Print PageAbnormally high seawater temperatures cause corals to expel their endosymbiotic algae (zooxanthellae), resulting in “coral bleaching” that may eventually lead to coral death if the stress persists. Global warming is expected to increase the frequency of devastating coral bleaching events, prompting diverse inquiries into the associated physiological and evolutionary processes. The influential adaptive bleaching hypothesis (ABH) proposed by Buddemeier and Fautin (1993) suggested that corals bleach to make space for more adaptive exogenous zooxanthellae strains. However, numerous studies seeking to validate this hypothesis revealed that adult corals are unable to trade their symbionts for a new strain not originally present within the coral. Here, I explore an alternative possibility: that bleaching is adaptive for zooxanthellae, since it facilitates their transmission to the next generation of host. This inverted adaptive bleaching hypothesis (iABH) is inspired by three observations: (1) zooxanthellae expelled in massive amounts during bleaching appear to be viable, (2) corals experience minor bleaching every summer, and (3) young aposymbiotic corals generally recruit to reefs shortly before the onset of maximum summer temperatures. Notably, iABH predicts that the evolution of higher bleaching resistance is maladaptive from the zooxanthellae standpoint, which implies host-symbiont conflict and may profoundly change the evolutionary mechanisms currently envisioned.
To test the iABH predictions, I set up an experiment in which Acropora millepora recruits were receiving inflow from heated and unheated jars containing branches of adult “donor” corals. Three corals containing different zooxanthellae genotypes ("clades"), C1, C2, and D, were used as donors.
Results/Conclusions
The recruits successfully acquired zooxanthellae, which is the first direct evidence of transmission of zooxanthellae expelled from an adult to recruits (note: at the time of abstract submission, this result is still to be verified by genotyping). Clades D and C1 are known to confer high bleaching resistance to the holobiont and, following the iABH logic, should not depend on bleaching for their transmission. Indeed, heat stress diminished the rate of transmission of both C1 and D zooxanthellae. In contrast, clade C2 (the most common zooxanthellae genotype on the Great Barrier Reef) is associated with low bleaching resistance, and in my experiment was appreciably transmitted only from the heat-stressed adult. Although the transmission rate was too low to draw a definitive conclusion, this result indicates that C2 zooxanthellae may depend on bleaching for transmission, as predicted by the iABH.
