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updated 2/17/2011 10:46:18 PM ET 2011-02-18T03:46:18

In the mucky bottom of the Hudson River dwell abundant Atlantic tomcod -- brownish, mottled fish a bit longer than a hand. There, too, can be found some of the highest concentrations of pollutants in the United States, the remnants of nearly 30 years of dumping from General Electric plants upriver.

These tomcod have evolved to survive their inescapable lot of swimming in the nation's biggest Superfund site, finds new research, published online today in Science.

Every one of the fish tested in the river now carries at least one copy of a mutation that reduces its susceptibility to polychlorinated biphenyls (PCBs) and dioxins, allowing it to live where tomcod from cleaner sites would almost surely die.

Around 30 years ago, around the time PCB dumping in the Hudson halted, researchers found that 94 percent of two-year-old tomcod collected in the Hudson carried a type of liver tumor. Isaac Wirgin began studying tomcod then.

"I started working on these fish with the hypothesis that they would be very sensitive to the toxic effects of PCBs," said Wirgin, now a professor at the New York University School of Medicine. "But the more work we did in controlled lab studies, we found that they were highly resistant to the toxic effects of PCBs and dioxin."

While it might seem like good news that these fish are surviving contaminated conditions, it probably isn't, researchers say.

"I don't think we should take any solace whatsoever in the fact that they can evolve resistance," said Jeffrey Levinton of Stony Brook University.

For one thing, he and others said, the fish are probably paying a price for the mutation, something like slower growth or less resistance to other stresses.

Also, the surviving fish are moving contaminants into the food chain.

"They serve as a prime prey for striped bass," Wirgin said. "You've got this fish that would normally be dead from PCBs or dioxin. It's alive and it's carrying around all this PCB and dioxin and it gets eaten."

The Genetic Culprit

In the new study, the researchers delved into the tomcod genome, identifying a genetic mutation responsible for the fish's resistance and showing how it functions.

The culprit is a gene for a type of molecule in fish cells (called a receptor) that, in tomcod without the mutation, binds to the toxic molecules, ultimately triggering a cascade of cellular reactions that cause big problems.

But the mutated gene in the Hudson tomcod reduces the binding of pollutant molecules to the receptor by fivefold, which lessens the toxic effects on the fish.

The researchers found that every Hudson tomcod they tested carried at least one copy of this mutated gene, and 95 percent had two copies of the mutated gene. Only 5 percent of fish in the nearest neighboring populations in Long Island and Connecticut had a copy of the mutant version.

"These are the guys that are most exposed," Wirgin said of the Hudson fish. "They are the ones that had to develop resistance."

Wirgin suspects that there are more animals out there like the tomcod.

 "I don't know what percent of species in a highly contaminated spot are going to be resistant," he said. "It's probably not that rare of an occurrence."

© 2012 Discovery Channel

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