Photoadaptation by photosynthetic organisms to lowered light intensities occurs in part through changes in pigment concentrations and in characteristics of the photosynthetic response curve. We have characterized photoadaptive responses of light- and shade-adapted colonies of the reef coral
Stylophora pistillata
, which possesses symbiotic algae (zooxanthellae) and grows naturally under a variety of light intensities in the highly cavernous reefs of the Red Sea. Shade-adapted corals have significantly more chlorophyll per individual zooxanthella cell than light-adapted corals (2.98 compared to 12.97 pg chlorophyll
a
per cell), but not a significantly different number of cells per unit area (1.00 × 10
6
cells per square centimetre), with the result that the mass of chlorophyll per unit area is greater for shade-adapted corals than for light-adapted corals. Tissue nitrogen content per unit area is significantly lower (
p
< 0.05) in shade-adapted corals, correlating with a decrease in polyp density (0.10 >
p
> 0.05) in shade forms. These biomass characteristics are concomitant with a variety of functional responses to natural light intensities. Rate of photosynthesis at saturating light intensities is the same per unit area in both forms (20.2 µgO
2
cm
-2
h
-1
for shade specimens; 18.8 for light specimens); but it is significantly different when measured by amount of chlorophyll (1.6 µg O
2
(chl
a
)
-1
h
-1
for shade specimens compared with 5.0 for light specimens). The initial slope of the
P: I
curve,
α
, is significantly
higher
for shade specimens by area (0.21 for shade corals compared with 0.12 for light corals), but significantly
lower
for shade specimens by amount of chlorophyll
a
(0.01 for specimens from shade compared to 0.04 for specimens growing in the light).
I
k
(the point at which maximum production begins) is significantly lower for shade specimens (138 µmol m
-2
s
-1
for shade compared to 273 for light), and likewise
I
c
(the compensation point at which net coral photosynthesis = 0) is also significantly less for shade specimens (30 µmol m
-2
s
-1
for shade compared to 141 for light). The average nocturnal respiration rate is significantly higher for specimens growing in the light (13.9 µg O
2
cm
-2
h
-1
for light specimens compared to 7.6 for shade specimens). Corals in intense sunlight respire at almost twice the rate of shade corals, probably in response to their higher total gross production. Owing to higher production rates and lower respiration rates, integrated
P
c
(gross)/
R
c
(24 h) ratios are greater for shade-adapted specimens either in direct sunlight (1.76
P/R
for shade specimens in the light compared to 1.10 for light specimens in the light), or in the shade (0.43 for shade specimens in the cave compared to 0.10 for light specimen in the cave). By using previously defined equations and biomass assumptions, it can be shown that light-adapted
Stylophora pistillata
can acquire all of their basal metabolic carbon through photosynthesis and translocation, but that shade-adapted
Stylophora
colonies growing in shade acquire slightly less than half. These results also show that if there were no photoadaptive response, shade-adapted specimens would acquire less than 4 % of their carbon from photosynthesis
Longevity strategies in response to light in the reef coral Stylophora pistillata
