Survival of fish with antifreeze in Antarctica

A unique group of fish that has evolved to live in Antarctic waters thanks to anti-freeze proteins in their blood and body fluids is threatened by rising temperatures in the Southern Ocean, according to a new study by Yale. The development of antifreeze glycoproteins by notothenioids, a fish family that adapted to newly formed polar conditions in the Antarctic millions of years ago, is an evolutionary success story. The three species of fish are an example of the diversity this lineage achieved when it expanded into niches left by fish decimated by cold water environment. Now the same fish are endangered by warming of the Antarctic seas. "A rise of 2 degrees centigrade of water temperature will likely have a devastating impact on this Antarctic fish lineage, which is so well adapted to water at freezing temperatures," said Thomas Near, associate professor of ecology and evolutionary biology and lead author of the study published online the week of Feb. 13 in the Proceedings of the National Academy of Sciences. The successful origin and diversification into 100 species of fish, collectively called notothenioids, is a textbook case of how evolution operates. A period of rapid cooling led to mass extinction of fish acclimated to a warmer Southern Ocean. The acquisition of so-called antifreeze glycoproteins enabled notothenioids to survive in seas with frigid temperatures. As they adapted to vacant ecological niches, new species of notothenioids arose and contributed to the rich biodiversity of marine life found today in the waters of Antarctica. Notothenioids account for the bulk of the fish diversity and are a major food source for larger predators, including penguins, toothed whales, and seals. Notothenioids have evolved a variety of interesting physiological and biochemical adaptations that either permit survival in, or are possible only because of, the generally cold, stable seawater temperatures of the Southern Ocean. Although many of the Antarctic species have antifreeze proteins in their body fluids, not all of the Antarctic species do. Some sub-polar species either produce no or very little antifreeze, and antifreeze concentrations in some species are very low in young, larval fish. While the majority of animal species have up to 45% of hemoglobin (or other oxygen-binding and oxygen-transporting pigments) in their blood, the Notothenioids of the family Channichthyidae have only 1%. They can still flourish in part because of the high oxygen content of the cold waters of the Southern Ocean and in part because oxygen is absorbed and distributed directly by the plasma. These fish must expend twice as much energy in cardiac output per second than the notothenioids with higher hemoglobin concentration. The new study suggests the acquisition of the antifreeze glycoproteins 22 to 42 million years ago was not the only reason for the successful adaptation of the Antarctic notothenioids. The largest radiation of notothenioid fish species into new habitats occurred at least 10 million years after the first appearance of glycoproteins, the study found. "The evolution of antifreeze was often thought of as a smoking gun, triggering the diversification of these fishes, but we found evidence that this adaptive radiation is not linked to a single trait, but to a combination of factors," Near said. This evolutionary success story is threatened by climate change that has made the Southern Ocean around Antarctica one of the fastest-warming regions on Earth. The same traits that enabled the fish to survive and thrive on a cooling earth make them particularly susceptible to a warming one, notes Near.