how the fish managed to make it from the Atlantic coast to the Gulf of Mexico. “We can only speculate,” said Whitehead. “It was almost certainly human assisted. These fish aren’t capable of swimming around the straits of Florida.” Perhaps an Atlantic killifish was scooped up in a ship’s ballast water and dumped into Galveston Bay.

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Embryo from a non-pollutant-resistant (left) versus pollutant-resistant (right) population, after exposure to pollution. The embryo on the right is faring better.

Other researchers were impressed by the paper. “This was really solid,” Michi Tobler, associate professor at Kansas State University, told Gizmodo. “This story builds on some decades worth of research. They really capitalized that, and very explicitly tested a hypothesis. I thought that was really neat. They sequenced a lot of genomes.”

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Tobler, too, was “flabbergasted” about how this could actually happen. While the ballast water explanation made sense, he wanted to see how common this sort of evolution via hybridization event is in polluted waters around the world.

Don’t get the idea that hybridization is always beneficial, though. It can also threaten some species with extinction, Karin Pfennig wrote in a commentary for Science.

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Your takeaway might be the old “life finds away,” or even “evolution is the solution to pollution.” But this isn’t really what’s going on here, explained Whitehead. Without an unlikely stroke of luck, Gulf killifish might have been completely killed out of the Houston Ship Channel. And far more species are dying out than getting lucky.

“Many of the species we care about are in big trouble when it comes to the pace and severity of environmental change and climate change,” Whitehead said. It’s unrealistic to rely on luck or even genetic engineering to maintain the species that humans care about, and other species more generally. “What’s feasible is to change our behavior as a species.”

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