It has been observed that there are two possible ways forthe fishto become male: they either develop directly into males or they are first born as female and then change their sex to male later on (Warner, 1982). Individuals are categorized based on their morphology and behavior. Initial phase (IP) fish are either male or female, whereas terminal phase (TP) fish are always male. TP males can further be subdivided into territorial (T-TP) and non-territorial (NT-TP) (Semsar et al., 2001). Those that make the transition from being IP females to TP males undergo permanent coloration; the yellow hue of their bodies turn green while their heads turn blue.

As their names imply, T-TP males demonstrate aggressive behavior whereas NT-TP males do not. T-TP males pair spawn with IP females and aggressively defend courting sites, thereby barring IP and NT-TP males from such access. Since NT-TP males do not display the aggressive courtshipbehavior that T-TP males do, they do not have a chance of entering spawning sites and mating with females. However, there are ways around this obstacle for IP males. The most viable option for IP males is to "streak" into
T-TP/IP female pairs by releasing their gametes into the water in hopes that the sperm will impregnate an IP female. Another alternative is for IP males to travel in large groups and gather immediately outside the spawning sites of the TP males, hoping to sneak access to the IP females (Warner, 1984). Thus, the aggression and courtship behavior that T-TP males exhibit serve as way to inhibit sex change in IP females (Perry and Grober, 2003). The existence of NT-TP males, however, proposes the theory that sometimes this suppression is not adequate enough; it is likely that NT-TP males originate from IP females that do not receive sufficient behavioral suppression from resident T-TP males. NT-TP males appear to have overcome the inhibition of gonadal conversion, as shown by their display of TP color, a trait that requires androgens. Because they fail to display aggression and courtship behavior, this indicaties that NT-TP males are unable to complete the neuroendocrine changes required for the display of T-TP male behavior.

Why do wrasses practice protogyny? Research has shown that this ability is a response to different attributes in social settings. For instance, it is the size of individuals that acts as a cue for sex change for the saddleback wrasse; fish will change sex if there are sufficiently fewer larger fish and enough smaller fish present (Warner, 1991). In the case of the bluehead wrasse, however, it is the loss of TP males from the social group that triggers the largest IP female to initiate the process of sex change (Warner and Swearer, 1991). While the environment is a stimulant during the critical period in which sex is differentiated, there is also a theory that suggests it may also be of a genetic nature: developmental homeostasis determines the gonadal ontogeny (Chen and Yeung, 1983). That is, the decision to change genders is the result of male and female sex determining genes. These genes control the developmental processes for the expression of different sex phenotypes and a developmental switch mechanism that determines which stage of development a certain sex should begin differentiation and maturation; this is a critical period for sex differentiation and may be of a genetic or environmental nature.

Key Terms

The developmental background of an organism over its lifetime, focusing particularly on how the organism’s environmental factors have an effect on its development.

Initial Phase (IP)
Fish that can be either male or female. Males are often small and remain the same sex their entire lives whereas females have the opportunity of beomcing a dominant male. They are of a bright yellow color.

Terminal Phase (TP)
The phase that IP females transition to when they are ready to become dominant males. TP fish can be further divided into those who are territorial (T-TP) and those who are not (NT-TP). They are of a blue and green color. Also known as a secondary male.

A steroid hormone that controls the development and maintenance of masculine characteristics.