Antarctic fish are capable of producing a type of antifreeze in their systems that keeps them from freezing, but has detrimental effects on the animal, as well.

Proteins found in many species of fish are able to attach to ice crystals in the bloodstream, preventing the tiny crystalline structures from clustering. However, these small crystals also stay solid, even at temperatures that would normally melt them, due to the actions of the chemicals.

"We discovered what appears to be an undesirable consequence of the evolution of antifreeze proteins in Antarctic notothenioid [ice-loving] fishes. What we found is that the antifreeze proteins also stop internal ice crystals from melting. That is, they are anti-melt proteins as well," Paul Cziko, a doctoral student at the University of Oregon and co-leader of the study, said.

Researchers investigated how ice crystals were affected when temperatures rose enough so that the tiny structures should have melted. They found many of the minuscule structures stayed together, even when warmed, an effect known as "superheated ice." This may be the first time this process has ever been seen in nature, according to investigators.

The Southern Ocean, surrounding Antarctica, is home to five families of notothenioid fish, which make up over 90 percent of the fish biomass in the waterway.

A specially designed underwater thermometer was placed at McMurdo Sound in the ocean, where it measured temperatures for 11 years, longer than a typical lifespan for the animals. Researchers found that the water never grew warm enough during those years to melt ice in the bloodstreams of the animals. Researchers believe these particles of ice could cause blockage of blood vessels in the fish, similar to the way blood clots affect human beings.

"Since much of the ice accumulates in the fishes' spleens, we think there may be a mechanism to clear the ice from the circulation," Cziko stated.

Arthur DeVries, a biologist at the University of Illinois, was the first person to discover the proteins in notothenioid fish during the late 1960s. He was also the first scientist to accurately measure their actions, as they bind to tiny ice crystals in the animals.

Researchers believe the danger of bloodstream blockage could be a drawback of an otherwise successful adaptation to the frigid marine environment. However, no such effect has yet been witnessed in the animals.

Study of Antarctic fish and how they are affected by antifreeze in their skin was detailed in the journal Proceedings of the National Academy of Sciences.