Bluehead wrasse are protogynous: individuals mature as females and some later become males. Social control is direct. If the large terminal-phase male is removed from a reef site, the largest female begins performing male courtship behaviour within about ten minutes to a few hours, well before any gonadal change is possible.
Gonadal transformation follows on a timescale of days. Sequencing work published in 2019 tracked gene expression through the process and found rapid downregulation of the ovarian programme, including aromatase which converts androgens into oestrogen, followed by activation of testis-specific genes and remodelling of the epigenetic machinery. Functional testes were present within roughly eight to ten days.
The direction is opposite to that of anemonefish, which begin as males, and the two together illustrate that sex in fish is often a developmental setting rather than a fixed state. Bluehead wrasse also show two distinct male strategies: large blueheaded terminal males defend spawning sites, while smaller yellow initial-phase males spawn in groups, and the reproductive success of the two routes differs with population density.