10,000 known species, but there may be as many as 4 million! Unknown number of species. These bacteria-like organisms were once thought to be rare. They are now estimated to be 50% of the species found in the open ocean — both the largest part of the ocean and the area that is still mostly unexplored. At least 60,000 known species. Though not a "true" group, these organisms are found in all habitats on Earth. Protists are the reason the ocean looks green near the shore. Of the estimated 1,500,000 species of fungi, scientists have described just 60,000. Only 500 species live in the oceans, where they prefer coastal environments. About 1,000,000 described species. Animals are unique life forms because they have nervous systems (except for sponges), which enable them to feel and touch. Over 250,000 described species. There are three kinds of plants: red, green, and brown. These three groups, although all plants, are not closely related. Green plants are common on land, and mostly red and brown plants live in the oceans.