In what way is the anomalous behaviour of water advantages to aquatic animals

Anomalous behaviour of water: On heating water contracts from 0°C to 40C. It means volume decrease and density increases from 0°C to 4°C.

Water has maximum density at 4°C. 

In cold countries during winter, the temperature is very low as lake cools towards 4°C, water near the surface becomes denser at 4°C and sinks to the bottom. Less dense water near the bottom rises. When lake cools towards 0°C, water freezes and forms ice layer. Since ice is a bad conductor of heat, it keeps the water at the bottom of lake still at 4°C. Hence aquatic animals like frogs, fish can survive in severe winter.