None can physically touch without killing each other, but many can grow in very close proximity with only a small no-man's-land separating them (e.g., a few mm of dead space). Even different individuals (i.e., different genotypes) of the same species will often react in the same way. Fusion between different individuals has been reported, but is quite, quite rare outside of larvae/newly settled corals. In general Hydnophora is really aggressive and will zap large areas of anything close to it, but many other 'sps' will tolerate growing into close proximity with only a small amount of conflict/dead tissue. Placing any two corals in physical contact will always result in the death of at least one at and near the point of contact though.
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