Corals are pretty, colourful and fluorescent. They produce their vibrant colours because they do not live alone, which is also what keeps them alive.
Over billions of years, they have worked out a special arrangement with algae: Corals give them shelter and algae convert light into food for the corals. Deep inside their tissue are little proteins that take the sun's ultraviolet light and turn it into a glowing green sunscreen, shielding from the sun these corals that live just below the water's surface. Corals do other things for the algae too.
But deeper in the water, it is dark and the little light that reaches that far down is only in the blue part of the spectrum. Somehow, there are corals that live far below the surface and also manage to glow burning hues of orange and red.
The reasons for this fluorescence have remained a mystery until now: These deep-sea corals glow to get more sunlight, according to a study published in the journal Proceedings of the Royal Society B. Their proteins soak up the scarce light and shine it back out as red-orange light that penetrates deep inside their tissues where their microscopic roommates take up residence. This means there is light for photosynthesis, and the algae creates energy and food for the coral. "This is a strategy that some corals pursue to cope with the challenges of a low-light environment," biologist Joerg Wiedenmann of Britain's University of Southampton, who led the study, wrote in an e-mail.
It is quite an adaptation, with a brilliant by-product.
The research could have implications for coral-reef conservation by highlighting how different species of coral adapt to various light conditions. For two decades, scientists have considered the idea that deep-sea reefs might provide a safe haven for shallow-water corals during threatening times of extreme heat. The thought is that shallow coral larvae pulled down by currents could survive long enough to reproduce and send their offspring back near the surface when temperatures returned to normal.
But "the depth might not offer a convenient escape road," Dr Wiedenmann said. He said he worries that shallow-water corals may not be able to adapt to the little light down deep.