Community organization of associates of the scleractinian coral Pocillopora damicornis

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Colony size and the inter and intraspecific interactions of the obligate symbionts of Pocillopora damicornis, i. e. xanthid crabs(Trapezia app.), a snapping shrimp(Alpheus lottini)and gobies(Paragobiodon app.), are examined in terms of their effects on the community structure of the coral associates. Species richness and diversity(H')and the number of associated animals increased with colony size, while eveness(J')was constant and high. In colonies with less than 1.5 cm3 interbranch space volume, no obligate symbionts were found. Obligate symbiont species and numbers increased with colony size : 1.5-12 cm3 --one individual of Trapezia or Cymo ; 15-23 cm3 --2 species, one individual each ; 24-71 cm3 --2 species including one heterosexual pair pf Trapezia or Paragobiodon ; 74-264 cm3 -- 2 or 3 species including 2 6r 3 heterosexual pairs ; . 331 cm3 --more than 4 species forming complex communities(maximum of 10 species in a colony of 1, 980 cm3). Females of Trapezia cymodoce expelled all introduced smaller individuals of Trapezia spp., but were frequently expelled by larger introduced crabs. Alpheus lottini sometimes expelled introduced Trapezia, but Trapezia rarely showed aggressive behavior toward A. lottini. A. lottini is most aggressive against conspecific intruder of the same sex, resulting in the occurrence of one heterosexual pair per colony. Carrying capacity of the habitat which is related to colony size, and species interactions between the residents and intruders are important factors in the organizing process of the associated animals community.