Mutant Fish Are Thriving in Massachusetts' Toxic Harbor

Massachusetts’ New Bedford Harbor is one of the country’s most polluted waterways. For 40 years, nearby factories and shipping vessels have been dumping pollution into it, and it's so grimy that there's been a ban on fishing in much of the area since 1979. So why is one species of fish thriving there?

According to the Environmental Protection Agency, two manufacturing plants near the harbor spent much of mid-20th century improperly dumping polychlorinated biphenyls (coolant fluids used in motor production) and heavy metals into the harbor, contaminating sediment as far as six miles away. The EPA has been working on cleaning up the harbor since 1982, but it remains a highly toxic area.

Like Blinky the three-eyed fish in The Simpsons, fish in the harbor have rapidly evolved to cope with the toxic waters. In a new paper published in BMC Evolutionary Biology, researchers from Woods Hole Oceanographic Institution say that the 3-inch-long Atlantic killifish (Fundulus heteroclitus) is dominating the harbor. Not only are they dominating the harbor, but they burrow into the toxic sediment for most of winter and spend most of their time in summer there, unlike other fish.

A genetic change in the fish has modified a receptor protein called AHR2. In normal fish, AHR2 regulates cellular functions—PCBs stimulate it, eventually causing toxicity and death. In killifish, the receptor isn’t fully turned off, but it is significantly dulled, making PCB ultimately less toxic to them. Only killifish living in the harbor seem to show this modified receptor—killifish living elsewhere have the normal receptor.

“The killifish have managed to shut down the pathway,” said Mark Hahn, a coauthor of the paper. “It’s an example of how some populations are able to adapt to changes in their environment. It’s a snapshot of evolution at work.”

It’s the latest evidence that rapid evolution occurs not just in bacteria or other smaller organisms, but in more complex ones, too.

“These studies are helping to dissect how evolution occurs on a contemporary scale and why some species are more likely to adapt to a rapidly-changing world,” Diane Nacci of the EPA said.

But, like Blinky, who seems to get along just fine in the Springfield River, the presence of killifish at New Bedford has had some impacts higher up on the food chain. Though they thrive in the sediment, they still carry extremely dangerous doses of PCBs that are transferred to larger fish, and ultimately humans, when they’re preyed on.

So while killifish have evolved in response to pollution, it's hardly evidence that pollution is anything but harmful. It's also important to note that the success of killifish could be due in part to the failures of other species.

EPA cleanup efforts are ongoing, and the AVX Corporation, one of the companies that originally dumped in the harbor, was just forced by a federal judge to pay $366.245 million to help clean the waterway. The hope is to hit cleanup goals within about 5 years so other fish can return to the area. Until then, the mutant killifish will keep doing what they do.