The over 80 species of parrotfish have their jaw teeth fused into a solid, parrot like beak. They also have heavy pharyngeal teeth consisting of solid units of bone. These structures permit parrotfish to scrape algae and invertebrates from hard surfaces of the reef and then to crush the ingested material. They move about the reef in small schools during the day and hide in caves and crevices at night. Perhaps the most remarkable feature of parrotfish is their color patterns which change dramatically with age and sex ( which may also change), reflecting extremely complicated mating systems.