You can take on missions to get rewards of gold and gems. You will always have 3 missions available for you to complete. Explore the available missions by tapping on the first 3 buttons at the bottom of the World Screen. Some missions can only be completed on specific levels. Look at the mission information boxes on the right to know where to go, select the right location and dive in! Missions you cannot complete on the level currently selected will be greyed out, while missions you can complete on that level will be blue. Some of the mission types you'll encounter are reaching high scores, hunting for specific prey and survival time. Some have to be completed in a single go whereas others you can completed over several go's.