Diving Guide to the World of Shimmering Fluorescence

Tube Anemone Fluoresce
Scientists are baffled why some marine creatures such as this tube anemone fluoresce

Concealed beneath the surface is a world of shimmering fluorescence. Scientists are starting to unravel the complex functions of these spectacular pigments. Louise Murray investigates

The muted brown and yellow colouration of a coral reef hides a spectacular secret – gem-like fluorescent colours in psychedelic greens, reds, oranges, yellows and reds. Bright sunlight overwhelms the visibility of these pigments, but they are easily seen while diving at night using a blue-light torch – it’s a light show to rival Oxford Street at Christmas. The source of the day-glo colours is fluorescent pigments embedded in the tissues of the corals and in some fish. These proteins absorb high-energy, shorter-wavelength light and then re-emit the light at the lower-energy, higher wavelengths. Once ignored by scientists as a charming but irrelevant curiosity or quirk of light, new research has shown that the glowing jewel-like fluorescent colours have many different roles and are perhaps critical to the overall health of the reef ecosystem and its ability to respond to stress. 

Galaxea fascicularis fluorescent pigments

It is now thought that for hard corals such as this Galaxea fascicularis the fluorescent pigments act as a sunscreen


Scientists are beginning to unravel the purpose of these photo proteins and it is clear that there is more than one explanation for their presence, and that the story is quite different between fluorescence in corals and its appearance in fish and other reef creatures. 

Professor Jörg Wiedenmann at the Coral Reef Laboratory of the University of Southampton has been studying the pigments in corals for more than 20 years. ‘Some act as sunscreens in the shallows, others as light enhancers at depth,’ he says. ‘It has taken some time to uncover the purpose of these pigments because they have different roles at different depths and there is a great deal of variation between individuals. You can see brightly coloured corals right next to colourless individuals of the same species. To begin with, it was a confusing and often contradictory picture.’ 

Not all species of shallow-water corals produce these fluorescent pigments, and not even all members of the same species will produce the same colour and quantity of fluorescent pigments. In the shallows, green fluorescent pigments often predominate. The amount produced by any individual is determined at the genetic level and is triggered by light levels and by the number of copies of the gene that are present. 

fluorescence of tassled scorpionfish

One theory for the red fluorescence of the tassled scorpionfish (Scorpaenopsis oxycephalus) is that it is a form of camouflage

When the reef is not under stress, the cost of producing this extra protein may be detrimental for those corals that do produce a lot of fluorescent pigments, causing them to grow more slowly than their less colourful neighbours. But when temperature and high light levels put the corals and their symbiotic algae under environmental stress, then the pigments can become positively advantageous, forming a sunscreen, or light umbrella, shielding the symbiotic zooxanthellae from the detrimental effects of excess sunlight. The less pigmented corals will respond to the stress by losing their algae and become bleached. 

‘The good news is that the variability that we find in pigment production levels means that in shallow-water reefs there will be at least some individuals who will suffer less under high stress conditions,’ said Wiedenmann. ‘At the reef crest that is subjected to the highest levels of light stress, we often find corals with the highest levels of fluorescent pigments’. 

Wiedenmann and his team believe that the metabolic cost of the production of photo-protective pigments by the corals might be relatively neutral between about 10m and 30m, where there is sufficient light for their algal symbionts. At greater depths, the possession of the fluorescent sunscreens might represent a disadvantage that slows down the growth of strongly pigmented corals. However, at the edge of the range of photosynthetic coral’s growth, where light levels are low and are mostly in the blue wavelength, different types of fluorescent pigments become more prevalent. Here, red and orange fluorescent pigments are more common. They might confer a growth advantage to those corals that produce the bright fluorescent proteins by amplifying the little light available to the zooxanthellae cells and improve their ability to photosynthesise and transfer sugars and other carbon compounds as food to the coral host.

hermit crab fluorescence

A hermit crab, Similan Islands, Andaman Sea, Thailand

giant moray eel fluorescence

Giant moray eel (Gymnothorax javanicus) hunting at night, Lembeh Strait, Sulawesi


Having collected many types of fluorescent proteins from coral samples in the field, Wiedenmann brought them back to the lab and has been able to produce purified solutions of the dye molecules in greens, yellows, pinks, reds and oranges. Each colour is produced by a single gene, which can be isolated from the coral and transferred into the cells of other organisms. If it is attached to a protein, the fluorescent pigment functions as a light label to allow biomedical researchers to track proteins or gene activity in living cells, improving our understanding of how they work. Fluorescence dyes like these are now used in a wide range of applications – from biomedical science, to develop and test new anti-cancer drugs, to imaging the internal structures of living cells with super-resolution. 

Whatever the colour, these all belong to the same molecular family as the first green fluorescent pigment   (GFP) isolated from the jellyfish, Aequorea victoria in the 1960s. In 2008 the Nobel Prize for chemistry was awarded to the scientists Osamu Shimomura, Martin Chalfie and Roger Tsien for their discovery of this GFP, and their development of its use as a marker in cell biology and medical research.

commensal shrimp fluorescence

Commensal shrimp (Periclimenes amboinensis) on host crinoid, Lembeh Strait, Sulawesi. The function of the shrimp’s orange/pink fluorescence remains a mystery

thorny seahorse fluorescence

Thorny seahorse (Hippocampus histrix), Lembeh Strait, Sulawesi


Many land animals, from birds to spiders, use fluorescent proteins to boost their natural colours and sex appeal. On land, peacocks and budgerigars use fluorescent pigments to make themselves more colourful, and more appealing to the opposite sex. The colours are so important that if you coat the cheeks of a wild budgerigar with sun screen, they immediately lose their sex appeal to potential partners. Underwater, some fish have fluorescent patterns on the parts of their bodies used in courtship displays. This might be part of the story for fish such as the pipefish, who mate for life, but there are probably several different reasons for the widespread appearance of these photo pigments in some species of fish, crustaceans, eels and the seahorse family.

