"The 25 species of ' and one of ' (family ) are obligate s of 10 species of facultatively symbiotic sea anemones. Throughout the tropical Indo-West Pacific range of the relationship, a fish species inhabits only certain of the hosts potentially available to it. This specificity is due to the fishes. Five fishes occupy six sea anemone species at , , Australia. harbors P. biaculeatus, A. melanopus and A. akindynos. ... ns cleared of symbionts disappeared within 24 h, probably having been eaten by reef fishes. Entacmaea, the most abundant and widespread host actinian at Lizard Island and throughout the range of the association, is also arguably the most attractive to es. I believe its vulnerability to predation was a factor in its evolving whatever makes it desirable to fishes. Experimental transfers pitted fish of one species against those of another, controlling for ecophenotype of host, and sex, size and number of fish. Competitive superiority was in the same order as abundance and over-all host specificity: P. biaculeatus, A. melanopus, A. akindynos. At least three factors are necessary to explain patterns of species specificity — innate or learned host preference, competition, and es."