Antarctic notothenioid fishes are protected from freezing by antifreeze proteins (AFPs) that bind to invading ice crystals and inhibit their growth. Paradoxically, accumulation of AFP-stabilized ice could be lethal. Whether and how fishes eliminate internal ice is unknown; one hypothesis is that it melts during summer warming episodes. However, prior in vitro evidence indicates that AFPs also inhibit melting. Our study establishes that pronounced melting inhibition occurs in vivo (i.e., superheated ice occurs inside notothenioid fishes). Our long-term temperature record of a high-latitude Antarctic fish habitat indicates that summer warming does not overcome AFP-induced superheating to reliably rid fishes of ice. Evolution of the life-saving AFPs exacts a cost: the risk of lifelong accumulation of damaging internal ice crystals. 
 
 Abstract 
 
 Antifreeze proteins (AFPs) of polar marine teleost fishes are widely recognized as an evolutionary innovation of vast adaptive value in that, by adsorbing to and inhibiting the growth of internalized environmental ice crystals, they prevent death by inoculative freezing. Paradoxically, systemic accumulation of AFP-stabilized ice could also be lethal. Whether or how fishes eliminate internal ice is unknown. To investigate if ice inside high-latitude Antarctic notothenioid fishes could melt seasonally, we measured its melting point and obtained a decadal temperature record from a shallow benthic fish habitat in McMurdo Sound, Antarctica. We found that AFP-stabilized ice resists melting at temperatures above the expected equilibrium freezing/melting point (eqFMP), both in vitro and in vivo. Superheated ice was directly observed in notothenioid serum samples and in solutions of purified AFPs, and ice was found to persist inside live fishes at temperatures more than 1 °C above their eqFMP for at least 24 h, and at a lower temperature for at least several days. Field experiments confirmed that superheated ice occurs naturally inside wild fishes. Over the long-term record (1999–2012), seawater temperature surpassed the fish eqFMP in most summers, but never exceeded the highest temperature at which ice persisted inside experimental fishes. Thus, because of the effects of AFP-induced melting inhibition, summer warming may not reliably eliminate internal ice. Our results expose a potentially antagonistic pleiotropic effect of AFPs: beneficial freezing avoidance is accompanied by melting inhibition that may contribute to lifelong accumulation of detrimental internal ice crystals. ||||| EUGENE, Ore., Sept. 23 (UPI) -- Fish species that make their home in the frigid waters of Antarctica have developed special antifreeze proteins that keep their blood from icing over. But evolution is a strange game in which seemingly every adaptive trait comes with a trade-off, and for the fish of Antarctica that trade-off is quite the paradox. 
 
 For the benefit of avoiding frozen blood, Antarctic notothenioid fish species must tolerate ice in their veins. That's right, the same antifreeze proteins that latch onto ice crystals and prevent them from growing also inhibit them from melting. 
 
 A team of marine biologists discovered the odd trade-off while studying the coping mechanisms of notothenioid species living in supremely cold conditions. They discovered the antifreeze proteins' anti-melting properties after raising water temperatures slightly above the freezing point and still finding some ice in the blood of the fish specimens. 
 
 "We discovered what appears to be an undesirable consequence of the evolution of antifreeze proteins in Antarctic notothenioid fishes," study author Paul Cziko, a doctoral student the University of Oregon's Institute of Ecology and Evolution, said in a press release. 
 
 If water temperature rises more than two or three degrees above the freezing point, these ice crystals are likely to melt. But Cziko and his colleagues at the University of Illinois found that ocean temperatures in the Antarctic rarely ever rose high enough to help clean the species icy veins. That means these fish carry ice crystals around with them for most of their lives -- a risky proposition that could help them stay cool under pressure but could also cause a life-threatening traffic jam inside small capillaries. 
 
 "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 added. "This is just one more piece in the puzzle of how notothenioids came to dominate the ocean around Antarctica." 
 
 The study was published in the Proceedings of the National Academy of Sciences on Monday. ||||| Scientists long have known that antifreeze proteins help a species of Antarctic fish survive in frigid waters. Now research has found that these proteins also prevent some fish from ever ridding themselves of the ice in their bodies. 
 
 The notothenioid fish uses evolutionary mechanisms to survive in anchor ice that exists on the sea floors of the Antarctic region. 
 
 Even after being studied for decades, the notothenioid, a bony fish that thrives in frigid waters, continues to hold surprises. 
 
 Notothenioids own the waters circling Antarctica known as the Southern Ocean; one of the few species that can survive in seawater temperatures as low as 29 degrees Fahrenheit, it makes up more than 90 percent of the fish biomass in the region. It's no wonder, then, that scientists began to investigate how these animals thrive in this icy environment without freezing over. 
 
