When you think about where light comes from, your mind probably goes straight to the almighty sun, or perhaps a household lightbulb. But one of the more interesting places that you can find luminescence is out amongst the plants and animals.  If you look very carefully,  preferably at night, you’ll find that there are species on our planet that can produce their own light through bioluminescence. This ability is spread apparently randomly across the tree of life, and has evolved over 40 separate times!

The most famous of these bioluminescent organisms is probably the firefly; even as someone who has never seen a firefly in real life, I’m well aware of their existence. However, despite being a land dweller, the firefly is not very representative of most bioluminescent organisms, the majority are actually marine creatures.

The generation of light holds a somewhat mystical position in my imagination, despite the fact I’ve studied it in-depth. This feeling is heightened when things are glowing, especially if they’re things which at first glance maybe shouldn’t, like a mushroom. Of course, light generation is not mystical and bioluminescence is actually pretty well understood.

How do they do it?

While there are a few physical processes that can generate light, bioluminescence is a byproduct produced during an exothermic chemical reaction. In an exothermic reaction excess energy is released to the environment, often in the form of heat. However, in the case of bioluminescence, the excess energy is light. This process is called chemiluminescence and in biological systems, it is achieved via an oxidation reaction wherein the chemical at the centre of the reaction, called the substrate, loses an electron.

The substrate, although it may have various chemical structures depending on the organism in which it is found, is called luciferin.  Its oxidation is assisted by an enzyme called a luciferase or a photoprotein, producing light which can be different colours. For example, if you were to visit the deep sea you’d see mostly blue flashes and glows. On land, bioluminescent critters tend to use yellowy greens.

The picture I’ve drawn above hopefully illustrates this spread of colours as well as the fairly diverse range of creatures that use this unique ability to achieve a variety of goals. Having first drawn them I, of course, had to do a bit of research to find out what makes each creature unique.

Fireflies

First, let’s take a look at the firefly, the one we’re all familiar with. Fireflies belong to the family Lampyridae and are in fact not flies but beetles! Within this family, there are many luminescent species which glow at different stages throughout their life-cycle. Often the larval eggs are luminescent, and sometimes one or both of the adults. Because of this variation, there are also differences in what the luminescence is used for. Commonly, its purpose is for males and females to find each other for mating.

For example, adult males and females of the ‘blue-ghost’ firefly (Phausis reticulata), found in the United States, use long-lasting blue glows to find each other among leaf litter on the forest floor. Only the male firefly has wings and will fly low to the ground searching for the faint glow the female produces.

Such a long glow is in contrast to another method of mate location practised by many firefly species who flash with precision. In some, this leads to the particularly intriguing phenomenon of synchronisation wherein adult fireflies spontaneously synchronise their flashes with hundreds of others nearby, making whole trees appear to pulsate rhythmically with light.

A particularly poetic article, investigating this was published in 1968. The authors travelled to Thailand to study the firefly species Pteroptyx malaccae. Using a series of experiments, they deduced that fireflies use a similar method to humans to achieve synchronicity. Like us, fireflies appear to anticipate the correct moment to flash, based on several of the previous flashes of close neighbours. Think of the way that you would match your claps with those of a partner, sometimes it can take several too-fast or too-slow claps before you’re perfectly in time.

Firefly luminescence can also be used for goals other than mate location. For example, in some species, only the larvae glow, presenting an intriguing mystery for some time. Research in the early 2000s however, suggested that larval glows are used in a similar way to the bright colours of poisonous frogs — acting as a deterrent to predators.

Another scientific puzzlement was how fireflies can use their glow all night, in some cases continuously, even though adults fireflies don’t eat. Surely the supply of luciferin would run out at some point? Not so! Recent research was able to model the entire chemical process of light generation, it showed that once oxidised, luciferin can, in fact, be regenerated and used again, allowing fireflies to search for a partner until the wee hours.

I must admit I’d love to live somewhere that had native fireflies, their nighttime displays seem utterly magical and I’m sure videos, although enchanting, don’t do justice to the experience.

Fungi

The bulk of my drawing is made up of fungi, and they were actually the organism which inspired this post. I saw an article about luminous fungi which had a moment on science media websites a few years ago. Scientists had completely characterised the genetic basis for bioluminescence in the fungus Neonothopanus nambi. This was exciting development because until then only bacterial bioluminescence had been similarly characterised. If scientists wanted to use these genes in new organisms, via genetic engineering, they were limited to bacteria in most cases.

Answering this question about how fungi generate luminescence leaves another yet to be fully resolved: the reason fungi glow at all. Some bioluminescent fungi glow continuously, day and night, while others only emit light after the sun goes down.

