What was done
The authors made intensive half-hourly measurements of shaded water temperatures at near-bottom depths of 1.0 and 1.5 meters in two pools of a backreef moat on Ofu Island in American Samoa for a period of an entire year (a different pool in two different years), after which they identified the coral species living there and determined their relative abundances.

What was learned
The mean summer temperature of the water was 29.3°C, but fluctuated by as much as 6.3°C daily, briefly soaring as high as 34.5°C.  "Hot" events - defined as times when the water temperature to which the corals were exposed reached 32°C or more - lasted an average of 2.4 hours each, with some of them lasting a full 5 hours.  These events occurred on an average of 35 summer days over each of the two years of their study.  There were 85 species of corals that were able to tolerate these conditions.

What it means
The authors note that a temperature increase of only 1-2°C can severely stress or kill corals if such conditions persist for several days, yet they observed that at least nine Acropora species and a diverse range of other taxa withstand the temperature regime of the site they studied, which regime clearly exceeds the extreme thermal danger criterion for most corals.  Furthermore, they say that Ofu's backreef corals are probably subjected to environmental stresses that are even more extreme than those recorded in their study, noting that low daytime tides often coincide with calm weather conditions that facilitate the transmission of high levels of solar radiation and that dissolved oxygen contents may be low when waters stagnate during nighttime low tides.  In addition, periodic heavy rains would be expected to reduce salinities in the backreef pools; and various of these phenomena acting alone or together could compound the seriousness of the extreme temperatures that often occur there.  Yet, as the authors emphasize, "bleaching in the moat is typically slight," and this high-temperature tolerance "is exhibited not by just a few hardy species but rather by a diverse assemblage of coral species," suggesting we still have much to learn about the bleaching phenomenon and its relation to high water temperatures.