If you have dived in the Red Sea, Indian or Eastern Pacific Ocean, you will probably have seen shoals of these small black fish. They live on coral and rocky reefs at depths of 1 to 55 m.
Do not rely on the name for identification. When young, the threespot dascyllus does indeed have three white spots: one on each side and one on the forehead. Adult fish lose the forehead spot, the side spots become less distinct and the black colour of the fish becoming less intense.
Youngsters often live in large sea anemones, alongside the instantly recognisable Clownfish. Adults, though, leave the anemone to congregate in small groups around prominant rocks or coral outcrops.
The groups of adult Dascyllus trimaculatus comprise around 10 individuals with one male to several females. When approached by an intruder, the male assumes a higher defensive position, while the females rest betwen the tentacles of the anemone. During spawning the male dances to attract females. They deposit eggs on coral branches which are watched and guarded by the male until they hatch.
Dascyllus trimaculatus is part of a complex of four species that vary in geographical ranges and colour patterns. Of these D. trimaculatus is the most widely distributed,
Coral Reef Fishes Indo-Pacific and Caribbean, Lieske and Myers