DEFENSE-MECHANISMS OF SCLERACTINIAN CUP CORALS AGAINST OVERGROWTH BY COLONIAL INVERTEBRATES

Colonial sessile invertebrates often dominate marine hard-substratum c ommunities by out-competing and overgrowing solitary organisms, yet so me solitary taxa such as scleractinian cup corals thrive in such habit ats. In this study we investigated the defense mechanisms which enable solitary, ahermatypic corals (cup corals) to avoid overgrowth and thu s facilitate coexistence with encrusting colonial invertebrates in spa ce-limited communities. Spatial competition between Balanophyllia eleg ans Verrill, a cup coral, and Trididemnum opacum Ritter, a colonial as cidian, was investigated in the San Juan Islands, Washington, to exami ne competitive interactions between cup corals and colonial invertebra tes. 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 int eraction with T. opacum. Colonial invertebrates occupied 55-75% of ava ilable space on subtidal vertical surfaces in this region. The roles o f four possible defensive mechanisms of cup corals (escape in size, ag gressive behavior, allelochemicals, and aggregation) in preventing ove rgrowth were examined. Results of manipulative field experiments and l aboratory pairwise interactions suggest that tentacular contact, which may represent a form of aggressive behavior, effectively deters overg rowth of B. elegans by T. opacum.