Wrasse (Labridae is the family) are a very large group that are mostly carnivores feeding on invertebrates with smaller fish feeding on crustaceans and large ones on molluscs.  Wrasses generally have a cigar shape and are very common on reefs swimming in groups around coral heads.  This is the bluehead wrasse ( Thalassoma bifasciatum ) and is the supermale or sexually mature male.  The adults are not as colorful with alternating white and dark bands but the juveniles are bright yellow on top with a white belly and a dark dot on the top or dorsal fin.