Damselfish differ from most other families in having only a single nostril on each side of the snout. The sergeant major's range extends from the Gulf of California to Peru, including offshore islands. They are abundant in the Gulf of California, though their number is reported to be reduced during winter months in the Bay of La Paz. During the spring and summer, the male clears a nest site of algae and other growth from a rocky surface in preparation for the female to lay eggs. He fertilizes them and guards them until they hatch.
Females reproduce with several males using several nest sites during a single breeding season.
The Ocean Oasis film features a male sergeant major swimming back and forth over the nest. The eggs look like purple patches over the rock. It is amazing to observe the energy of this fish, constantly chasing off predators to protect the eggs. It seems like an endless task. The male spends weeks guarding the nest until the eggs hatch.