This experiment coincided with a natural (but moderate)
bleaching event that began in December 2010, such that healthy
and bleached corals were collected form the field; Healthy corals
were heavily pigmented, while bleached corals were visibly pale. To
ensure that the colonies used in our experiment were actually
bleached and not just naturally pale, zooxanthellae densities were
quantified for both bleached and healthy corals, following
McCowan et al. (2012). Zooxanthellae densities were quantified
using individual branches (ca. 7 cm long) taken from the centre of
each colony upon collection (n ¼ 30 colonies per species). Sample
branches were fixed in 10% buffered formalin for at least 1-week
and then decalcified using dilute (5e10%) formic acid. Once
decalcified, two replicate 5 5mmtissue sections were cut from of
each sample branch, homogenized with 1 ml of 70% ethanol and
then immediately placed on to Neubauer Improved Tiefe Depth
Profoundeur (0.100 mm) haemocytometer to count the number of
zooxanthellae in 3 replicate 0.0025 ml aliquots per tissue section.
Zooxanthellae densities (number per cm2) for each branch were
then calculated by averaging across sections and aliquots, following
McCowan et al. (2012). Bleached colonies of both species had
zooxanthellae densities that were 50% lower than healthy colonies
which was a significant difference (ANOVA F1,57 ¼ 150.181,
p < 0.0001).