Clownfish are protandric hermaphrodites that mature first as a male and then later change sex to become a female.
Each anemone is usually home to a pair of adults and up to four smaller, immature individuals. Dominance is based on size, the female being the largest and the male being the second largest.
As long as the adults are alive, the smaller fishes will not grow or become mature.
If the female dies, the male gains weight and immediately becomes the female for that group. The largest immature fish also begins to grow rapidly, quickly becoming a mature male.