Corals tend to live close to their upper thermal tolerance range and if sea waters get above 32˚C, bleaching tends to occur. This results from the breakdown of the symbiosis between the zooxanthellae and the coral, leading to a decrease in nutrition for the coral and a resultant increased susceptibility to disease. The effects of bleaching are not uniform as bleaching tolerance is variable both within the colony and within and between species of corals. Variation, of course, is one of the raw materials of evolution.
There are three different genetic types, or clades, of zooxanthellae living in the corals of the Great Barrier Reef (GBR): Clade A, C and D. Clade C is the most common in the GBR but clade D performs better at higher temperatures. However, corals with clade D have a slower growth rate compared with clade C. Further studies are underway to examine the genetic differences between these clades and factors that affect their gene expression.