This Fish Has Adapted To Lethal Levels Of Water Pollution By Making Hybrid Babies

The Gulf killifish (Fundulus grandis) might sound like the name of a ferocious marine predator, but lethal levels of pollution on its home turf has put its very survival under threat.

And yet, in an extraordinary tale (should that be tail?) of adaption and hybridization, one population of fish has evolved to exist in the dirty waters of the Houston Ship Channel thanks to genes acquired from its relative, the Atlantic killifish. This is the finding of a study recently published in the journal Science

Despite measuring just 18 centimeters (7 inches) head-to-tail, the Gulf killifish is one of the largest of its kind. It can be found in coastal estuaries running along the northern Gulf of Mexico and the Atlantic coast, where it serves as a favorite mealtime snack for many of the region's sport fish – the speckled trout, red drum, and flounder included.

It can also be found in the Houston Ship Channel, a stretch of water heavily polluted from more than six decades of industrial activity. Here, the Gulf killifish serves as a conduit of sorts, providing a route for pollution in contaminated soil sediments and macroinvertebrates to enter the human food chain. 

To find out how the species is adapting to this extreme (and rapid) environmental change, a team of scientists from Baylor University sampled Gulf killifish from 12 sites in the Houston Ship Channel and Galveston Bay, and had them cultured in the university's aquaculture facilities to test their tolerance to pollution. To do so, embryos from each population were exposed to model pollutants mimicking those of chemicals found in the Houston Ship Channel. 

They found that those collected from sites with the highest levels of pollution were also the most resistant to that pollution, a fact that in itself might not be all that surprising. Yet, when they sequenced the entire genomes, they found that those best adapted to the pollution contained genomic regions that could be traced to the Atlantic killifish – not earlier generations of the Gulf killifish. 

"The huge population sizes of Gulf killifish allow them to retain a large amount of genetic variation," study author Elias Oziolor said in a statement

But “under radical pollution pressure, the ultimate solution was not their own genetic variation, but variation they were lucky enough to capture from their sister species, the Atlantic killifish, through hybridization.”

This hybridization is thought to have taken place as a result of human activity, potentially in the ballast water from a large ship. But while it is exciting to see adaption to changing surroundings take place at such a rapid speed, this is not a solve-all solution to the problem of human-caused environmental degradation, the researchers warn.

"Hybridization is not likely to be a commonly utilized mechanism of evolutionary rescue," explained senior author Cole Matson, an associate professor of environmental science and a member of the Center for Reservoir and Aquatic Systems Research (CRASR) at Baylor University. "But this study clearly shows that it is possible."

The team hopes to find out how Gulf killifish from other polluted sites have responded to the change, as well as the impact of pollution on other fish residents of the Houston Ship Channel. 

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