Decadal environmental ‘memory’ in a reef coral?

West sides of the coral Coelastrea aspera, which had achieved thermo-tolerance after previous experience of high solar irradiance in the field, were rotated through 180o on a reef flat in Phuket, Thailand (7o50´N, 98o25.5´E), in 2000 in a manipulation experiment and secured in this position. In 2010, elevated sea temperatures caused extreme bleaching in these corals, with former west sides of colonies (now facing east) retaining four times higher symbiont densities than the east sides of control colonies, which had not been rotated and which had been subject to a lower irradiance environment than west sides throughout their lifetime. The reduced bleaching susceptibility of the former west sides in 2010, compared to handling controls, suggests that the rotated corals had retained a ‘memory’ of their previous high irradiance history despite living under lower irradiance for 10 years. Such long-term retention of an environmental ‘memory’ raises important questions about the acclimatisation potential of reef corals in a changing climate and the mechanisms by which it is achieved.