Antifreeze proteins might be antimelt, too. (Paul Cziko)

In the icy waters of the Antarctic, most of the native fish have special proteins in their blood that act like antifreeze. The proteins bind to ice crystals, keeping them small to prevent the formation of fish popsicles. New research in the Proceedings of the National Academy of Sciences reports that these antifreeze crystals have one downside: They don't melt when temperatures warm.

In the relatively warm waters of the Antarctic summer, the researchers found, the fish still had ice crystals in their blood. When ice stays solid above its usual melting point, it's "superheated" — and this may be the first example of the phenomenon observed in nature. So in addition to keeping all of the fish's liquids from freezing, the proteins were keeping those controlled ice crystals from melting.

If the ice crystals don't disappear in the summer, where do they go? That remains unclear. It's possible, the researchers report, that a life-long accumulation of ice crystals could be dangerous, or even fatal — but for n0w, they haven't observed any obvious effects.

It does make sense that there would be some downsides to an ever-increasing number of antifreeze crystals in ones blood. If the fish really do spend their entire lives fighting to keep the crystals at bay, the researchers wrote in their study, it would be a fascinating evolutionary tale: This adaptation had made the family of fish great at surviving in Antarctica compared to other species, but might cause a host of health problems in return.