From the perspective of the three-inch fish, the Houston Ship Channel might resemble a post apocalyptic world — a tattered reminder of a coastal salt marsh paved over by petrochemical plants, a tidal estuary dredged for heavy ship traffic, and a waterway polluted by industrial contaminants. Superfund sites line the channel, and industrial chemicals like polychlorinated bicenyls, or PCBs, which are known to disrupt cardiovascular development in vertebrates like fish, persist in the environment.
Many vertebrate species can’t survive this polluted ecosystem. Yet the Gulf killifish, a silver-flecked yellowish minnow native to the Gulf of Mexico, can. The question that many scientists have, is how.
Andrew Whitehead, an environmental toxicologist at the University of California, Davis, has been studying this question for more than a decade. He looks at killifish genetics for clues on how some species can withstand rapid, human-caused changes in their environment. The answer for killifish, according to his research, is evolution. In a series of studies, including a paper published last year in Science, Whitehead and his colleagues have shown how the Gulf killifish has rapidly evolved to adapt to what would normally be lethal levels of toxins for most other vertebrates.
“[Gulf killifish] are the exception to the rule,” Whitehead says. “We want to understand what makes them an exception.”
The Houston Ship Channel connects the mouth of Galveston Bay to the Port of Houston and the city’s extensive bayou system. Elias Oziolor, a postdoctoral researcher in Whitehead’s lab who studied Gulf killifish in the Houston Ship Channel as a PhD student at Baylor University, identified 12 sites along a pollution gradient — from relatively “clean” sites near Galveston Island to the heavily industrialized upper portions of the Houston Ship Channel and bayous that run through downtown Houston.
Oziolor then used seine nets to catch killifish at these sites, spawned them in a lab at Baylor, and tested their embryos against various concentrations of toxicity. Polychlorinated biphenyls, along with other chemical classes found in the channel, typically result in heart deformities that stunt vertebrate development and can be lethal to most fish species. But Oziolor found that killifish from polluted sites responded relatively well to toxic exposure, while killifish from cleaner sites remained highly sensitive to pollution. In fact, in results he published in Aquatic Toxicology, Gulf killifish from polluted sites in the upper ship channel were 1,000 times more resistant to toxicity than fish from Galveston.
Evidently, the Gulf killifish has developed a genetic resistance to changing conditions. And, according to Whitehead, they did it recently and quickly, and with help from another species of killifish, the Atlantic killifish.
In their recent study in Science, Whitehead and his colleagues looked closely at killifish genetics to explain the mechanisms for surviving polluted waters. For one, Whitehead explains that for a vertebrate, Gulf killifish population sizes are massive. “If you think of genetic diversity for being the fuel for evolution, this species has populations with massive fuel tanks,” he says.
But the fuel tank needed a spark to ignite the evolution. That’s where the Atlantic killifish came in. Past studies have shown that Atlantic killifish can rapidly evolve to adapt to the type of environmental pollution typical of our urban coastlines. “Even at the most contaminated…sites, where pollutants exceed lethal levels and killifish are not expected to persist, they appear to thrive,” Whitehead wrote about Atlantic killifish in Evolutionary Applications. To explain why, Whitehead suggests that certain populations of Atlantic killifish are abundant, and, since they are nonmigratory, they have small home ranges. These two factors increase both genetic variability and the ability to rapidly evolve locally specialized phenotypes to withstand human-caused change in urban environments.
Whitehead found evidence that sometime in the last few decades, Atlantic killifish passed along that genetic mutation to Gulf killifish in a process called introgressive hybridization. That mutation is the genetic key to withstanding pollution.
But the native range of Atlantic killifish is 1,500 miles away from the Houston Ship Channel, so how did these Atlantic killifish get there? “Your guess is as good as mine,” says Whitehead, though he speculates that human transport had something to do with it.
The Port of Houston is one of the most trafficked ports in the country, connected via shipping routes to other ports along the Atlantic. It’s possible that Atlantic killifish or their embryos were transported to Houston in ship ballast — or even in bait buckets. But Whitehead knows for sure that Atlantic killifish arrived in the coast sometime in the 1970s, just as some of the Gulf killifish’s range in the Houston Ship Channel was reaching extreme pollution levels, and that their arrival triggered an “evolutionary rescue.” For a Gulf killifish, their cousins from the Atlantic came just at the right time.
“We as a species do a good job at moving other species around the planet, and usually that causes problems for the native species,” says Whitehead. In this instance, however, a nonnative species saved a native species. The irony is not lost on Whitehead: As human-caused change has contaminated the killifish’s habitat and put a strain on their survivability, human-caused change via introduced species also delivered the killifish its genetic savior.
So is evolution the solution to pollution? Whitehead says no. Rather, his research on Gulf killifish serves as a cautionary tale. By showing the variables that allow Gulf killifish to survive extreme pollution — a combination of large genetic fuel tanks with sheer luck — he points out that most vertebrates aren’t like the Gulf killifish. “I’m careful not to communicate the message that evolution is the answer to our problems that abdicates us from our environmental responsibility of cleaning things up,” he says.
Gulf killifish is the exception to the rule, he reiterates, explaining that, for the most part, evolution is unlikely to keep up with the pace and severity of human-induced ecological change. On top of pollution, there’s climate change, which is pushing many species closer to extinction through temperature and seasonality changes, ocean acidification, and so on. “Some species are going to be able to evolve their way out of it, but most species are much less likely to be able to,” he says. “It’s easier to clean up PCBs than to expect evolution to do its thing.”
Still, the killifish is a reminder of what species resilience through genetic diversity can look like. It’s also a reminder that even in the most radically transformed urban environments, like the Houston Ship Channel, a small fish lives on in a landscape that’s been rebuilt for us.