There are over 30 species of dolphins found worldwide. They live mainly in saltwater oceans and seas, though some species can live in freshwater. Dolphin species vary in size, coloration, fin shape, and skull structure. The most common species is the bottlenose dolphin, known for its intelligence. River dolphins live in freshwater habitats in South America and Asia. The rough-toothed dolphin prefers deep waters and lacks a melon crease found in other dolphins. Species identification can be determined by characteristics such as the Risso dolphin's square head or the striped dolphin's blue and white stripes.