Description:
The gag, Mycteroperca microlepis is an oblong, robust grouper that may reach 38 inches in length and 50 pounds or more.  The head is long and convex in profile, with the lower jaw projecting beyond the upper.  Canine teeth are well developed on both the jaws.  Scales are ctenoid and smooth, except for those on the pectoral fins.  Fishes over 15 inches develop a rounded lobe at the angle of the preopercule that bears large serrae.  In adult fishes, the posterior nostrils are larger than anterior ones.  The dorsal fin is rounded and continuous, with 11 spines in the anterior portion and an interspinous membrane that is deeply incised.  The soft dorsal has 16-18 soft rays.  The rounded anal fin has 3 spines and 10 – 12 soft rays. 

Body color varies based on sex, age, and activity level.  Adult females and juveniles are typically a pale gray to brown-gray with darker blotches and wormlike markings that lend a marbled appearance to the dorsal surface and sides.  The pelvic, anal and caudal fins have black-blue outer margins.   In resting fishes, 5 dark brown “saddles” are separated by white bars below the dorsal fin.  Large adult males are generally pale to medium gray in color, with faint reticulations below the dorsal fin.  The ventral surface is darker gray or black, as is the margin of the soft dorsal fin, caudal fin, and posterior margins of the pectoral and pelvic fins.  Some also exhibit a “black-back” phase in which the posterior body, dorsal caudal peduncle, and all of the soft dorsal and anal fins are black. 

Habitat:
Eggs and early larval stages of the gag are found in the offshore water column.   On becoming juveniles, young fish enter estuaries and inhabit seagrasses, salt marshes, oyster reefs, and mangrove creeks where they remain for 3-5 months.  As they grow, later juveniles migrate to offshore reefs and ledges.  Adults are typically found in offshore reefs and hardbottom areas, shipwrecks, coral reefs, and rock ledges in depths of  100 - 329 feet. 

Similar Species:
Juveniles under 16 inches that have not yet developed the rounded lobe on the preopercule are often confused with a related species, the black grouper (Mycteroperca bonaci).  However, the gag and black grouper are easily distinguished based on a scale count along the lateral line:  Black grouper have 78-83 lateral line scales, while the gag has 88-89. 

Range:
Gag range from North Carolina south through Florida, the Gulf of Mexico, Yucatan Peninsula, as far south as Brazil.  Juveniles have been observed as far north as Massachusetts and New York.  Gag are rare in Bermuda, and are generally absent from the West Indies.