ABC. It’s easy as Protandry
The largest, most aggressive female anemonefish rules the roost - she is the dominant member of the social group, and with one breeding male, the only one who reproduces. In anemonefish society, everybody knows where they stand in the hierarchy! So when she leaves the group, everybody moves up one spot - including the largest, most aggressive male, who undergoes a sex change to become the leader (and breeding female) of the group. While undergoing a sex change might sound drastic to humans, it is actually not all that uncommon in other species. Many do it, either naturally or due to environmental factors: fish, invertebrates, amphibians, some reptiles, even the occasional chicken!
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