Many species of coral glow — some subtly, some more vibrantly — adding to the mystique of the otherworldly reef habitats that they form. Scientists have known for some time why most of them glow: fluorescent proteins can act as sunblock. For corals living in shallow water, under constant assault from the sun, that's a lifesaver. (Imagine if you were stuck in one spot on the beach with no sun block for your entire life.)

But corals still need sunlight to survive, because they're kept alive with the help of microscopic algae that photosynthesize 90 percent of their nutrients for them. The florescent sunblock provides the perfect filter so these organisms get the sunlight they need to photosynthesize, but not so much that they get scorched by radiation.

This balance shouldn't work the same way for deep water corals, however. These corals still rely on a symbiotic relationship with their algae buddies in order to survive, but there's far less light that reaches down to the depths where they live. They need to maximize the sunlight that they get, not block any of it out. And yet, deep water corals still glow, just like their shallow water relatives. What gives?

A remarkable adaptation

Scientists from the University of Southampton have finally discovered the amazing reason behind the mystery of these shimmering marine animals. It turns out, they're glowing because they're actually producing their own sunlight, according to a press release.

Deep water corals produce a different kind of florescent protein that, instead of acting as sunblock, actually absorbs the blue light of the deep sea and then re-emits it as orange-red light, which is more like sunlight and is easier for the algae to photosynthesize. It's a remarkable adaptation.

The research team also found that brightly red fluorescent corals had a better chance of survival in the long run compared to their non-pigmented counterparts, providing further corroboration that the added luminosity is helping to feed the symbiotic algae.

“This is an important step forward in understanding how the mysterious fluorescent pigments in corals work," said Jörg Wiedenmann, head of the Coral Reef Laboratory at the University of Southampton. "Our finding help us to understand how the amazing diversity of coral colors structures the communities on coral reef.”

The study was published in Proceedings of the Royal Society B.