Toxic Waters Provide 'a Snapshot of Evolution'

The killifish's adaptation is not unique, but marine animals rarely inhabit contaminated places. Hudson River fish called tomcod have evolved a tolerance for dioxin, and various populations of mosquitofish live in insecticide-laden waters.

"But all can become vehicles for the transfer of chemicals through the food web," said Peter Van Veld, a toxicologist at the Virginia Institute of Marine Science in Gloucester Point, Va. Killifish, for example -- which absorb PCBs -- are eaten by wading birds such as herons and by larger fish such as eels, bluefish and striped bass.

Nacci, Hahn and Gloria V. Callard of Boston University have studied the New Bedford killifish for a decade, hoping to discover the mechanism by which the fish evolved resistance to PCBs -- and what that means for the ecosystem.

The average life span of a killifish is three years; they reproduce when they are a year old. Since PCBs first fouled the harbor, tens of generations have lived there, giving them time to adapt.

"Killifish in New Bedford Harbor seem to be doing well," Hahn said, "which is amazing when you consider that fish usually metabolize PCBs and break them down into a chemical that's even more toxic. But these killifish have managed to shut down that pathway. They're able to survive because the secondary harmful chemical isn't formed." No other species in the harbor, as far as scientists know, is able to do that.

"What we're witnessing," Callard said, "is a snapshot of evolution at work."

Teasing out the mechanism that allows the fish to coexist with PCBs involved comparing killifish from New Bedford Harbor with killifish from a clean environment, Scorton Creek in Sandwich, Mass.

Hahn and Callard are using genetic techniques to trace the evolution of genes that altered the response to PCBs. "Fish have more receptor genes [places to bond] for chemicals like dioxins and PCBs than do mammals, possibly explaining the extreme sensitivity of most fish to these pollutants," Hahn said. "Small changes, called SNPs, in these genes likely are involved in PCB resistance in New Bedford killifish."

SNPs (pronounced "snips"), or single nucleotide polymorphisms, are tiny genetic variations that occur in a DNA sequence. DNA's genetic code is made up of four letters or bases: A for adenine, C for cytosine, G for guanine and T for thymine. SNP variation occurs when a single letter, for example, an A, replaces one of the other three letters, C, G or T. The researchers have identified several combinations of SNPs in the New Bedford killifish and are working to pin down what role they play in the fish's ability to metabolize PCBs safely.

In addition to holding, perhaps, a key to the mystery of how killifish survived in New Bedford Harbor, SNPs are being investigated in humans for disease diagnosis and new drug treatments.

The killifish's adaptation to pollution may come at a high cost, Nacci said. When the PCB-laden fish are maintained in clean laboratories, they have less ability to respond to other stresses, such as exposure to bacteria or low oxygen levels, and are often unable to capture prey.

"Traits that are 'good' under certain conditions may have disadvantages, too," Nacci said.

Resistance to toxic compounds may sap energy from growth and reproduction, although that does not seem to have happened -- yet -- in New Bedford's killifish.

"We're looking at one fish species, killifish, in one place, New Bedford Harbor," Nacci said. "But what about the implications of animals adapting to potentially irreversible ecological effects like these [from PCBs] on a large scale?"

If similar stories are unfolding in other geographic areas and other ecosystems, she said, pollutants may prove to be unexpectedly powerful forces of evolution.

Van Veld shares Nacci's fears. There is a big "side effect," he believes, to adapting to toxic conditions. "Since in nature you can't have it all, something has to give."

Nacci wonders what will happen to New Bedford's killifish if the harbor is finally cleaned up. "Will these fish be so adapted to a polluted environment that they won't be able to live in a clean one?"