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 hydrocoral Millepora dichotoma, no buffer zone is formed and a natural fusion between branches of the same colony is recorded.
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