During the day, the sergeant major lives in shoals of several hundred individuals, at a depth of 0 to 15 m, above the rocky bottom. It takes refuge at night in cavities. Juveniles are found closer to the surface, near floating objects, in mangroves or small lagoons.
The sergeant major feeds mainly on zooplankton that it captures in the open water. During the periods when they are close to the bottom, they ingest algae and small invertebrates.