Abstract
Corals form an obligate symbiosis with a wide range of genetic types within the genus Symbiodinium, a genetically extremely diverse group of single-celled algae. In this chapter we review global patterns in the distribution of Symbiodinium diversity, variability in the levels of specificity of the coral--algal symbiosis among corals with differing life histories, temporal change versus stability in the Symbiodinium community harboured by corals, particularly following bleaching events, and the extent to which Symbiodinium type defines physiological attributes of the coral holobiont. We further discuss evidence for shuffling versus switching under thermal stress and how coral--algal symbioses are likely to respond to ocean warming associated with climate change.