Intraspecific competitive networks in the Red Sea coral Stylophora pistillata

Intraspecific competitive interactions were studied underwater in the Red Sea coral Stylophora pistillata during 1976-1981, through a series of field observations (FO) and field experiments (FE). The FO series were conducted on more than 200 pairs of colonies (with a gap of 1-3 cm between the nearest branches in a pair), which were checked monthly for possible interactions, during approximately five years. The FE series consisted of allografts, isografts and colony to colony attachements. S. pistillata exhibits two basic colour morphs, in which purple colonics are found to be superior to yellow morphs and competitively exculde them, even when they are not physically touching. When differences in size between the competing colonies were in the range of 2-3 orders of magnitude, a significant superiority of big colonies over little ones was recorded, irrespective of colour morphs. Five major schematic routes of intraspecific interactions are drawn and discussed. The outcome of interactions between two competing colonies is the synergistic effect of different aggressive forms, such as nematocyst discharge, overgrowth on branches or basal plates, a "retreat growth" phenomenon (possibly caused by pheromones), formation of border lines, abnormal growth forms and others. SEM observations indicate the existance of a gap (up to 30 μm) between allografts that appeared to be fused in naked-eye observations and the appearance of plasmic (?) filaments immediately above and within the contact zones. In contrast to the FE series the FO were free from any stress caused by experimental procedures and provided the opportunity to record additional forms of aggression, which were not observed in the FE series. In many cases, the duration of processes and the final outcome of interactions were much faster in FE than in FO. It is concluded that intraspecific interactions involve significant energetic expenditures that otherwise would be channelled into other metabolic requirements such as reproduction and growth. Self-recognition mechanisms and the role of immunological processes are discussed. The FO series indicate that in the vast majority of interactions no physical contact (cell to cell) is needed for self-identification.