Control of different mating systems in a coral reef fish by one environmental factor

The Red Sea coral-dwelling damselfish Dascyllus marginatus is organized in stable terri torial groups with males dominating all females. The fish live in pairs, harems or multi-male groups. Group size, and the number of sexually active males in a group, are correlated with coral size. Males monopolize and control the coral, attracting potential mates. The experimental alteration of coral sizes, using spherical Stylophora corals, resulted in an increase of male numbers, in artificially composed groups, with the increase of coral size. Thus pairs and harems were produced on small corals, multimale groups on big corals. Experiments with unisexual groups revealed a higher survival of females; males are more aggressive to each other, and cause a stronger emigration of subdominants. In D. marginatus, group size and the number of sexually active males are essentially restricted by one environmental factor: the number of available hiding places on a coral.