ABSTRACT
Anemonefishes are monogamous protandrous hermaphrodites that live in societies structured by size-hierarchies that function as breeding queues. Within each group, the dominant individual is always the female, and her disappearance offers subordinate fish the opportunity to upgrade their rank and grow. The largest remaining individual changes sex to take over the vacated dominant female position, while the second-largest differentiates into a mature male to complete the breeding pair. This chapter dissects the coordinated cascade of behavioural, physiological, and morphological changes needed to transform a functional male into a functional female. We describe the distinct stages occurring in the brain during sex change in anemonefish, including the initiation, the behavioural shifts, and the changes occurring in the preoptic area (POA) of the hypothalamus. We discuss how these changes are transmitted through the hypothalamic-pituitary-gonad axis, ensuing in a dramatic reorganization of the gonadal tissue. We highlight histological changes and molecular mechanisms and pathways orchestrating social sex change in anemonefishes and point at significant gaps in our knowledge of how sex change takes place in this group of fishes.
