- Coral reefs have become extremely vulnerable to rising ocean temperatures, with mass bleaching events increasing in frequency and severity. If bleaching events persist annually, models predict that more than 90% of reef species will face long-term degradation. However, recent evidence has shown that corals may be able to thermally acclimate to increasing sea surface temperatures through mechanisms in the coral host, symbiont, or both. In this study, we examined the composition of symbionts as well as monitored changes in the algal symbiont community over three time periods during a thermal acclimation and stress experiment. Not only is the type of symbiont important, but the density of symbionts over time can influence the ability of the host to withstand stressful conditions (Baker 2001). These associations contributed to thermal tolerance of the coral host. Higher acclimation temperatures lead to higher symbiont density retention. Understanding of these associations and how they change over time can help elucidate mechanisms of thermal tolerance. Therefore, measuring their abundance over time can provide insights into whether the symbiont is beneficial during stress.