Background
The authors write that "following extreme and localized coral mortality events, coral populations may be repopulated by recruits from nearby, less impacted reefs." In situations where nearby reefs are likewise decimated, however, another source of recruits is needed; and they note, in this regard, that "deep reef habitats are more protected from temperature/light-induced coral bleaching events (Glynn, 1996; Feingold, 2001; Glynn et al., 2001) as well as a range of other disturbances such as storms and loss of major herbivores (Bongaerts et al., 2010)," indicating that these deeper reefs "may therefore serve as coral refuges and subsequent sources of recruits."

What was done
To further explore this deep reef refugia hypothesis, van Oppen et al. studied ten coral host microsatellite loci and sequences of the host mtDNA putative control region, as well as ribosomal DNA (rDNA) ITS2 sequences of the coral's algal endosymbionts (Symbiodinium) in order to "examine population structure, connectivity and symbiont specificity in the brooding coral Seriatopora hystrix across a depth profile in both northwest (Scott Reef) and northeast Australia (Yonge Reef)."

What was learned
No migration from genetically divergent deep slope populations into shallow habitats was evident at Younge Reef. However, the five researchers state that there was evidence for "a considerable amount of recent vertical migration" at all three of the Scott Reef locations they studied; and they say that the implication of this finding is that "connectivity exists between shallow populations, where corals are most at risk of bleaching because of high light levels, and populations at depths, where lower light reduces the risk of bleaching."

What it means
Bongaerts, P., Ridgway, T., Sampayo, E.M. and Hoegh-Guldberg, O. 2010. Assessing the 'deep reef refugia' hypothesis: focus on Caribbean reefs. Coral Reefs 29: 309-327.

Feingold, J.S. 2001. Responses of three coral communities to the 1997-98 El Niņo-Southern Oscillation: Galapagos Islands, Ecuador. Bulletin of Marine Science 69: 61-77.

Glynn, P.W. 1996. Coral reef bleaching: facts, hypotheses and implications. Global Change Biology 2: 495-509.

Glynn, P.W., Mate, J.L., Baker, A.C. and Calderon, M.O. 2001. Coral bleaching and mortality in Panama and Ecuador during the 1997-1998 El Niņo-Southern Oscillation event: spatial/temporal patterns and comparisons with the 1982-1983 event. Bulletin of Marine Science 69: 79-109.