Growth or reproduction: Intrasexual competition in a colonial damselfish Stegastes nigricans

Intrasexual competition within colonies of rite damselfish Stegastes nigric ans was investigated. Both males and females held individual territories, a nd these territories were adjacently distributed, forming distinct conspeci fic colonies with different size compositions. In both large and small colo nies, only relatively large individuals participated in reproduction. Breed ing males and females left their territories to court, spawn and aggregate more frequently than non-breeders of the same colonies. Breeders were less frequently attacked by other fish during these excursions than were non-bre eders. Most attacks were conducted by breeders of the same sex. This intras exual aggression may suppress the reproduction of non-breeders at least in large colonies. Non-breeders in large colonies had smaller gonads than bree ders of both large and small colonies, but they grew faster than breeders o f the same body size in small colonies. This suggests that nonbreeders in l arge colonies allocate energy to growth in order to be reproductively compe titive in the future, although only a few of them remained in the same colo nies upon maturity. Colony size thus has an effect on the reproductive life histories of individual damselfish.