Antarctic Fish Have Antifreeze Blood and Ice Crystals in Their Bodies

Antarctic fish may swim in freezing waters, but they don't freeze themselves. Now, scientists have taken a closer look at why this is, uncovering "antifreeze" proteins that allow the fish to survive in the icy Southern Ocean. Yet there's an unfortunate side effect of this antifreeze; it turns out that protein-bound ice crystals that accumulate in the body resist melting even after temperatures warm.

"We discovered what appears to be an undesirable consequence of the evolution of antifreeze proteins in Antarctic notothenioid fishes," said Paul Cziko, the lead researcher, in a news 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."

A total of five families of notothenioid fishes inhabit the Southern Ocean.  In fact, they make up more than 90 percent of the fish biomass in the region due to the fact that they're some of the only types of fish that can survive under the icy conditions present in the ocean. Yet despite this amazing antifreeze ability, the fish seem to have other complications.

In order to understand this antifreeze ability a bit better, the researchers examined whether antifreeze protein-bound ice crystals inside the fish would melt as expected when temperatures warmed. Despite being warmed to above the expected melting point, though, the crystals failed to melt. Ice heating above its melting point is referred to as "superheated."

While the researchers believe that the accumulation of ice crystals could have adverse physiological consequences, so far none have been discovered. If the fish are destined to carry these crystals in their bodies throughout their lives, it's possible that ice particles could obstruct small capillaries or trigger inflammatory responses.

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

The findings are published in the journal Proceedings of the National Academy of Sciences.