Bluehead wrasse males with yellow females of the species. If the blueheaded male is removed from a t

During a short swim over a coral reef you can observe a diversity of social interactions among individuals of many species. At dusk, one may see a school of fish move steadily toward an opening in the reef, heading into the open sea for a night of feeding. Approached underwater, the edge of the school looks like a giant translucent curtain stamped with the silhouettes of thousands of seemingly identical fish. Their coloration, countershaded dark above and silvery below, highly coordinated movements, and great numbers give the fish within the school some protection from predators. Though seabirds and fish attack the school as it makes its way, only a small proportion of the schooling fish are eaten. Meanwhile along the reef, damselfish are distributed singly on territories. The damselfish retain possession of their patches of coral rubble, living coral, and sand by patrolling the boundaries and driving off any fish attempting to intrude. At certain times, some territory-holding males are joined by females, where they court and deposit eggs and sperm on the nest site prepared by the male. Higher along the reef face a male bluehead wrasse mates with one of the females that live within his territory.