ALLOGENEIC AND XENOGENEIC INTERACTIONS IN REEF-BUILDING CORALS MAY INDUCE TISSUE-GROWTH WITHOUT CALCIFICATION

Tissue growth without the deposition of calcium carbonate skeletons wa s recorded in 2 Red Sea hermatypic cnidarians during competitive inter actions. Tissue contacts between allogeneic colonies of the hydrocoral Millepora dichotoma resulted in unilateral overgrowth. In 39% of the assays the overgrowing tissue did not secrete a skeleton for up to 10 wk. These tissues were loosely attached to the overgrown branch, and r apidly advanced by up to 25 mm within the first 2 wk. Thereafter, tiss ue growth slowed down or stopped and calcium carbonate was deposited o ver the subordinate branch, starting at the original contact area. In xenogeneic interactions between the scleractinian coral Cyphastrea cha lcidicum and the cirriped barnacle Savignium dentatum, tissues of the coral always overlaid the plates of the barnacle without depositing ca lcium carbonate as long as the barnacles were alive (up to 5 yr). Calc ium carbonate was deposited by the coral's tissue on the barnacle's pl ates only following barnacle death. In both cases, the non-calcifying overgrowing tissues lacked polyps but appeared normal in histological sections and contained typical cnidarian cells and endosymbiotic zooxa nthellae. This type of tissue growth without calcification is a newly described allogeneic/xenogeneic response elicited by hermatypic cnidar ians.