In order to survive in frigid Arctic waters, certain fish species have evolved to keep their blood flowing rather than freezing. "Antifreeze"-like proteins pump through the veins of these notothenioid fish in order to prevent blood from freezing over, but with that special adaptation comes an interesting side effect: these fish have ice in their veins.

Yes, you read that right. Actual pieces of ice are continually pumped throughout the veins of these Antarctic fish, pieces of ice that never melt due to the antifreeze proteins. While those proteins keep the blood from entirely freezing, they also prevent small amounts of ice in the bloodstream from ever melting.

The news comes from a study recently published in the Proceedings of the National Academy of Sciences. Scientists made the discovery after a team experimented with warming the fish, only to find that some ice crystals refused to melt when brought several degrees above freezing point. Researchers also gathered fish from warmer waters and discovered that they too had ice crystals.

Roughly 90 percent of fish in the world's southern oceans are among one of five families of notothenioid fish. Scientists believe these antifreeze proteins to be responsible for the fish reigning supreme in the region, which allows the fish to live in water temperatures of negative two degrees Celsius. 

So far the team conducting the research has yet to find any real negative side effects of the fish having icy veins, but University of Illinois professor Chi-Hing Cheng says that the ice crystals could potentially build up and block small blood vessels. Scientists do know that the ice crystals accumulate in the spleens of the fish, which leads the researchers to believe that this particular type of Arctic fish have some kind of mechanism in the spleen to remove the ice from circulation. 

Cheng says the discovery is an example of the imperfect nature of evolution. 

"This is just one more piece in the puzzle of how notothenioids came to dominate the ocean around Antarctica... 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," Cheng says. 

In order to obtain all the fish for study, researchers had to go fishing in the oceans of Antarctica by drilling holes in sheets of ice in sub-zero temperatures. It thankfully looks the payoff for all their legwork was well worth it.