Defense mechanisms of scleractinian cup corals against overgrowth by colonial invertebrates

Colonial sessile invertebrates often dominate marine hard-substratum communities by out-competing and overgrowing solitary organisms, yet some solitary taxa such as scleractinian cup corals thrive in such habitats. In this study we investigated the defense mechanisms which enable solitary, ahermatypic corals (cup corals) to avoid overgrowth and thus facilitate coexistence with encrusting colonial invertebrates in space-limited communities. Spatial competition between Balanophyllia elegans Verrill, a cup coral, and Trididemnum opacum Ritter, a colonial ascidian, was investigated in the San Juan Islands, Washington, to examine competitive interactions between cup corals and colonial invertebrates. Both species are abundant on vertical rock walls in this region (B. elegans mean density = 34 corals/0.25 m(2), T. opacum mean cover = 12%) and 25% of examined B. elegans were involved in a competitive interaction with T. opacum. Colonial invertebrates occupied 55-75% of available space on subtidal vertical surfaces in this region. The roles of four possible defensive mechanisms of cup corals (escape in size, aggressive behavior, allelochemicals, and aggregation) in preventing overgrowth were examined. Results of manipulative field experiments and laboratory pairwise interactions suggest that tentacular contact, which may represent a form of aggressive behavior, effectively deters overgrowth of B. elegans by T. opacum.