Key functional role of the optical properties of coral skeletons in coral ecology and evolution

Abstract

Multiple scattering of light on coral skeleton enhances light absorption efficiency of coral symbionts and plays a key role in the regulation of their internal diffuse light field. To understand the dependence of this enhancement on skeleton meso- and macrostructure, we analysed the scattering abilities of naked coral skeletons for 74 Indo-Pacific species. Sensitive morphotypes to thermal and light stress, flat-extraplanate and branching corals, showed the most efficient structures, while massive-robust species were less efficient. The lowest light-enhancing scattering abilities were found for the most primitive colonial growth form: phaceloid. Accordingly, the development of highly efficient light-collecting structures versus the selection of less efficient but more robust holobionts to cope with light stress may constitute a trade-off in the evolution of modern symbiotic scleractinian corals, characterizing two successful adaptive solutions. The coincidence of the most important structural modifications with epitheca decline supports the importance of the enhancement of light transmission across coral skeleton in modern scleractinian diversification, and the central role of these symbioses in the design and optimization of coral skeleton. Furthermore, the same ability that lies at the heart of the success of symbiotic corals as coral-reef-builders can also explain the 'Achilles's heel' of these symbioses in a warming ocean.

Keywords: colonial growth form; coral morphology; coral skeleton; holobiont emergent traits; multiple scattering.

Figure 1.

Light scattered by a branching coral illuminated with a laser beam. Reflected on an acrylic structure, the size of the laser beam can be seen in the lower and right side of the image. (Online version in colour.)

Figure 2.

(a) Mean ± s.e. of the three colonial growth forms analysed in the Faviidae family; (b) mean ± s.e. of two coral skeleton morphologies, chalice and meandroid morphs, within the Faviidae family; (c) box plot describing the distribution of the variability displayed by the type of budding (intra- versus extra-calicular); and (d) box plot describing the importance of coenosteum (plocoid versus cerioid) within each type of budding. Boxes encompass the 25% and 75% quartiles of all the data, the central line represents the median, bars extend to the 95% and 5% of confidence limits. No values beyond those confidence limits were observed. (Online version in colour.)

Figure 3.

Mean ± s.e. of the LEF measured in (a) Moseleya latistellata along a perpendicular axes at the centre of the polyp; 0 is the shallowest area and 1 refers to the deepest centre of the polyp; and (b) Fungia sp. along two axes, one at the mouth of the organism (solid line) and the second within septa (dashed line). (Online version in colour.)