VARIATION IN LIGHT INTENSITY CAUSES PLASTIC CHANGE IN HERMATYPIC CORAL MICROMORPHOLOGY

Coral reefs constitute the most diverse marine ecosystem; however, they are also among the most threatened by climate change, ocean acidification, and other anthropogenic sources. Recent molecular analyses have only recently contributed to our understanding of the evolution of the primary reef-framework builders, scleractinian corals, and have shown that 11 of the 16 traditional macromorphological families are polyphyletic. New micromorphological characters (features requiring scanning electron microscopy) have been found to be synapomorphies separating Atlantic and Pacific members of the traditional families Mussidae and Faviidae into clades congruent with molecular phylogeny. Environmental variation in skeletal growth within scleractinian corals is well known, but micromorphological plasticity has rarely been examined. Light intensity, which oceanic depth and water clarity directly influence, is an important determinant in skeletal variation. Here we examine the effect of light intensity on one mussid species (Acanthastrea lordhowensi) and three faviid species (Caulastraea furcata, Favia sp., Cyphastrea ocellina). We conducted environmentally-controlled aquarium experiments using three levels of light intensity (420,360,280 µmol m^-2s^-1) over a period of 60 days. All three species displayed a plastic response within their micromorphology to the differences in light intensity. We measured the variation of micromorphological features in response to differing light intensity and found that the mussid species shows an increase in septal granulation under low light, whereas faviid species show a decrease in the overall thickness of the wall and septa under low light. These trends correspond with a decrease in thickening deposits, which suggests a decrease in the calcification rate. The observed responses differ in magnitude and pattern from micromorphological differences among clades, and support the use of micromorphological characters in phylogenetic analysis.