Several species of wild Antarctic fish produce their own “antifreeze” proteins that bind to ice crystals and stop them from growing in the blood. While this helps fish survive in icy waters, those same proteins also stop ice crystals from melting even when temperatures warm up, according to a new study published in Proceedings of the National Academy of Sciences this week. The build-up of protein-stabilized ice crystals in their bodies could damage tissues during warmer conditions.
Five families of notothenioid fishes inhabit the frigid Southern Ocean and make up more than 90 percent of the fish biomass of the region. In the 1960s, researchers described how proteins bind to ice crystals in the blood to prevent these fish from freezing, allowing them to dominate the ocean around Antarctica. But, “adaptation is a story of trade-offs and compromise,” says Paul Cziko from the University of Oregon. “Every good evolutionary innovation probably comes with some bad, unintended effects."
Cziko and colleagues wanted to see if the ice crystals bound to antifreeze proteins inside fish would melt during warmer seasons. They measured seawater temperatures for 11 years using temperature-logging devices in McMurdo Sound, one of the world's coldest Antarctic fish habitats. They also tested the antifreeze proteins in the lab.
"We discovered what appears to be an undesirable consequence of the evolution of antifreeze proteins in Antarctic notothenioid fishes," Cziko says in a news release. "The antifreeze proteins also stop internal ice crystals from melting. That is, they are anti-melt proteins as well."
A decade is a substantial portion of the notothenioid lifespan, and not once in that time did temperatures increase enough to overcome the anti-melting effect and rid the fish of their internal ice completely. Even those swimming in relatively warmer Antarctic summer waters -- at temperatures where they would be expected to be free of ice -- had ice crystals. When the researchers warmed them up to temperatures above the expected melting point, some internal ice crystals still failed to melt. When ice doesn't melt at its normal melting point, it's called "superheated," and this work may be the first example of ice superheating in nature.
The researchers suspect that the accumulation of ice could be dangerous and possibly even fatal, but no adverse physiological consequences have been discovered yet. If the fish are destined to carry ice crystals around all their lives, ice particles could obstruct small capillaries or trigger undesired inflammatory responses, says study coauthor Chi-Hing Christina Cheng of the University of Illinois, Urbana-Champaign. The potential danger is like the threat posed by asbestos in the lungs or blood clots in the brain, Cziko explains. But since a lot of the ice accumulates in the spleen, maybe the fish have a mechanism to clear it from circulation.
Images: Paul Cziko (top), Elliot DeVries (middle)