Glowing corals deep in the sea

The dark depths of the Red Sea have been concealing an array of rainbow corals that put on a vibrant show under torchlight. But given that light barely reaches them, why do they respond to it?

The question has researchers scratching their heads. “We were stunned to see these fluorescing colours,” says Jörg Wiedenmann, biological oceanographer at the University of Southampton, whose team first discovered the deep sea coral’s secret. The researchers published their glowing deep-sea coral discovery in the journal PLOS ONE in June.

Shallow-water corals, which live in sundrenched coastal areas, produce green fluorescing proteins as a kind of sunscreen. These proteins soak up damaging UV rays, then re-emit them at longer, less harmful wavelengths.

Wiedenmann saw something similar 50 metres below the surface of the Red Sea. Very few scuba divers ever reach these depths but corals thrive here. When Wiedenmann shone his light on them, he was amazed to see them glow intensely in a range of colours, from yellow and red to blue.

When samples were brought back to the lab and kept in complete darkness, they still fluoresced. So the corals were not using their pigments as sunscreen. “We thought: ‘It must be something else!’” he says. So why do these corals bother producing these glowing proteins at all? A few theories are floating around:

Share with the neighbours: The corals may convert the sparse blue sunlight reaching these depths into other wavelengths that their symbiotic algae can use for photosynthesis.

Lure food: Corals are active predators – their polyps feed on plankton. Perhaps the plankton flock towards the fluorescing coral like moths to a light bulb.

Call for help: Some larger fish help “weed” coral gardens, by grazing on algae to prevent it overgrowing a coral reef. The corals might fluoresce when they sense there’s too much algae in the area, as a way of attracting helpful fish.

The researchers will continue to investigate why the corals glow. The fluorescent proteins are more than a feast for the eyes –they’re potentially useful in the lab. Researchers label cells with fluorescent proteins in petri dishes to track their movements. Pigments extracted from deep-sea corals could add a new palette of colourful proteins to the researchers’ collection.