*2.6. Adaptation*

Adaptation, strictly defined, refers to changes in the genetic composition of a population that are passed onto the next generation through natural selection [17,44,105]. The major concern regarding global climate change is that the current rate of environmental changes will outpace the evolutionary capabilities of corals to adapt [12,14,16,19,106]. Recent evidence has shown that, in addition to phenotypic plasticity and acclimatisation, other adaptive responses in corals, such as trans-generational plasticity [107], epigenetics [108,109], and somatic mutations [110] might contribute to resilience under thermal stress. Moreover, the fast rate of asexual reproduction within the Symbiodiniaceae (days to weeks *in hospite*) [111] in combination with large population sizes within corals (~1–5 × 10<sup>6</sup> cells cm<sup>−</sup>2) [112] provide the potential for mutations to develop that might enable corals to resist thermal stress [110].

Few studies have examined adaptation to local thermal history in Symbiodiniaceae dinoflagellates [113,114]. Howells et al. [113] demonstrated adaptive capacity in the symbiont *C. goreaui* (formerly type C1) in corals from two sites in the GBR with dissimilar thermal histories. Corals hosting *C. goreaui* from the cooler site presented photodamage and bleaching, while those from the hotter site exhibited no signs of stress and greater growth [113]. Chakravarti et al. [114] tested adaptation to thermal tolerance of *C. goreaui* through experimental evolution. Dinoflagellates were cultured in vitro at elevated temperature of 31 ◦C for ~80 generations (2.5 y), while wild-types were reared at 27 ◦C ambient temperature, then both cultures were tested at both temperatures. To measure physiological responses *in hospite*, both types (thermally selected and wild types) were inoculated into aposymbiotic recruits of three coral species and were exposed to both temperatures similar to in vitro experiments [114]. Symbionts reared in vitro performed better in photophysiology and growth at both temperatures, and showed lower levels of extracellular ROS. In contrast, wild-type symbionts were unable to photosynthesise or grow at high temperatures, and produced 17 times more extracellular ROS [114]. The differences were less obvious *in* hospite than in vitro. Cultures of corals inoculated with the thermally tolerant symbionts showed no difference in growth between 27 and 31 ◦C, while those inoculated with wild-types showed a negative growth trend at 31 ◦C, confirming an adaptation to thermal stress in *C. goreaui* after many generations living under high temperature [114].

Dixon et al. [115] revealed genetic data from the coral host that forms the heritable basis of temperature tolerance by performing a cross-fertilization experiment with coral colonies from two thermally divergent locations in GBR. The authors measured heat tolerance using the survivorship rate of larvae exposed to high temperatures and found that parents from the warmer location conferred significantly higher thermo-tolerance to their offspring, up to 10 fold increase in odds of survival, in comparison to parents from the cooler location. Dixon et al. [115] also identified "tolerance-associated genes" (TAGs), whose expression before stress predicted high survivorship rates in larvae under thermal stress, dissimilar from frontloaded genes [103]. When TAG expression was compared with parental colonies after three days of heat stress, they were negatively correlated with long-term heat stress response similar to the larval response, indicating that the larval heat tolerance results from the absence of pre-existing stress and not from prior up-regulation of heat stress genes through frontloading [115].

Krueger et al. [116] presented evidence that *Stylophora pistillata* underwent selection for heat tolerance in the Red Sea, after spending 47 days at 1–2 ◦C above their long-term summer maximum and showed an increase in primary productivity. Fine et al. [117] demonstrated how different corals species showed no signs of stress after exposure to 33 ◦C for four weeks and proposed that corals that colonised the Gulf of Aqaba after the last ice age had to cross exceedingly warm waters (>32 ◦C in the summer) at the entrance of the Red Sea, maintaining this adaptation to heat tolerance until the present day.
