Specificity and stability in high latitude eastern Pacific coral-algal symbioses

The Gulf of California (Sea of Cortez) acts as a refuge for zooxanthellate coral communities at high northern latitudes in the eastern Pacific. The diversity of dinoflagellate endosymbionts living with cnidarians (9 genera, 16 species) was examined using denaturing gradient gel electrophoresis (PCR-DGGE) fingerprint analysis of the rDNA Internal Transcribed Spacers (ITS) 1 and 2. Although geographically isolated from the rest of the Pacific, coral-algal associations in the Gulf of California were similar in cladal composition (dominated by clade C Symbiodinium). However, levels of host-symbiont specificity appeared to be greater than in the western Pacific, where host diversity is considerably higher and environmental conditions are relatively stable. As a result, the cnidarian community lacked a host-generalist symbiont. Most colonies of the ecologically dominant genus Pocillopora contained uniform populations of either Symbiodinium D1 (in ∼70% of colonies) or C1b-c (∼ 30%). The proportions of D versus C colonies (i.e., D and C holobionts) remained stable between the years 2004 and 2006. The specificity, stability, and prevalence of Symbiodinium D1 found among individuals of Pocillopora spp. in a region minimally impacted by coral bleaching indicate that these symbiotic combinations, exhibiting specific variability, appear to be products of long-term ecological and evolutionary processes.