'Superheated ice' found in Antarctic fishes

Fish living in the freezing waters of the Antarctic have evolved a method to cope with ice building up in their bodies that actually increases the risk of their dying, according to a New Zealand-US study on Tuesday.

The study revealed an evolutionary paradox in the protective function of Antarctic fish antifreeze proteins, which came at the cost of actually increasing their internal ice burden, according to a University of Auckland scientist who took part in the study.

"What we have found is that antifreeze proteins not only stop ice from growing inside Antarctic fishes, but they also stop it from melting," Dr Clive Evans, of the School of Biological Sciences, said in a statement.

"The ice inside the fish is actually 'superheated'; that is, it remains stable above its expected melting point, and this is a direct consequence of the binding of antifreeze proteins to the ice crystal surface," said Evans.

"This is the first demonstration of the existence of ' superheated ice' inside living organisms, but it presents a significant problem for the fishes since the only known way for them to melt internal ice is through warming of the freezing seawater they inhabit."

The researchers from the universities of Auckland, Oregon and Illinois found in data going back more than a decade that seawater in Antarctica's McMurdo Sound had never reached temperatures that would melt all the superheated ice in the resident fish.

"Internal ice, protected by antifreeze proteins, seems a permanent burden for most fish in this area and is likely to prove lethal," said Evans.

"What we have here is an evolutionary paradox. The adaptive evolution of fish antifreeze proteins has enabled Antarctic fishes to survive in freezing seawater by stopping internal ice from growing, but at they same time by inducing a superheating capacity these proteins have reduced the opportunity for melting the ice burden, thus increasing the risk of dying."

The long-term temperature record from McMurdo Sound would help scientists study the responses of a wide range of Antarctic organisms as the waters warm in response to climate change.