Bluehead wrasse are protogynous: most individuals mature as females and some later become males. The species is also diandric, meaning a proportion of fish mature directly as males and never pass through a female phase. Social control of the change is direct: remove the large terminal-phase male from a reef site and the largest female begins performing male courtship 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 show that sex in fish is often a developmental setting rather than a fixed state. Bluehead wrasse also run two male strategies: large blue-headed terminal males defend spawning sites, while smaller yellow initial-phase males spawn in groups, and which route pays better shifts with population density.