ABSTRACT
A vast majority of fi shes are gonochores; individuals develop and remain as either female or male throughout their life time. A few fi shes are unisexual (Fig. 9); they are females; their haploid or diploid eggs are activated by sperm ‘borrowed’ from males of the sympatric species. Hermaphrodites are bisexuals and this sexual pattern is broadly categorized into those in which individuals to a greater or lesser degree synchronously function as both sexes, i.e., simultaneous hermaphroditism and those, in which individuals develop and function as one sex or the other and subsequently change to the opposite sex, are sequential hermaphrodites. The latter form of hermaphroditism may further be divided into sexual patterns that involve either one or two pathways of sexual development for the second sex. For example, in the case of protogyny, i.e., female to male sex change, either all males are derived from adult females (monandry or a single pathway of male development, Fig. 9) or some males develop directly as males from a juvenile phase, while others are derived from adult females (diandry or two pathways of male development). The reverse is true to protandry, i.e. male to female sex change, either all females are derived from adult males (monogyny or a single pathway of female development) or some females develop directly as females from a juvenile phase, while others are derived from adult males (digyny or two pathways of female development). Serial (Grober, 1998) sex change includes the bidirectional and cyclical (Kuwamura et al., 1994) sex changes during the life time of adults, and the most extreme sexual plasticity is characterized by back and forth sex change. This has come to light only during the recent years. A population or species is considered to be functionally hermaphroditic, if a substantial proportion of individuals functions as both sexes, either simultaneously or sequentially or serially, at some point during their life, the emphasis is placed on reproductive function to distinguish between gonochoristic and non-gonochoristic strategies (Sadovy and Colin, 1995).
