*2.6. Epigenetics*

Organism responses to any environmental challenge develop through either genetic change (e.g., allele frequency alternations between generations, mutational accumulation) and/or nongenetic (i.e., epigenetics) processes. Epigenetics refers to external modifications in genes (e.g., methylation, acetylation, histone modifications and small RNAs; without any modification in gene sequences) that cause change in gene expression. The literature attests that many of the environmentally induced epigenetic changes are, as a matter of fact, heritable [118], thus facilitating the acceleration of adaptation processes.

It is generally assumed that epigenetics allows corals a greater ability to buffer the impacts of environmental changes and of various stress conditions (Table 1), by fine-tuning gene expression, thereby providing additional time for genetic adaptation to occur. A recent study [83] has revealed that epigenetics significantly reduced spurious transcription in the Indo-Pacific coral *Stylophora pistillata*, diminishing transcriptional noise by fine-tuning gene expressions and causing widespread changes in pathways regulating cell cycle and body size, with impacts on cell and polyp sizes as well as skeletal porosity. In a similar way, probable epigenetic signatures (a) imposed diminished bleaching responses when comparing two of the most severe episodes (17 y period) of global-scale seawater temperature anomalies [79], and (b) assisted transplanted gravid coral colonies to release an order of magnitude more coral larvae than local colonies for at least 8 reproductive seasons post transplantation ([46]; unpubl.). Coral epigenetics as a managemen<sup>t</sup> tool, alleviating impacts of global climate change on reef corals, and as a potential tool for improving reef restoration outcomes, has further gained support from studies showing links between coral adaptation and epigenetics [46,77–83].

Interestingly however, epigenetic changes may also be induced under 'healthy', more pampered situations, such as under parental care and improved nutrition [119–121]. Various epigenetic impacts have already been suggested to develop in coral colonies or coral fragments subject to different environmental conditions [46,77,83,84], most interesting of all are the impacts on heightened long-term coral reproductive capabilities [46]. Thus, favorable biological and physical conditions at the nursery stage, including: optimal light conditions, increased water flow, minimized sedimentation, enhanced planktonic supply, reduced intra- and interspecific competition, and controlled corallivory [15,26–28, 45,58,122], may impose lasting epigenetic changes on fitness and on ecological traits of transplanted corals, enhancing their ability to counter global climate change impacts and other less-favorable environmental conditions. It should be noted however that while meriting further experimental investigation, the discipline of epigenetics and epigenetic impacts in corals is still in its infancy.

### *2.7. Coral Chimerism*

A new potential tool in reef restoration (Table 1) that stems from the phenomenon of coral chimerism (Figure 2 [85]). The coral chimera is a biological entity that simultaneously consists of cells originating from at least two sexually-born conspecifics, a natural tissue transplantation phenomenon intermingling complex ecological and evolutionary mechanisms and concepts [123,124]. With regards to reef restoration, coral chimerism is presented as one of the best applied tools for accelerating adaptive responses to global climate change impacts [85], thus improving reef restoration tactics. The adaptive qualities are based on the suggestion that coral chimerism counters the erosion of genetic and phenotypic diversity, by presenting high flexibility on somatic constituents following changes in environmental conditions. This enables all partners in a chimera to synergistically present the best-fitting combination of genetic components to the environment [85,123,124]. In most cases, chimerism in corals is restricted to specific short windows at early ontogenic stages [125,126] and chimeric impacts are evident from early stages of development [86].

**Figure 2.** Coral chimerism. (**a**) Two contacting young spats (about 1 month old) of the Red Sea branching coral *Stylophora pistillata*, during the process of fusion (bar = 2 mm); (**b**) a several months old chimera of *Stylophora pistillata*, before the initiation of up-growing branches. Morphologically undistinguished area of fusion.

The literature documents a wide range of ecological advantages and benefits incurred to coral chimeras. Chimerism endows the chimeric entity, primarily at early life-history stages, with an instant survival advantage, like enhanced growth rates by virtue of the abrupt increase in size when the two organisms merge [84,86–88], and facilitation of the healing of exposed coral skeletons by enhanced preferential gregarious settlement of coral planulae [89]. The development of asexual chimeric coral planulae [90] together with the phenomenon of planulae fusion in the water column [88,91] may further mitigate the loss of genetic diversity of small colonizing populations [85,90].

The phenomenon of coral chimerism (Figure 2) is probably one of the least explored potential pathways corals take to bu ffer the impacts of capricious environmental conditions. Studying coral chimerism is not a trivial task and much has to be investigated before a better understanding can be achieved regarding this unique natural phenomenon and its inclusion in the coral restoration toolbox, another added facet to the gardening approach for active reef restoration [1,17].
