The gag grouper is typical among the groupers with an oblong-shaped elongate body. The head is long while the mouth is large with a protruding lower jaw. The bases of the dorsal and anal fins are covered with scales and thick skin. The caudal fin is large and has a slightly concave margin. This grouper is often confused with the black grouper (M. bonaci), however it is has some distinguishing characteristics. These include the shape of the caudal fin - the gag grouper has a slightly concave margin along the posterior edge of the caudal fin while the black grouper has square-shaped caudal fin. The preopercle (a boomerang-shaped bone whose edges form the posterior and lower margins of the cheek region; the most anterior of the bones comprising the gill cover) is angular and slightly notched with a distinct lobe. This characteristic also helps to distinguish the gag grouper from black grouper which has a gently rounded preopercle. Body color of the gag grouper is dependent upon the sex and age of the fish. Juveniles and mature females are pale to brown-gray with dark blotches and worm-shaped markings resulting in a marbled appearance. The caudal, anal, and pelvic fins have dark black-blue outer margins. Inactive individuals sometimes display a camouflaged pattern with dark brown "saddles" separated by white bars just below the dorsal fin. Large mature males are pale to medium gray in color with barely visible reticulations below the dorsal fin. The ventral surface is darker gray to black in color. The soft dorsal fin, caudal fin, pectoral, and pelvic fins are also dark gray to black while the margins of the anal and caudal fins are white. Individuals may exhibit a darker phase in which the posterior of the body, penduncle, soft dorsal fin, and anal fins are black in color. The gag grouper is often confused with the black grouper, however it may be distinguished based on the color of the fin margins. The caudal fin of the gag grouper has white margins on the anal and caudal fins while the black does not.