New Bedford Killifish

by Tracy Hampton

Beneath the waters of the New Bedford Harbor lies one of the most highly PCB contaminated sites in the nation. PCBs, or polychlorinated biphenyls, were dumped there as a byproduct of manufacturing plants that operated from the 1940s to the1970s. The harbor has been declared a Superfund site because PCB levels are over four times higher than what the Environmental Protection Agency says is biologically safe. Surprisingly enough, a species of fish is thriving there despite long-term exposure to high levels of these toxic chemicals.

Mark Hahn of the Woods Hole Oceanographic Institution enjoys showing off his collection of killifish. His lab is like a small aquarium -in each tank, there's a different group of fish on display.

Mark Hahn: "This is Fundulus heteroclitis, which is the Atlantic killifish, sometimes called the mummichog. And when the males are mature, they have this beautiful yellow color that you might see in some of these guys, whereas the females are more of a dull, but pretty, white."

Hahn gets his killifish from nearby estuaries and marshes, and he brings them back to his lab where he breeds them and studies their genetic makeup. One of his research interests is how these little fish - as well as other vertebrates " are affected by pollution.

Mark Hahn: "It's a very interesting ecological question. What happens when animals are exposed for generations to very high levels of contaminants. One thing that could happen is that the population could be eliminated, but another thing that could happen is that the population is able to adapt through genetic changes to the high levels of pollution.

And that's just what Hahn has found at the New Bedford Harbor. There're plenty of killifish living there, and they seem to be eating and reproducing without any trouble. But upon closer examination, he noticed there was something abnormal about a particular genetic pathway in these fish that acts in response to chemicals like PCBs. Say a fish takes PCBs into its body. Normally, the PCBs are metabolized and broken down, and the resulting chemicals are even more toxic to the fish than the original PCBs themselves. The fish in the New Bedford Harbor have somehow managed to shut down this pathway, and they're able to survive because the toxic breakdown products aren't formed. Other species of fish in the New Bedford Harbor weren't as adaptive as the killifish. Diane Nacci of the EPA:

Diane Nacci: "The whole story about the genetic adaptation, well does that mean it's really ok, well, no, it doesn't mean it's really ok. Because one species happened to have sufficient genetic variation that it was able to evolve. There probably were many fish species that were unable to evolve, were unable to live there as the PCBs moved in, and are unable to live there now. It is one of the most highly PCB contaminated sites in the nation."

And just because killifish have been able to adapt, that doesn't mean they're perfectly healthy. Sarah Cohen is a population geneticist at Harvard University, and she says these fish may now be even more susceptible to the toxic effects of other chemicals.

SarahCohen: "There's a cost for that kind of alteration. And it can be a severe cost. For example, we see that if there's other kinds of dangerous compounds in the environment, if the organism has shut down the pathway, then it can't metabolize other harmful things. So there's a tradeoff to adapting to these extreme situations. You can't have it all."

Another cost is the effect these fish are having on other animals. The PCBs build up in the fish's bodily tissue and are passed along up the food chain. So far, scientists don't know the full implications of that. Cohen and Hahn are also interested in seeing how their fish studies can apply to other areas of research and medicine.

Mark Hahn: "There's a field called pharmicogenetics, which looks at variability among humans in their sensitivity to chemicals and drugs. There are genetic differences in humans that determine this sensitivity. So we're using these fish to give us two populations that are very different in sensitivity that might serve as examples of the two ends of the spectrum in humans in their sensitivity to a chemical."

Cohen also says studying fish is much simpler than trying to study humans.

Sarah Cohen: "Because humans do so many different things -their behavior is so much more complex. For example, smoking might be something that might interact with immunogenetics or compounds and these fish don't smoke - we happen to know. So that allows us to partition out what kinds of effects are happening."

The researchers are also curious about what will happen to these fish when the New Bedford Harbor is finally cleaned up - something the EPA plans to do in the near future. The question is, will they be so well - adapted to an extreme environment that they'll no longer be able to live in a healthy one.


Tracy Hampton is a reporter for WCAI-WNAN.