Social control of terminal phase transition in primary males of the diandric wrasse, Halichoeres poecilopterus (Pisces: Labridae)

In many diandric fishes, large territorial males with bright body coloration (terminal phase (TP) males) are derived either from initial phase (IP) females that change sex to male or from IP primary males that change color and behavior, but do not change sex. The mechanism controlling the transition of IP primary males into TP males is not well understood. We conducted cohabitation experiments to examine social conditions favoring TP transition by primary males in the diandric wrasse, Halichoeres poecilopterus. IP primary males always started TP-specific sexual behavior in the presence of a smaller subordinate, and subsequently acquired TP body coloration. In contrast, primary males under subordinate conditions often performed female-like sexual behavior. In pairs with similar body sizes, both individuals initiated TP male behavior. The results suggest that TP transition in primary males may be closely related to a dominance relationship (or size order) within social groups, as it is in the case of sex change by females.