The article below is by Matthew Genge, a research scientist at the Natural History Museum.
Fish on the slab might be happier than we think, says Matthew Genge
Tursday January 3, 2002
At the fish counter in your local supermarket, you will probably find an attractive display of fish laid out on a carefully prepared bed of ice. You would probably not be surprised to learn that this is not their usual habitat. Some fish, however, would be more at home amongst the jumble of ice cubes and even come pre-frozen straight from the wild.
In the freezing waters of the Antarctic, beneath the solid pack ice, fish abound and they contain tiny crystals of ice actually within their bodies. In virtually all other animals outside the polar regions, internal ice would spell certain death. The survival of fish in these icy waters is a testament to the power of biochemistry to adapt to even the most extreme conditions. It is also a demonstration that the chemical machinery of living things can be the key to an entire ecosystem.
The growth of crystalline ice within the cells of organisms is about as destructive a process as it is possible to imagine. When water freezes, the ice crystals that form are less dense than the original water and thus occupy a much larger space.
Place a milk bottle full of water in the freezer and you will see how much the water expands as it freezes by the column of ice that extrudes from the top of the bottle, then imagine that occurring within the cells of your body.
Within a cell the expansion of ice and the growth of crystals tears delicate structures such as the cell wall and the nucleus. This freezing damage cannot be repaired when the ice melts and consequently the cell dies.
Antarctic notothenioid fish can tolerate the presence of ice crystals within their bodies. This is lucky since, in the frozen waters of the southernmost ocean, seawater exists in a delicate balance between freezing and melting and ice crystals are everywhere. In their day-to-day lives, fish in the Antarctic eat and breathe the tiny crystals of ice that are always present in the water around them.
The water is always on the edge of freezing and the slightest change could cause ice crystals to grow and result in serious damage to tissues. Within Antarctic fish, ice crystals are found in the skin, stomach, gut and kidneys but are absent from the muscles, blood and the remaining internal organs, even though they should be present. Fish in the Antarctic have developed a biochemical arsenal to allow them to survive this deep freeze and prevent ice crystals from growing.
Dr Art Devries studies the freeze-resistance strategies of notothenioid fish caught in McMurdo Sound, Antarctica, to understand exactly how these animals survive the icy conditions. Notothenioid fish, it seems, have anti-freeze in their blood and tissues that allows them to survive in these chilly waters. Special proteins, known as anti-freeze glyco proteins, are secreted to prevent ice crystals from forming and growing where they are not wanted within the fish's blood and tissues.
The anti-freeze proteins work by smothering the ice crystals and, like all proteins, are complex folded ribbons made up of amino acid building blocks. The anti-freeze proteins within notothenioid fish are just the right shape and their atoms snugly fit those on the surface of the ice grains. The presence of the proteins on the surface of ice crystals blocks water molecules and prevents the ice from growing. In notothenioid fish the natural anti-freeze is so effective that Devries has cooled fish in his laboratory at McMurdo Station to -8íC with no apparent ill effects.
The anti-freeze fish of the Antarctic, which spawn in shallow waters, are locked to the icy continent by the expanse of the southern oceans. A few escapees from the Antarctic are, however, found in the coastal waters of New Zealand and were probably transported on ocean currents as larvae. These have largely lost their deep-freeze ability.
Because of their isolation, it is strange that fish at the opposite end of the world in the Arctic, such as the Arctic cod, share almost exactly the same anti-freeze proteins. There are, however, differences in the size of the proteins that Antarctic and Arctic fish use to keep the chill from freezing them solid.
The similarities, however, point to a common ancestor somewhere around 10 to 15 million years ago during the midst of the glaciations that covered much of the Earth's surface with ice. It has only been in the repeated warm periods between glaciations, the most recent of which has lasted 11,000 years, that the northern and southern polar fishes have been separated by the warm waters of low latitudes and have followed their own separate evolutionary paths.
Notothenioid fishes are leftovers from our planet's recent icy past: the mammoths of the oceans that, unlike the unfortunate pachyderm, have clung on in the coldest conditions the world has to offer. We should be glad they have pulled through.
In the Antarctic, fish are the key to the survival of virtually all large animals. If it weren't for their anti-freeze chemistry, the animal kingdom might not include delights such as the penguin, many species of seal and the killer whales that feed on them. So next time you are at the fish counter in the supermarket, if you see one that appears to be smiling on its bed of ice it is probably a pre-frozen notothenioid fish.