If you’re going to live in the icy waters of the Antarctic, you’re going to want something to keep your blood from turning into solid ice. According to researchers from the University of Illinois, Antarctic notothenioid fishes have a protein in their blood that does just that.

However, in a weird evolutionary twist, the same protein that keeps ice crystals from forming may also keep them from melting, leading to health issues over time.

“We discovered what appears to be an undesirable consequence of the evolution of antifreeze proteins in Antarctic notothenioid fishes,” said University of Oregon doctoral student Paul Cziko, who led the research with University of Illinois animal biology professors Chi-Hing “Christina” Cheng and Arthur DeVries. “What we found is that the antifreeze proteins also stop internal ice crystals from melting. That is, they are anti-melt proteins as well.”

When water remains in frozen form despite being heated to temperatures well above 32 degrees Fahrenheit, it’s known as “superheating.” The researchers believe this may be the only known instance of superheated water in nature.

The problem with non-melting ice crystals is that they don’t exactly belong in a living organism’s bloodstream. Over time, the crystals could accumulate and cause serious damage. How the fish survive this is unknown, but scientists believe they have a mechanism for clearing the ice from the spleen, where they accumulate.

“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.”