The study investigated the effects of temperature on carbonic anhydrase activity and population density in four clades (A, B, D, F) of cultured Symbiodinium. Carbonic anhydrase activity increased in clade D as temperature rose, indicating adaptation to warmer environments, while activity declined in clades A, B and the Cx culture at higher temperatures. Population density increased in clades B and F with rising temperature up to 29°C, suggesting this is optimal, while clade D density declined at all temperatures possibly due to nutrient limitations. Different responses between clade A cultures made generalizations difficult, but sensitivity to high temperatures was observed. Future studies will examine responses to rising CO2 levels.