The unprecedented level of diversity recorded among the clones of self-fertilizing gynogenetic unisexuals and self-fertilizing simultaneous hermaphrodites challenges current ideas on the predominant role of recombination in promoting evolution of biological diversity. Though limited to a few species, the existence of self-fertilizing and cyclical sex changing hermaphrodites remians a puzzle to many geneticists and endocrinologists. Within a few days after the removal of a harem master or mistress, a successor changes sex and takes over the harem; the causes for the sex changing mechanism pose many challenges to socio-ecologists and behaviour specialists.This book is a synthesis of relevant and recent information on sexuality of fishes and points towards new directions of desired research.