Competition by anemone fishes for host actinians

Asterisks (*...*) surround words or phrases that are to be italicized.Ten species of sea anemones are symbiotic with 26 species of anemonefishes (Pomacentridae). Five of the former inhabit six of the latter at Lizard Island, Great Barrier Reef, Australia. Direct challenge field experiments for actinians from among fish of all species normally inhabiting them support the hypothesis that competition is important in governing which species of symbionts occur together. The competitive hierarchy for *Entacmaea quadricolor* is ordered as is numerical abundance of fish individuals and proportion of anemones occupied by that species: *Premnas biaculeatus*, *Amphiprion melanopus*, *A. akindynos*. Congruence between competitve dominance of the territories (= anemones) with number of individual fish and with proportion of the actinian population occupied by each holds as well for *A. perideraion* and *A. percula* (in that order) in *Heteractis magnifica*. Dominance is also a direct correlative of host specificity in both cases. Actinians of the other four species at Lizard Island (all members of family Stichodactylidae) host only the fish *A. akindynos*, which occupies the second greatest number of hosts. Thus, in a particular locality, proportion of individuals of a host species occupied by its various symbionts, and number of specimens of each fish, are direct reflections of the dominance hierarchy among the fish. In addition, attractiveness of host actinians to anemonefishes is reflected in the number of species of fishes sharing that resource, either in one locality or over the range of the symbiosis as a whole. By this measure, *Entacmaea quadricolor* is the most attractive host. Probably not coincidentally, it is the only host actinian that has been demonstrated to benefit from its association with anemonefishes