In clownfish groups, the largest individual is usually the female.
Clownfish live in size-based social hierarchies centered on a sea-anemone host. The largest and most dominant fish is the breeding female, followed by the breeding male. Smaller nonbreeders occupy lower ranks and generally remain reproductively inactive while waiting for an opportunity to move up the hierarchy.
This arrangement is linked to clownfish biology: they are protandrous sequential hermaphrodites. Individuals develop first as males, and the dominant male can change sex and become female if the established female disappears. The largest remaining nonbreeder then moves up to become the breeding male.
Only the dominant pair normally reproduces. The female lays eggs on a cleaned surface near the anemone, and the male fertilizes, guards, cleans, and fans them. The anemone provides shelter from predators, while clownfish may defend it, clean it, and bring nutrients through their activity.
A common mistake is to assume that a clownfish’s sex is fixed from birth. In many species, social position and body size influence sex change, so the hierarchy helps determine both reproduction and social order.