One reason fish are so diverse is that 70 percent of the planet is covered in water. The animals in this group live in a variety of habitats ranging from coral reefs and kelp forests to rivers, streams, and the open ocean. Another is that fish are very old on the evolutionary scale. According to fossil records, they have been on Earth for more than 500 million years! The total number of living fish species—about 32,000— is greater than the total of all other vertebrate species (amphibians, reptiles, birds, and mammals) combined.