Fish have no symbiotic zooxanthellae to protect, so why do some fish glow red? The fact that red light is progressively absorbed by seawater and that red objects appear grey or black below about 15m, has long led fish biologists to believe that the colour red is not important to fish and that they are not even able to see it. But German evolutionary ecologist and diver, Professor Nico Michiels, from the University of  Tübingen discovered that many fish fluoresce orange and red. ‘I was diving in the Red Sea with a filter on my mask to block all but red light, to see just how far down the red was visible,’ he says. ‘Because red light is absorbed by seawater and disappears entirely by about 15m down, biologists had long assumed that deeper-water fish could not see red. Once my eyes got used to the gloom, I could see red light sparkling everywhere. This had to be fluorescent red light, but the big question was why the red should be so widespread.’ 

mushroom coral fluoresces

The mushroom coral, (Fungia sp) fluoresces brightly at night

porcelain crab

A porcelain crab feeding inside the safety of its host Pocillopora coral branches, is lit by the bright green fluorescence of the coral. Tondoba Bay, Marsa Alam, Red Sea, Egypt

Red light does not travel very far through the water, so one theory is that fish are using the pigments to signal to each other, particularly members of the opposite sex, without giving away their position to any possible predators in the neighbourhood. 

‘Red-eyed gobies have conspicuous small fluorescent red markings around their eyes and swim around in little swarms, and we think that they use red light signals to indicate where they are and keep the swarm together,’ said Michiels.

Other fish seem to use the coloration as a way of hiding themselves, by breaking up their outline – at least five species of the normally cryptically camouflaged scorpionfish family fluoresce brilliantly in red and orange, with some juveniles shining out a strongly coloured red. They have a patchy fluorescence that helps them to blend in with the blotchy fluorescence of algal chlorophyll on the rocks and parts of the reef where they lie in wait for their unsuspecting prey. 

Michiel’s research also found that some species of small fish, such as triplefins and gobies that feed on small-eyed prey, show red fluorescence around the eye area. He and his team hypothesise that this could help in hunting and prey detection where the fluorescence is strong enough to produce a revealing eyeshine in their tiny prey animals.

But there are still many instances of glorious light shows in marine life where the purpose of these beautiful pigments remains unknown. The reason for the psychedelic display on the rhinophores and skirts of some nudibranchs, or why some hunting giant moray eels fluoresce a bright orange, or a common seahorse fluoresces red, where some other individuals do not fluoresce at all, remains a mystery. But that will not stop me visiting the reefs of the world with my blue lights. Every diver should experience the night time reef like this at least once. 

reef hermit crab fluorescence

Reef hermit crab (Dardanus lagopodes) fluoresces orange and green, while its home shell is orange  Perched on top of Pocillopora coral.

mushroom anemone fluorescence

A mushroon anemone  (Amplexidiscus sp), Lembeh Strait, Sulawesi

Night Fever

Many resorts now keep blue lights and the appropriate yellow mask filter that will allow you to explore the reef in vibrant fluorescent colours at night. If, like me, you want to do a lot of night diving, here are some tips to help you select a night-diver friendly resort. First, bear in mind that most guides will have been working all day and are usually hungry and fairly keen not to get wet again and to eat a late dinner. Often, if the resort requires a guide to accompany you, the only member of staff willing to do so may be lacking in experience. I had one memorable dive in Egypt being guided towards the (best avoided) Saudi Arabian coast. Twice. Your ideal resort should have its own self-guided house reef which you should opt to explore in the daylight first before choosing to navigate it at night with blue lights. It will look very different and it is easy to get lost. A compass is a very good idea. It’s best to contact the resort beforehand to make sure that they can accommodate you. Many resorts will only put on one or two night dives a week. The ideal is to have the option to mix self-guided house reef dives with night boat dives for a bit of topographical variety.

polycerid nudibranch fluorescing

A polycerid nudibranch (Nembrotha kubaryana) fluorescing on the gills and rhinopores and margin

Where to fluo dive

Malaysian Borneo

Celebes Beach Resort on the eastern Borneo island of Pom Pom has a house reef packed with unusual cephalopods and other rarities. www.celebescuba.com


Onong Resort on Siladen Island inside Bunaken Marine Park and its sister resort Mapia on the main island both have a great house reefs for night diving just off their jetties. www.celebesdivers.com

Wakatobi in Sulawesi has a world-class house reef just in front of the resort. www.wakatobi.com

Lembeh Resort has an excellent house reef plus boat night diving is also offered. Great for macro critters and muck diving. www.lembehresort.com

Red Sea

Marsa Shagra resort has a superb house reef and boat drop-offs can be arranged to cut down on swimming. www.oonasdivers.com/region-egypt/15-resorts/marsa-shagra

South of Marsa Alam, Tondoba Bay has a beautiful and easily accessible house reef. www.blueheavenholidays.com


The sheltered and shallow waters of the Similan Islands Marine National Park have many excellent night dive sites and can be accessed by night-diving friendly liveaboards. www.scubacat.com





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