Symbiodinium spp. in colonies of eastern Pacific Pocillopora spp. are highly stable despite the prevalence of low-abundance background populations

A shift in the dominant Symbiodinium species within a coral colony may allow rapid acclimatization to environmental stress, provided that the new symbiont is better suited to prevailing conditions. In this study, the Symbiodinium diversity in Pocillopora corals was examined following a cold-water bleaching event in the Gulf of California. Individual colonies were differentially impacted by this event based upon their association with either the Symbiodinium ITS-2 type C1b-c (sensitive) or ITS-2 type D1 (tolerant). Real-time PCR indicated a high prevalence of an alternate and compatible Symbiodinium sp. (i.e. C1b-c or D1) residing at low-abundance background levels within many colonies both during and after a 1 yr recovery interval (46 to 52%). However, despite the potential for 'switching,' the dominant resident symbiont remained at high abundance during the recovery, with only 2 of 67 colonies (3%) under-going a change to the other Symbiodinium type. Pocillopora residing in the Gulf of California therefore maintain long-term associations dominated by a specific Symbiodinium sp., where potential competition by a second symbiont type is suppressed despite the temporary change in environmental conditions that would favor a shift in symbiosis toward a more stress-tolerant species.