Forcible eviction and prevention of recruitment in the clown anemonefish

How big an animal group will be depends on how the group's sizeis regulated and on the costs and benefits of living in thegroup. To determine which individuals regulate group size ofthe clown anemonefish, Amphiprion percula, I investigated thestrategies involved in the formation, maintenance, and dissolutionof its groups. Groups composed of a single breeding pair andof zero to four nonbreeding subordinates occupied individualsea anemones (Heteractis magnifica), which provided the fishwith oviposition sites and protection from predators. Groupsize increased linearly with anemone size. I used the residualsof this relationship as a measure of the degree of saturationof each anemone. Residents evicted low-rank subordinates andprevented the recruitment of additional subordinates at anemoneswith a high degree of saturation, but not at anemones with alow degree of saturation. These strategies indicate that residentscontrol group membership of their subordinates, and suggestthat residents might incur costs from the presence of subordinatesin more saturated anemones. In general, whenever residents cancontrol group membership, the prevention of recruitment andthe eviction of subordinates will set an upper limit on groupsize.