BULLETIN OF MARINE SCIENCE, 36(2): 319-324, 1985 CORAL REEF PAPER CORAL ISOMONE: A PROPOSED CHEMICAL SIGNAL CONTROLUNGINTRACLONALGROWTH PATTERNS IN A BRANCHING CORAL Baruch Rinkevich and Yossi Loya ABSTRACT A chemical signal (semiochemical) system which regulates the growth pattern is proposed to be the decisive factor for colony configuration in some reef corals. Field observations and experiments indicate that when intracolonial branches of the hermatypic coral Stylophora pistillata grow towards the others, a buffer zone in the immediate vicinity of each branch is formed, and as a result a decrease in calcification rates of the internal branches or a change in growth direction (retreat growth) are produced. These results strongly indicate the possible appearance of a new type of chemical signal(s) carrying biological activities: This chemical agent is emitted by tissue cells, secreted outside into the water, and then controls the growth pattern (a physiolOgical process) in other parts of the same individual. The term "isomone" is proposed for this newly indicated chemical-communication. In others, such as the hydro- coral Millepora dichotorna, no buffer zone is formed and a natural fusion between branches of the same colony is recorded. While morphogenesis in some coelenterates (such as hydra) seems to be con- trolled by at least four morphogenetic substances which activate or inhibit head and foot formation (Schmidt and Schaller, 1980), the presence of similar activating chemical signals in reef corals has not been demonstrated. The factors affecting colony construction in such corals are believed to be primarily related to abiotic- environmental factors like storm activities (Stoddart, 1974; Gilmore and Hall, 1976; Shinn, 1976; Highsmith et aI., 1980; Tunnicliffe, 1981; Highsmith, 1982), currents (Chamberlain and Graus, 1975; Jokiel and Cowdin, 1976; Jokiel, 1978), sediment (Yonge, 1935; Hiatt, 1951; Kornicker and Boyd, 1962; Loya, 1976b), or light levels (Goreau and Wells, 1967; Graus and Macintyre, 1976), and only partly to biotic-factors such as biological disturbance, mortality, fission and fusion (Hughes and Jackson, 1980; Kaufman, 1981), or critical radius (Barnes, 1973). Stylophora pistil/ata, one of the most abundant reef-corals in the Gulf of Eilat, Red Sea (Loya, 1972), commonly exhibits an axially rod-like growth form. New up-growing branches (UGB) are primarily added by dichotomous fission at the tip (anterior end) ofa branch. In addition to these UGBs, many new lateral (outside and inside) branches are formed. The lateral branches facing outside the colony elongate similarly to the up-growing branches, so the resulting symmetry of a typical S. pistil/ata colony approximates a sphere (Loya, 1976a). This typical shape is surprisingly maintained even after the symmetry is lost through partial breakage of branches: "within the same colony damaged branches grow faster than intact branches, which results in a tendency to regain symmetry lost" (Loya, 1976a). The lateral branches facing the internal volume of the colony (internal- side branches = ISB) are developed from the middle and bases of the UGBs and it could be predicted that after prolonged elongation of these ISBs, an adjacent UGB would sometimes encounter and fuse with them. However, such fusion was never observed in unharmed colonies growing in the reef (Fig. 1). The apparent absence offusion between branches within S. pistil/ata colonies, stands in contrast to the fast rate of fusion in isografts, artifically replaced on their original colonies (Rinkevich and Loya, 1983). It is hypothesized from field observations on hundreds 319