An anemone typically houses a strict size-ordered queue: one large female, one smaller breeding male, and a tail of even smaller juveniles that do not reproduce at all. Rank is enforced by aggression and by growth suppression, so subordinates stay physically small as long as the fish above them remain.
When the female dies or is removed, the breeding male changes sex over a period of weeks to months, his gonads remodelling from testicular to ovarian tissue. The largest juvenile then matures into the new breeding male. The change runs in one direction only: no anemonefish has been shown to revert from female back to male.
The mechanism is social rather than seasonal, driven by the removal of dominance cues rather than by a calendar or a temperature trigger. What remains less settled is how quickly the behavioural switch precedes the gonadal one, since laboratory fish begin acting as females well before their ovaries are fully functional, and how much of the change is reversible during that early window.