All life on earth needs water for its survival. So the oceans play one of the most important roles for life on earth. Water on Earth is two-third of the Earth’s surface. Due to its depth, sea habitat covers about 300 times the habitable volume of the globe. About 230,000 species live in the sea. Also new species are being discovered almost every day and there doesn’t seem to be an end to this process. Sea animals are very strange and very different from the animals on land. They are very complex and hard to understand. Marine animals have a great deal of diversity, from the microscopic, like the amoeba to the huge whales which reach up to 98 feet in length. All types of life are found in sea, including birds (penguins), reptiles (sea turtles, sea snakes), invertebrates (jelly fish, shellfish), mammals (whales, dolphins), fungi, plants and of course the fish. Large areas beneath the ocean surface still remain unexplored. Marine life is very useful, providing food, medicine, and raw materials, in addition to helping to support tourism all over the world. Marine life helps determine the very nature of our planet. Many species are economically important to humans, including food fish. Also the well-being of marine organisms and other organisms are linked in various ways. Our knowledge regarding the relationship between life in the sea and life on the land is rapidly growing, with new discoveries being made nearly every day. These relationships include those of matter (such as the carbon cycle) and of air (such as Earth’s respiration). Marine organisms contribute significantly to the oxygen cycle, and are involved in the regulation of the Earth’s climate.