GLOWING CORALS: Like a forest canopy, the branched corals can scatter and absorb light at multiple levels. In this study, branching corals scattered light over such a large area that the researchers could not measure it with their detector. Coral colonies that form flat structures (not shown), such as Echinopora lamellosa, also have high scattering. Like solar panels, flat structures allow coral to maximize absorption of light coming from above; they also permit scattering of light among the colony’s polyps. This inter-polyp light sharing also occurs in coral species that form massive colonies, which have moderate scattering. The phaceloid corals have low scattering; their thick, upright walls prevent them from sharing light among polyps. (Branched coral: Pocillopora damicornis; massive coral: Orbicella faveolata; phaceloid coral: Eusmilia fastigiata)COURTESY OF SUSANA ENRÍQUEZ


Around 2005, Susana Enríquez discovered “something extraordinary” in her work on blue crust corals. She reported that corals housing photosynthetic algal symbionts absorbed light more efficiently than plant leaves, and that the algae collected significantly more light when inside the coral skeleton than they did when isolated. More light absorption, it was known, would lead to greater algal photosynthesis. In other words, the skeleton somehow made it possible for the algae, which provide corals with glucose and other nutrients, to produce more hydrocarbons with the same amount of sunshine.

Looking into the phenomenon further, Enríquez, a photobiologist at the National Autonomous University of Mexico, found that the coral skeleton enhanced light absorption through scattering. The more light scatters and bounces around within the coral skeleton, the greater the chance it will hit a chlorophyll molecule within an algal cell and stimulate photosynthesis.

Recently, Enríquez and her colleagues tested whether the skeletal architecture of different coral species affects their scattering abilities. Using cleaned museum specimens, the researchers placed a light near each skeleton and measured how much light the skeleton scattered and reflected back to an omnidirectional sensor. Then, as a control, they repeated the experiments with black fabric placed in front of each skeleton. A high ratio between the experimental and control values indicated a coral with high scattering—one that, together with its algae, would likely use light efficiently.

“The corals that were more efficient were the structures that tend to be flat and the branching corals,” Enríquez says. “The structures that were less efficient were the morphologies that are considered more primitive, the phaceloids.” Phaceloid corals are collections of vase-shape individual polyps that grow side by side but do not share nutrients or light. Light-sharing among polyps is possible, in contrast, within the planar and massive colonial corals.

“This stuff is interesting,” says Chris Langdon, a coral researcher at Florida’s University of Miami. He cautions that because the researchers didn’t do experiments with live coral, “they don’t prove anything. . . . But it is thought-provoking and could stimulate [researchers] to look at these things in more depth.”

Enríquez notes that using light more efficiently might not be a harmless benefit, however. Corals that use more of the light that hits them are also more likely than less-efficient corals to suffer light stress, which can cause bleaching, she says. “The evolution of this symbiosis may have defined a trade-off between the development of efficient structures for enhancing light-scattering and then enhancing photosynthesis and calcification and enhancing the growth rates of these organisms, but at the same time increasing the fragility to light stress and, now, to heat stress.”

S. Enríquez et al., “Key functional role of the optical properties of coral skeletons in coral ecology and evolution,” Proceedings of the Royal Society Bdoi:10.1098/rspb.2016.1667, 2017.