The underwater disco effect of UV lights in the dark ocean

If you thought Cayman’s reefs were colourful by day, you’ll want to take a look at them at night, when they’re illuminated with ultraviolet lights.  

We’re talking about the same UV lights that make teeth and clothes glow an eerie shade of white in nightclubs. These disco lights are just taking off in the underwater world and Ocean Frontiers was the first dive shop in the Cayman Islands to invest in special UV dive lights. They don’t come cheap, but the effect is unlike anything you have seen before.  

Looking down at a group of divers from the surface, with their beams of violet light slicing back and forth is a pretty strange sight in itself. Once you reach the bottom and get a close up view of the reef; however, it takes on a science fiction quality: as you pass your light over a coral head, certain corals seem to jump out at you, glowing neon yellow, orange and green.  

Star corals seem to have pinpricks of light shining out from the hundreds of tiny openings in their surface and the anemones look like the tip of each tentacle has a tiny lightbulb inside it. What the UV lights do, as well as generally lighting up the scene, is they reveal fluorescence – this apparent glowing – that occurs naturally in some creatures. Not every creature or coral fluoresces, but those that do really ‘pop’ with the vibrance of the colours.  

Night diving is not everyone’s cup of tea, of course. Jumping into a black-looking sea, some way off shore in the pitch dark goes against some people’s instincts, but there is good reason to do so: night diving opens up a whole different experience of the underwater world.  

By the time the sun has set, the creatures you usually see by day have settled down to sleep. Fish wedge themselves into crevasses between coral heads and lie perfectly still while stingrays bury themselves in the sand. Meanwhile, the creatures of the night come out to feed. Almost as soon as we get to the bottom we spot an octopus, meandering over the reef in search of dinner. Lobsters and crabs emerge from their holes and roam freely by night, and pairs of red glowing eyes alert you to the presence of tiny shrimp.  

On closer inspection of the reef at night you see that almost every coral takes on a ‘furry’ appearance as the polyps emerge, waving minute tentacles around in the hope of catching a microscopic meal of passing plankton. 

Under UV light it’s hard to tell what the normal colours of what you are looking at would be and something you might swim right past in daylight stops you in your tracks when lit up by UV lights: the pattern of ridges in brain coral glow emerald green and the bristles on the fireworms’ legs look like rows of miniature tiki torches.  

Ocean Frontiers was the first dive shop in Cayman to bring in UV flashlights. They look and work just like a regular flashlights but the effect at night is mesmerising.  

The science behind how fluorescence works is a little complex but essentially, certain substances have the ability to receive light of one wavelength and transform it into another so that, rather than simply reflecting the light they receive, they emit a different wavelength of light. This is what we call fluorescence. The most dramatic cases of fluorescence are those in which the received light is almost invisible (such as ultraviolet light) and the emitted light is transformed to a totally different wavelength, such as green.  

In the underwater world, certain corals, anemones, shrimp and other organisms contain a protein called GFP which causes fluorescence. The GFP is not evenly distributed across a coral colony or any one creature so only some corals, and certain spots or stripes on creatures will glow under UV light.  

For those wanting a very different view of the reefs at night, the science is not really important. The point is that it’s like swimming around a reef created by Disney, where the colours are so vivid and the critters so bizarre-looking that you feel you have fallen down the rabbit hole and emerged into a land of pure fantasy. 

Alex Mustard 1

Coral fluoresence as seen under ultraviolet lights. – Photos: Alex Mustard

Alex Mustard 4

Great star coral at night, with polyps extended and feeding, glows green under UV.

Elly Wray UV1

Corals take on a different appearance by night anyway, but when illuminated by UV lights, the difference is dramatic. – Photo: Elly Wray

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