The family Labridae, commonly known as wrasses, is one of the largest and most diverse families of fish in terms of color, shape, and size. Within this family, there are more than 600 species spanning over 82 genera (Westneat and Alfaro, 2005), including Thalassoma, the genus which the bluehead wrasse belongs to. A recent study has shown that Thalassoma originated about 8-13 Mya, with a speciation explosion that occurred approximately 5-10 Mya (Bernardi et al., 2003). It is unclear when the ability to change sex appeared in the evolution of the Labridae family and the Thalassoma genus although it has been proposed that a change in developmental timing has been a mechanism in the evolution of sequential hermaphroditism (Oldfield, 2005). Additionally, developmental plasticity might have evolved in response to differences in the reproductive success of dominant males in populations of differenct sizes (Munday et al., 2006).

Research has shown that protogynous hermaphroditism is the basic pattern for the Labridae family (Diener, 1977). Still, like with all things, not every species within the Thalassoma genus strictly adheres to this behavior. That is to say, various species have signified different cues that prompt their sex change or have developed different adaptations to accommodate the transition from female to male. For instance, the rainbow wrasse (Thalassoma lucasanum) rarely changes sex and does so only under specific allocation of size of both sexes (Ross, 1983). The saddleback wrasse (Thalassoma duperrey) is promiscuous rather than harem-living like all protogynous hermaphrodites. Because the saddleback wrasse is promiscuous, the change from one sex to the other is much slower as compared to the bluehead wrasse. On average, it takes two months for the former to complete the transition and a mere seven days for the latter (Warner, 1982). Thus, the ability to change sex is activated and functions in distinctive ways for different species of the wrasse.

Evolution of the Labridae

Figure 1. A chronogram illustrating the ages of origin and diversification of the Labridae
(Cowman et al., 2009)

Key Terms

The evolution of an organism's trait.

Having more than one sexual partner.