Bi-directional sex change in marine fishes

Sex change (sequential hermaphroditism) is well known in fishes, where its occurrence and evolutionary advantage have been the focus of numerous reviews since the early 1960s (e.g., Atz 1964; Ghiselin 1969; Warner 1978, 1988; Kuwamura & Nakashima 1998; Munday et al. 2006a; Sadovy de Mitcheson & Liu 2008). Typically, individuals of sex-changing species either first function as female and then change sex to male (protogynous sex change) or they first function as male and then change to female (protandrous sex change). Bidirectional sex change — where both males and females change sex in the same population — was not thought to occur, either because the conditions favoring sex change by both males and females did not exist within the same species, or because physiological constraints prevented individuals changing sex more than once. The discovery that individuals of some species can change sex more than once has overturned these assumptions and opened up a whole new area of research into the adaptive significance and proximate mechanisms of sex change in fishes (Kuwamura & Nakashima 1998; Munday et al. 2006a)