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A mild bleaching event was observed among Pocillopora spp. in the southern Gulf of California in the spring of 2006. Uniform bleaching occurred in numerous colonies on the upper portions of their branches. Most (∼90%) colonies that exhibited bleaching contained a species of endosymbiotic dinoflagellate, Symbiodinium C1b-c, which differed from the Symbiodinium D1 found inhabiting most unbleached colonies. Analysis of chlorophyll fluorescence, indicated a decline in photosystem II photochemical activity, especially among colonies populated with C1b-c. By early August, most affected colonies had recovered their normal pigmentation and fluorescence values were once again high for all colonies. No mortality was observed among tagged bleached colonies nor did symbiont species composition change during recovery. This unusual episode of bleaching did not appear to be a response to thermal stress, but may have been triggered by high levels of solar radiation during a period of unseasonally high water clarity in the early spring. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]