Adjusting the light exposure and capture of their
symbiotic photosynthetic dinoflagellates (genus
Symbiodinium Freud.) is central to the success of
reef-building corals (order Scleractinia) across high
spatio-temporal variation in the light environment of
coral reefs. We tested the hypothesis that optical
properties of tissues in some coral species can provide
light management at the tissue scale comparable
to light modulation by colony architecture in
other species. We compared within-tissue scalar irradiance
in two coral species from the same light habitat
but with contrasting colony growth forms:
branching Stylophora pistillata and massive Lobophyllia
corymbosa. Scalar irradiance at the level of the symbionts
(2 mm into the coral tissues) were <10% of
ambient irradiance and nearly identical for the two
species, despite substantially different light environments
at the tissue surface. In S. pistillata, light
attenuation (90% relative to ambient) was observed
predominantly at the colony level as a result of
branch-to-branch self-shading, while in L. corymbosa,
near-complete light attenuation (97% relative to
ambient) was occurring due to tissue optical properties.
The latter could be explained partly by differences
in photosynthetic pigment content in the
symbiont cells and pigmentation in the coral host
tissue. Our results demonstrate that different
strategies of light modulation at colony, polyp, and
cellular levels by contrasting morphologies are
equally effective in achieving favorable irradiances
at the level of coral photosymbionts.
Key index words: irradiance; morphology; photoacclimation;
scale; scleractinian coral; Symbiodinium
Abbreviations: a
chl a, specific absorption coefficient
of chl a; Ddn, diadinoxanthin; Dtn, diatoxanthin;
GBR, Great Barrier Reef; GFP, green fluorescent
protein; Kd, light attenuation coefficient of downwelling
irradiance