We reported and quantified the phenomenon of chimerism in Mediterranean red coral (Corallium rubrum) in semi-natural conditions. A total of 1688 larvae were maintained in close circuit in the presence of a suitable settlement surface (marble tiles). Post settlement survival and chimera formation were monitored for 1 year. When polyps settled close enough to each other, a high frequency of chimerism was observed (32%). After 1 year, only 33% of chimeric individuals survived but they were 40% bigger than non-chimeric individuals, suggesting that chimerism could confer a competitive advantage linked to an increased growth rate.