Near Antarctica, the seawaters are congested with ice. And so are the local fish, as they live out their lives with ice crystals flowing through their bloodstream. How do they live with ice in their veins? As it turns out, they make their own antifreeze – in the form of specialized proteins found in their blood. These antifreeze proteins prevent excessive ice build-up, but research has found that they come with a cost.
About 45 years ago, anti-freeze proteins were discovered in notothenioid fishes, the dominate group of fish species around Antarctica. These proteins bind to small ice crystals in the blood and, in the process, prevent them from growing into larger ice crystals that could damage tissue. This was a necessary evolutionary adaptation for survival in freezing temperatures. Similar proteins have been found in other cold weather dwellers as well, including insects and other fishes. Just last September, however, scientists discovered that these specific antifreeze proteins also prevent ice crystals from melting when the seawater warms.
“We discovered what appears to be an undesirable consequence of the evolution of antifreeze proteins in Antarctic notothenioid fishes,” says Paul Cziko, lead researcher on the study. “What we found is that the antifreeze proteins also stop internal ice crystals from melting. That is, they are anti-melt proteins as well.” Cziko, a doctoral student from University of Oregon, performed the research under University of Illinois animal biology professor Chi-Hing Christina Cheng, with co-authors Clive Evans and Arthur DeVries. DeVries first discovered these antifreeze proteins in 1969.
The researchers were interested in what happens to the ice in the fish over time, and whether it actually melts away in the warmer season. In their research, the scientists looked at whether the ice crystals persisted when the fish were swimming in water that was above the usual melting temperature. They found that the “superheated” ice did not melt at the higher temperature. This was a surprising finding that had previously been demonstrated in the lab but not in nature - until now.
The other major contribution from this story is the thorough temperature recording data over a span of 11 years from the seawaters in the McMurdo Sound, Antarctica where these fish reside. This information was used to conclude that the fish rarely encounter temperatures warm enough to actually melt the ice in their bodies, but it is also useful information for monitoring changes in seawater temperatures in the future.
What happens to the accumulating ice? The researchers did not observe any obvious negative physiological effects, so there may be some yet-to-discovered biology at play. Ice is often observed accumulated within the fish spleen, so maybe it is removed from circulating blood and sequestered within that organ. It also remains a question as to whether the ice ever fully melts, or whether it remains for the life of the fish. Why did the fish evolve this trait if it has an undesirable consequence? “This is just one more piece in the puzzle of how notothenioids came to dominate the ocean around Antarctica,” says Cziko. “That is, adaptation is a story of trade-offs and compromise. Every good evolutionary innovation probably comes with some bad, unintended effects.”
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