Killifish and Pollution

Killifish and Pollution Photo Credit: Brian Gratwicke [CC-BY-2.5], via Wikimedia Commons.

The E.P.A.’s Superfund sites are properties contaminated with some of the most dangerous pollutants known. But at one of these sites, scientists have found that some small fish are beating the odds, thriving in what should be deadly waters. In this Science Update, you’ll hear more about this bizarre situation.


The secrets of some hardy fish. I'm Bob Hirshon and this is Science Update.

A unique group of minnows called killifish are thriving in Virginia’s Elizabeth River. What’s so unusual about this is the water’s loaded with creosote -- a highly toxic, cancer-causing wood preservative.

Duke University researcher Richard Di Giulio says such contamination would normally kill a killifish. But somehow, they’ve adapted to the pollution. Now he and his colleagues are trying to find out how.

Di Giulio:

By what defense mechanism have these fish attained that ability to better withstand those chemical stressors?

They haven’t worked out all the details, but they have found some clues. For example, while creosote does weaken the fish’s immune system, it also kills off any parasites or bacteria that would normally make the fish sick. And although many of the fish eventually succumb to cancer, they’ve already had several broods by then. So the population as a whole continues to thrive.

Di Giulio says the research is teaching scientists how pollution can change wildlife populations in complex ways.

Di Giulio:

Over long periods of time, pollutants can act as sort of evolutionary drivers. They’re actually, to some extent, driving the way animals evolve.


Making Sense of the Research

You can’t get much worse than a Superfund site. It’s kind of like the E.P.A.’s “Most Wanted” list of seriously polluted areas. If you stir up the water in the Elizabeth River, oil can actually rise to the surface. So this is not a place you’d want to wade through, much less live in and get your oxygen from.

And the pollution isn’t exactly breeding healthy, ultra-strong Superfish. The killifish that live there are 30 to 50 percent likely to get liver cancer by the time they hit old age. They’re also more sensitive to oxygen deprivation and light-activated toxic chemicals than fish from clean sites. Finally, if you put these fish back in clean water, they’re more likely to get infections and diseases than fish that grew up in unpolluted areas.

Still, considering that a normal fish from a clean river wouldn’t last too long in the Elizabeth, these fish have clearly built up some special defenses. Di Giulio and his colleagues are trying to figure out exactly what those defenses are, and whether they’re caused by changes in the killifish’s genetic code, or by physiological adjustments to constant exposure to the pollution.

So far, it looks like it’s a little of both. When the scientists capture killifish from the Elizabeth River and breed them in the lab, they find that their offspring have some of the same strengths and weaknesses that their parents had, including a resistance to creosote and other pollutants. But by the time the grandkids come along, about half of that resistance disappears. That suggests that some of the resistance may come from genetic changes, but not all of it. Di Giulio suspects that exposure to heavy pollution from birth may also force an individual fish’s body to process certain chemicals differently.

If there is a genetic component, though, it could mean that pollution is actually changing the way these fish evolve. And while that may seem trivial for a little bait fish in a Virginia River, if you think of how many other unnatural substances humans release into the environment, the problem becomes much bigger.

Now try and answer these questions:

  1. What’s different about the killifish in the Elizabeth River?
  2. How can scientists figure out if the differences between these fish and normal fish are genetically based?
  3. Why is it worth finding out if the changes are genetic? Do you think that would affect the process of cleaning up the river, and restoring it to its original state?
  4. Can you think of other circumstances in which human activities might affect the evolution of a plant or animal species?

For Educators

CROAK is an online science mystery for grades 6-9, which asks why all the frogs in a town park are on the verge of extinction.

Read National Geographic’s article Fish Free Zone, about a huge area in the Gulf of Mexico where fish are dropping like flies.

NOVA Online’s Life in the Abyss has information about creatures that thrive in a punishing natural environment: deep-sea hydrothermal vents.

Natinal Geographic’s Rivers has lesson plans and activities relating to the way humans affect rivers.

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