Allogeneic and xenogeneic interactions in reef-building corals may induce tissue growth without calcification

Tissue growth w~thout the d e p o s ~ t ~ o n of calcium carbonate skeletons was recorded in 2 Red Sea hermatyp~c cnldarians dunng competitive interactions Tissue contacts between allogeneic colonles of the hydrocoral Millepola dichotorna resulted in unilateral overgrowth In 39% of the assays the overgrowing tissue d ~ d not secrete a skeleton for up to 10 wk These tissues were loosely attached to the overgrown branch, and rapidly advanced by up to 25 mm ~ v ~ t h ~ n the f~ r s t 2 wk Thereafter, tissue growth slowed down or stopped and calcium carbonate was depos~ted over the subordinate branch, starting at the original contact area In xenogenelc interactions between the scleract~nlan coral Cyphastrea chalc~dicurn and the c ~ r n p e d barnacle Savignium dentatum, tlssues of the coral always over la~d the plates of the barnacle without depos~ting calc~um carbonate as long as the barnacles were ahve (up to 5 yr) Calcium carbonate was depos~ted by the coral s tlssue on the barnacle's plates only following barnacle death In both cases, the non-calc~fy~ng overgrowing t~ssues lacked polyps but appeared normal in histological sections and contalned typlcal cnldarian cells and endosymbiotic zooxanthellae Thls type of tissue growth without calc~fication is a newly descnbed allogeneic/xenogeneic response ellcited by hermatypic cnidanans