Soft-coral natural chimerism:: a window in ontogeny allows the creation of entities comprised of incongruous parts

Chimerism, the evolutionary perplexing outcome of fusion between conspecific individuals, is widely documented in nature. Several reports assign a variety of benefits to the state of chimerism, especially in cases of fusions between kins, rating a chimera as a congruous entity. For the first time, we describe here a follow-up study on chimeras of soft corals that occur only between ontogenetically immature conspecifics, prior to development of a histocompatibility recognition system. Four soft coral species from the Red Sea were investigated: Nephthea sp., Heteroxenia fuscescens, Parerythropodium fulvum fulvum and Clavularia hamra. Co-settlement of planulae resulted in high frequencies of spontaneous allogenic fusions between primary polyps. Tissue fusion between allogenic partners was confirmed histologically. During the observation periods (up to 450 d) chimeras were detached, or chimerism resulted in the death of 1 or more partners, or in morphological resorption of the partners. The results also document slower growth and growth-retarding disorders such as disruption of the structural patterns of polyp budding and polyp configuration. These cumulative effects were only manifested by individuals comprised of incongruous components. Such chimeras are likely to be less suited to field conditions than genetically homogeneous individuals, raising the ecological-evolutionary question of why soft-coral chimeras arose in the first place. We propose that juvenile cnidarian chimerism represents a case in which ontogenetic allorecognition is not infallible, and that it is further promoted by the gregarious settlement of larvae characteristic of many coral species.