 Scientists have been revealing startling facts about the notothenioid for more than four decades now, including the discovery, in the 1960s, of antifreeze proteins inside these fishes, a vital tool for their survival. But according to a new study published in the journal Proceedings of the National Academy of Sciences, researchers have revealed that these proteins may not be as helpful as they were once thought to be. The proteins do indeed help prevent water from freezing in the fish, but it turns out they also prevent ice from melting. 
 
 "We've known for a long time that the antifreeze proteins are at the essence of how these fishes survive in their freezing environment. But we're still working out the details, and this is one of those details," Paul Cziko, lead author of the paper, told the Monitor. 
 
 Mr. Cziko, a doctoral candidate at the University of Oregon, worked with Christina Cheng and Arthur DeVries, a husband-and-wife team of animal biologists at the University of Illinois. Devries revealed in the late 1960s that antifreeze proteins existed in the bodies of notothenioids. In the 1990s, he discovered that the fishes were also carrying ice crystals in various organs, such as the gills and the spleen. 
 
 In this new study, Cziko's team tested fish in the lab by warming them to a point at which the ice in their bodies completely melted. These were temperatures, though, that the notothenioid never experiences in its own environment. They also examined temperature data logged continuously over the last 11 years in McMurdo Sound, Antarctica. 
 
 The research group found that the ice crystals didn't melt at their expected melting point of about 30 degrees Fahrenheit, but instead remained intact. The fish continued to contain ice even after researchers raised the temperature further another 1.8 degrees Fahrenheit, and this was primarily due to the antifreeze protein. 
 
 Though the life-saving protein was given the name "antifreeze," it works differently from, say, the liquid you mix with your windshield wiper fluid, according to Cziko. Rather than acting like a salt and lowering the freezing point, the notothenioids' antifreeze proteins bind themselves to the ice crystals in their bodies and alter the surface of the crystals so that they have a hard time growing any larger. 
 
 This mechanism doesn't just make it very hard for the ice crystals to grow, though. The proteins bind themselves so tightly that they also make it difficult for the ice to get any smaller, even in above-freezing temperatures. 
 
 "When people are studying antifreeze proteins they're looking at the ability of the organisms to deal with cold temperatures and not warm temperatures," says Cziko. "All their focus is towards 'How much does it protect against freezing?' and people just kind of missed that there could also be an anti-melting effect that's important." 
 
 In controlled settings, scientists have been able to heat ice above its melting point without having the ice melt. But Cziko says this may be the first example of "superheating" found in nature. 
 
 So what does this superheated ice mean for the fish? Even in the slightly warmer summer months when the notothenioid has a chance to thaw out, some ice crystals linger. 
 
 Whether or not there are any adverse effects on the fishes' bodies is still unknown. The long-term cost of having the protein, Cziko says, is a question for further research. 
 
 "If they can tolerate it, maybe they've evolved some other mechanism to deal with these ice crystals inside their bodies," says Cziko. "And maybe in the long-term it does kill the fish and they're not that good at dealing with it." 
 
 The highest concentration of ice crystals resided in the fishes' spleens. Cziko says it's possible that the spleen is trying to filter these ice crystals out of the blood to prevent their causing problems in other parts of the body. 
 
 Cziko also says research on this particular evolutionary puzzle provides us with a better understanding of the progression of all species. 
 
 "This shows that with every good evolutionary innovation, some potentially bad or unintended consequences come along with that," says Cziko "Evolution is a stepwise process. You take two steps forward, and take one step back."
– How do the fish that thrive in the waters around Antarctica prevent their blood from turning to ice? Turns out at least some of them don't. Scientists have long known that the group of fish species known as notothenioids have an antifreeze protein in their blood that prevents them from being frozen to death, but new research has revealed that the same protein keeps ice crystals in their veins, apparently permanently, UPI reports. The crystals melted at a few degrees above freezing, but the fish may spend their whole lives without encountering temperatures that warm, according to research published in the Proceedings of the National Academy of Sciences. The antifreeze proteins that prevented the blood from completely freezing also bound itself to the ice crystals, keeping them from getting smaller, the scientists say. Lead researcher Paul Cziko tells the Christian Science Monitor that it's not clear whether the fish have evolved some way of dealing with the ice crystals in their bodies, or if the ice ends up killing them in the long term. "This shows that with every good evolutionary innovation, some potentially bad or unintended consequences come along with that," he says. "Evolution is a stepwise process. You take two steps forward, and take one step back." (Researchers recently solved the decades-old mystery of a quacking noise detected in Antarctic waters.)