Variety of Sex Change in Tropical Fish

Abstract

Among vertebrates, teleost fish are of particular interest for their sexual diversity and plasticity. During ontogenesis in gonochoristic fish, the undifferentiated gonadal primordium develops into an ovary or testis, a process referred to as sex determination or sexual differentiation. After primary sex determination, the sexes of gonochoristic fish remain fixed for the remainder of their lives. Other fish are hermaphrodites, however, and can change their sex in adulthood, as either simultaneous hermaphrodites, which possess both functional ovarian and testicular tissue, or sequential hermaphrodites. Sequential hermaphrodite species can be divided into three groups. The first are protogynous (female-first) hermaphrodites; the fish begins its life as a female but later becomes a male. The reverse is the case for protandrous (male-first) species (the second group). A third group is composed of those few species that can change their sex serially (bidirectional sex change). This diversity of sexual plasticity is unique to fish and, as such, provides excellent model systems with which to investigate the mechanisms of sex determination and differentiation in vertebrates. In this chapter, we first discuss gonadal sex differentiation in hermaphroditic fish in comparison with gonochoristic fish and then describe various types of sex change in fish from the viewpoints of morphology and physiology.