What is anomalous expansion of water and why is it important?

In general, the volume of a liquid increases and it's density decreases with rise of temperature . But in the case of water some anomaly is noticed . If a quantity of water is heated from 0°C ,it's volume gradually decreases up to about 4°C. With further rise of temperature, the volume increases following general rule. The behaviour for liquid between 0°C and 4°C is found to be opposite to the general behaviour for liquids. Such thermal expansion of water is known as the anomalous expansion of water. As water occupies the smallest volume at 4° C, it's density is a maximum at this temperature. The anomalous expansion of water is greatly important in the survival of marine life in cold countries. During winter in these countries ,the water in the lakes, rivers etc. gradually falls to 4°C. On further cooling of the surface, the upper layer becomes lighter and therefore remains on the surface. The water on the surface thus cools to 0°C and freezes into ice ,but the water at the bottom remains at 4°C. The ice floats on the surface and ice and water being bad conductors of heat, practically no heat is lost due to conduction from the water below to the surroundings at a lower temperature through the ice. Thus water continues to exist below the surface layer of ice and it's temperature slowly increases downwards from 0°C to 4°C. Thus the lives of fish and other aquatic animals are saved.