Many invertebrate organisms are hermaphroditic. Snails and earth worms, for example, can copulate with any other individual that they meet. This is a real advantage. Imagine how fast a population could grow if every individual in the group can produce eggs and any other individual can fertilize them. Fish are not quite that advanced as hermaphrodites.
Protogynous hermaphrodites have populations in which every individual starts out life as a female.
When the fish reach sexual maturity the strongest and most aggressive individual becomes the functional male. This phenomenon is well documented in marine fish such as wrasses, groupers, anthias and angelfish. Protogyny is also the type of hermaphroditism that my Dicrossus filamentosus seem to exhibit. The first freshwater fish to be identified as a protogynous hermaphrodite is Crenicara punctulata Protogyny is very common in marine reef fishes. The classic example involves the Caribbean bluehead wrasse, Thallosoma bifasciata .