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.
At the time of breeding, the male goes to the rocks and cleans a small area. He then starts a courtship to attract females. The females deposit their eggs which adhere to the substrate.
The male fertilises them and guards the nest, taking care to aerate the eggs until they hatch after a week. During this period, the males have a bluish background colour.
Juvenile sergeant major may hold cleaning stations where they deworm other species.
