Multiple symbiotic partnerships are common in scleractinian corals, but not in octocorals: Comment on Goulet (2006)

The ability of reef corals to adapt or acclimatize to projected climate change is a critical, but controversial, question in contemporary coral reef science and conservation. Much research has focused on whether or not corals are able to flexibly associate with diverse symbionts ('zooxanthellae') whose different physiologies may impart greater resistance to environmental extremes. Goulet (2006; Mar Ecol Prog Ser 321:1-7) concluded that most coral species only host zooxanthellae from a single Symbiodinium clade, and that these symbionts do not change in response to environmental disturbance, disease, or temperature stress. We re-evaluate her data and show that: (1) almost two-thirds of the coral species investigated have been severely undersampled (N <= 5); (2) more than three-quarters of the scleractinian (stony) coral species sampled more adequately (N > 10) host multiple Symbiodinium clades, or multiple types within a clade; (3) scleractinian corals are more flexible than octocorals; (4) almost all scleractinian coral families can host more than one symbiont clade. Moreover, the molecular approaches used by many studies are likely to have underestimated Symbiodinium diversity, because they are unable to detect symbionts at low abundance and because they cannot distinguish between closely related types. In addition, various other studies show that: (1) scleractinian corals commonly host multiple symbiont types within a Symbiodinium clade; (2) symbiont communities in scleractinian corals are dynamic; (3) 'cryptic' symbiont taxa found at low abundance within colonies are unlikely to be molecular artifacts. We suggest that the majority of scleractinian coral species, including virtually all of the dominant reef-building taxa, can be found hosting multiple symbiont types, even at the clade level. This flexibility is important in understanding the past evolutionary success of these species and their future survival trajectories in the face of projected climate change.