Forcible eviction and prevention of recruitment in the clown anemonefish

How big an animal group will be depends on how the group's size is regulated and on the costs and benefits of living in the group. To determine which individuals regulate group size of the clown anemonefish, Amphiprion percula, I investigated the strategies involved in the formation, maintenance, and dissolution of its groups. Groups composed of a single breeding pair and of zero to four nonbreeding subordinates occupied individual sea anemones (Heteractis magnifica), which provided the fish with oviposition sites and protection from predators. Group size increased linearly with anemone size. I used the residuals of this relationship as a measure of the degree of saturation of each anemone. Residents evicted low-rank subordinates and prevented the recruitment of additional subordinates at anemones with a high degree of saturation, but not at anemones with a low degree of saturation. These strategies indicate that residents control group membership of their subordinates, and suggest that residents might incur costs from the presence of subordinates in more saturated anemones. In general, whenever residents can control group membership, the prevention of recruitment and the eviction of subordinates will set an upper limit on group size.