Most corals may not change their symbionts

Many corals (stony corals and octocorals) rely on their symbiotic algae (zooxanthellae) for survival. Under stress, zooxanthellae are expelled, resulting in coral bleaching. The hypothesis that corals may survive climate change by exchanging algal types is shown here to be potentially applicable only to a minority of corals. Data on 442 coral species from 43 studies reveal that only a few coral species may be able to change their symbionts. The ability to change symbionts seems to be linked to whether a coral species can host multiple zooxanthella clades, either at different depths on the same reef, on different reefs or at different geographic locations, or concurrently within the same colony. The combined data set shows that only 23% of coral species host multiple zooxanthella clades. Most coral species (77%) exhibit fidelity to a narrow subset of a single zooxanthella clade, some even to specific algal genotypes within a clade. These specific algal genotypes in coral species hosting a single algal clade do not change over time. Furthermore, no algal change occurs when a coral colony is either transplanted to different environments, or subjected to stressors such as disease or increased temperatures. For the majority of corals, therefore, algal switching does not appear to occur. These coral species will survive only if the existing host–symbiont combination withstands the changing conditions. If climate warming continues, coral reefs may undergo a change in biodiversity such that only a subset of symbiotic corals may persist.