The evolutionary changes that have kept fish swimming in the bitterly cold waters of the Arctic have led scientists to one the biggest quirks in biological development, report researchers at the University of Oregon.

New findings detailed in the online Early Edition of the Proceedings of the National Academy of Sciences explain the life-saving proteins in Antarctic notothenioid fish that keep ice crystals from expanding inside the fish's body also end up preventing those same ice crystals from melting, even when the surrounding environment is notably warmer.

Paul Cziko, a doctoral student in the UO Institute of Ecology and Evolution, along with co-author Arthur DeVries, professor emeritus of animal biology at the University of Illinois at Urbana-Champaign discovered antifreeze proteins -- first identified over 40 years ago -- help the fish avoid freezing by binding to ice crystals that have entered their bodies and preventing them from expanding.

"We discovered what appears to be an undesirable consequence of the evolution of antifreeze proteins in Antarctic notothenioid fishes," Cziko said in a press release. "What we found is that the antifreeze proteins also stop internal ice crystals from melting. That is, they are anti-melt proteins, as well."

The researchers determined that when they warmed the fish to temperatures above the expected melting point, some ice stayed frozen inside their bodies.

Ice that doesn't melt in such conditions is referred to as being superheated.

Cziko and DeVries next tested wild fish in Antarctica, during a period when the when the usually-freezing seawater had warmed a little during the summer, and they found no change: ice remained inside the fish in that region as as well.

Back in the lab, the team's added testing revealed the the antifreeze proteins were, paradoxically, responsible for the superheating effect.

"Our discovery may be the first example of ice superheating in nature," said co-author Chi-Hing "Christina" Cheng, a professor of animal biology at the University of Illinois.

The researchers suspect that the newly-discovered accumulation of ice inside the fish poses adverse physiological consequences -- but, so far, they're not sure what that would be.

"This is just one more piece in the puzzle of how notothenioids came to dominate the ocean around Antarctica," Cziko said. "It also tells us something about evolution. That is, adaptation is a story of trade-offs and compromise. Every good evolutionary innovation probably comes with some bad, unintended effects."