How do cold-blooded animals survive subfreezing water temperatures as low as 27.1oF without literally being shattered by ice crystals? Salt water with a salinity of 35 ppt (parts per thousand), the average salinity of the open ocean, freezes at 28.5oF. As sea water freezes, the salt becomes more concentrated in the remaining unfrozen water. This makes Antarctic water extremely salty, more so than most of the world's oceans causing it to freeze at a lower temperature.
Marine invertebrates in the Antarctic have the same concentration of salt in their bodies as the water. Antarctica has a surprising variety of invertebrates. Scientist David G. Campbell, reporting in the November, 1992 issue of Natural History Magazine ("The Bottom of the Bottom of the World") says that taxonomists have identified 875 species of mollusks, 650 polychaete worms, 299 isopods, 100 pycnogonids (sea spiders), 129 tunicates, and more than 300 different kinds of sponges. Many of these invertebrates grow to gigantic proportions. The giant Antarctic isopod Glyptonotus antarcticus is 4 inches long. A giant Antarctic sea spider is the size of a human's palm. Both the isopod and the sea spider in other seas are easily-overlooked specimens no bigger than a fingernail.
In contrast, there are relatively few fishes. Only 120 species of fish out of 20,000 occur in Antarctic waters. Fishes' bodies have a lower concentration of salt than the ambient sea water, and their tissue freezes at about 31.7oF. If a fish just brushed against ice crystals in 28o sea water, the ice crystals would spread, and penetrate the fish's skin like a spear. Most invertebrates stay on the ocean floor, thus avoiding the floating ice, but a fish swims dangerously close to the pack ice. What's a fish to do to keep from freezing?
One of the strangest-looking, but most successful fishes in Antarctica is the ghostly-looking ice fish. Antarctic whalers called it the white crocodile fish because of its large mouth with many long teeth. Ice fish have a natural antifreeze that keeps them from freezing in Antarctic seas. "The antifreeze consists of glycopeptides, molecules made of repeating units of sugar and amino acids, which depress the freezing point of water 200 to 300 times more than would be expected from the physical properties of the dissolved substances alone." (Natural History Museum, November 1992)
The red blood cell-hemoglobin, doesn't carry oxygen well in low temperatures. Ice fishes don't have red blood cells. (This accounts for their deathly pallor.) Instead, ice fish have a large heart, wide blood vessels, and thin blood. These work well enough in cold water, but in temperate waters leave the fish sluggish and unable to compete with more energetic, red-blooded fishes. Therefore, the 16 species of ice fishes are confined to Antarctica.
See how the sea freezes
- Discuss how salinity changes affect marine animals. For example, lobsters die if kept in pens close to land during spring runoff. On the other hand, oysters can tolerate wide ranges of salinity as can other estuarine animals. Ask students, Does the sea ever freeze?
- Make up four solutions of different salt concentrations. Put a cup of fresh water in each of the four containers. Mix one tablespoon of salt into one container, two tablespoons of salt into another container, and three tablespoons of salt in another. The saltiest solution is representative of the Antarctic sea.
- Ask students to predict which container will freeze first and how long it will take. Record the students' predictions.
- Place the containers inside a freezer. Measure the temperature of the freezer.
- Take the temperature of each solution every 15 minutes.
- Graph the temperature and incremental time for each solution. Use a different color pencil for each solution.
- Note when each solution starts to freeze.
- Discuss how the students' findings relate to the purpose of putting road salt on icy roads in winter.
- Discuss how different our lives, economy and ecology might be if the coast of the Gulf of Maine froze the way the Antarctic coast does.
- Why is the Antarctic Ocean saltier than the Atlantic Ocean? (taste the ice that formed-there is no salt in it)
- Discuss how cold-blooded Antarctic animals such as invertebrates and fishes cope with the cold water.
What's left behind?
Place the different solutions of salt on a window sill and allow them to evaporate over a few days to show how much salt was in each container. Collect a cup of sea water from the Atlantic Ocean, and allow that to evaporate as a comparison.
Commercial extraction of salt
Many places extract salt from the sea. Investigate how and where it is done on a commercial scale.
- four small containers of equal size (i.e., single-serve yogurt containers)
- a thermometer
- a freezer
- a clock
- graph paper