Antarctic fish 'make antifreeze' in pancreas

Seawater freezes at -1.91C and fish fluids at -1C. Picture / RNZAF.

A New Zealand researcher and two American colleagues have solved the mystery of how Antarctic fish manufacture the "antifreeze" which allows them to live in icy waters.

Clive Evans, a professor of molecular genetics and development at the University of Auckland, and two researchers at the University of Illinois, Paul Cziko, and Chi-Hing "Christina" Cheng, found the antifreeze glycoproteins (AFGP) originate primarily from the exocrine pancreas and the stomach.

The work was done on fish in New Zealand's Ross Sea at McMurdo Sound and Terra Nova Bay.

AFGPs were first documented in Antarctic notothenioid fish 35 years ago by Arthur DeVries of the University of Illinois.

"Ever since the discovery of the antifreeze proteins, it was wrongly assumed to be they had to be produced in the liver, since the vertebrate liver is well known as a source of secreted plasma protein, so there was no reason to think otherwise," Professor Cheng said in a statement.

"It turns out that the liver has no role in the freezing avoidance in these fishes at all."

Instead, antifreeze glycoproteins originate primarily from the exocrine pancreas and the stomach, the three researchers said in a paper released online ahead of regular publication in the Proceedings of the National Academy of Sciences later this week.

The exocrine pancreas is the larger of the two parts that make up the pancreas. It consists of glands that primarily manufacture and secrete digestive enzymes that break down food in the intestine so it can be absorbed.

In this case, AFGPs are secreted into the intestine, where they protect the intestinal fluid from being frozen by ice crystals that come in with seawater and food. Internal fluids in notothenioids are about one-half as salty as seawater. From the intestine, the AFGPs are, apparently, absorbed into the blood.

While seawater reaches its freezing point at -1.91degC, fish fluids freeze at about -1degC, and the species studied live in water that rarely rises above the freezing point and is regularly filled with ice crystals.

The researchers also studied a variety of fish from Arctic waters that express antifreeze in their livers, and found all of them also express antifreeze in the pancreas.

The research team made another surprising discovery earlier this year, when it found that survival of the hatchlings of Antarctic fish with "antifreeze" in their blood relied on slow development of gills.

Larval fish of at least two species of the antifreeze fish did not have the antifreeze from the time they hatched, instead developing it over about three months.

The average freezing point of the larval fish fluids was about -1.3degC, yet the fish hatched into water at almost -2degC: Theoretically, their internal fluids should freeze instantly and the baby fish die. The researchers showed the gills of all three species were undeveloped at hatching, reducing the risk of ice crystals getting into the fish.

- NZPA