Arrau turtles are the largest river turtles in South America. It has a broad, domed and streamlined carapace for active swimming in moderate river currents. The colour may be influenced by the algae that is attached to it, but is usually olive green to brown in colour. It has a long neck which can be withdrawn horizontally within the shell, leaving it partly exposed, rather than retracting it in a vertical ‘S’ bend as in most other turtles.