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 ac - climatization to environmental stress, provided that the new symbiont is better suited to prevailing conditions. In this study, the Symbiodinium diver- sity in Pocillopora corals was examined following a cold-water bleaching event in the Gulf of Cali- fornia. Individual colonies were differentially im - pacted 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 there- fore maintain long-term associations dominated by a specific Symbiodinium sp., where potential com- petition 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.