The butter hamlet or yellowtail hamlet, is a species of marine ray-finned fish, a sea bass from the subfamily Serraninae which is part of the family Serranidae, which also includes the groupers and anthias. It occurs in the western central Atlantic Ocean and occasionally makes its way into the aquarium trade. Hypplectrus unicolor has a deep body and head which is highly laterally compressed with a straight forehead and a rather short snout and a protrusible upper jaw. It has an angular preoperculum which has serrations on its edge and a number of small forward pointing spines on its lower margin close to the angle. The continuous dorsal fin has ten spines and 14-17 soft rays. It has long pelvic fins which extend as far as or beyond the anus. The caudal fin is slightly forked. The body is greyish white to yellow in colour with the back being darker, with a sizeable black saddle-like marking on the base of the caudal fin, this extends over the lateral line on each side. Horizontal, iridescent blue lines run along the head, occasionally these extend as far as front edge of the pelvic fins. There is an obvious blue line around the eye. Some individuals may show two black spots on the snout. The pelvic fins are yellowish in colour while the pectoral fins are transparent. This species attains a maximum total length of 13 centimetres (5.0 in). Hypoplectrus unicolor is found in the western Atlantic. It extends from Cape Canaveral in Florida south to the Bahamas and into the Gulf of Mexico as far west as Cedar Key and along the mainland coast of Mexico from Tuxpan to the northern Yucatan Peninsula and from northwestern Cuba south through the Caribbean Sea.