Additionally, which part of the fungus glows can vary as well. Above the log, I’ve drawn fungi which glow from their fruiting bodies — the classic ‘mushroomy’ part. The fruiting bodies are the part of a fungus involved in sexual reproduction and they glow when the fungus is ready to create its spores. However, I’ve also included a glow on the mycelium which covers the log. This is the main part of the fungal organism and it may also glow.

To understand the purpose of fungal luminescence, scientists consider these characteristics of where and when the glow occurs, as well as the environment in which particular species live. However, a lack of consistent evidence led to a growing consensus that fungal luminescence actually originated as a byproduct of oxidation reactions that serve another purpose. This is also true for bacterial bioluminescence where bioluminescence might have evolved as a byproduct of molecular processes to repair DNA and prevent cellular damage.

There are some species, however, where it appears that what might have started out as coincidental luminescence, has become useful. For example, in habitats where there is not much wind, glowing fungi may attract insects that can carry their spores far and wide in the same way that bees carry flower pollen. Research on the fungus Neonothopanus gardneri supports this, showing that insects are attracted to luminescent replicas of the fungus during the night when their glow is present.

However, experimental work on the ghost fungus Omphalotus nidiformis, which is the flatter of the two fruiting bodies I’ve drawn, did not have the same attractive properties; it seems there are still fungi species for which bioluminescence is probably just a beautiful byproduct. Personally, I find this suggestion quite lovely; the glowing is merely a coincidence, there to be appreciated for its aesthetic value and nothing more.

In the ocean

So far the organisms I’ve discussed can be seen from land. However, bioluminescence is actually a relatively common phenomenon in the ocean. A recent study which looked at 17 years worth of observations from all ocean depths, determined that between 69-78% of all the organisms that were observed were bioluminescent! Many of these exist at extreme depths, where sunlight does not reach, so bioluminescence makes a great way to communicate, be it for attracting a mate, fending off predators, or to help with hunting.

Plankton

Despite the fact that most marine bioluminescence is found way down in the water column, the first marine glow on my drawing is something you can see from land: twinkling waves. This is not some strange trick of the light or chemical contaminant, rather the sparkling that can be seen on some shorelines is created by tiny organisms called plankton.

Bioluminescent plankton can be enjoyed all over the world,  including a number of bays in Puerto Rico, Mission Bay in San Diego, and the coasts of Wales. In this video, plankton glitter in blue pinpricks on the shoreline of the Isle of Anglesey in North Wales. It’s pretty clear why these tiny organisms are often referred to as Sea Sparkle.

‘Plankton’ is a general term for any drifting organism, whether a plant or animal, and there are many different kinds that can be bioluminescent. However, the most common is a dinoflagellate called Noctiluca scintillans.

The blue glitters produced by N. scintillans are a response to agitation of the surrounding water, as would be produced by its predators. The sparkle is a protection mechanism, acting to both startle would-be attackers, as well as attracting larger predators that might pick them off first.

The bright blue flashes are created along a network of fine strands that bridge the single-celled, bubble-like structure of  N. scintillans. The large ‘open’ space inside this cell makes it buoyant, ensuring that it always returns to a position near the surface of the water. This fun property might help it come in contact with prey, which it eats by engulfing them through an opening in its cell wall which acts like a mouth. It directs prey, caught in a sticky slime, towards this ‘mouth’ using a flagellum, or tail-like structure.

In some strains, N. scintillans also gets food via a symbiotic relationship with a microalgae called Pedinomonas noctilucae. As a result, these strains are green, instead of the typical red. Red N. scintillans are more widely distributed throughout the world’s oceans, whereas the green variant is restricted to warmer climates which are around 25-30 °C. Both variants can be found in the eastern, northern and western Arabian Sea, but abundance is seasonal — the green variant is more common in winter and red in summer.

While Sea Sparkle is extremely pretty, its presence isn’t always pleasant. In fact, the prevalence of N. scintillans has increased dramatically in a number of places around the world as a consequence of global warming and pollution. For example, in Sam-Mun-Tsai Beach, Hong Kong large blooms have been seen, described as ‘tomato soup consistency‘.

As you can probably imagine, in large quantities this thick goop is not good for marine life – killing fish by depleting the water of oxygen and excreting toxic levels of ammonia. Blooms have also been seen in Australia in places which have recently become warm enough to accommodate them thanks to climate change. The bioluminescence of N. scintillans has actually come in use for tracking these hazardous blooms, and more recently satellites have taken advantage of their red hue to look at huge areas of the ocean at once.

Squid

Below the sparkling waves, I’ve drawn three examples of bioluminescent organisms from the deep. The cephalopod is Watasenia scintillans, or the firefly squid. This small creature (only 7.6 cm long on average), is found in Toyama Bay, Japan and has become a draw for tourists between March and June when they come to shallow waters to spawn.

Squid, like other marine animals, make their bioluminescence in special organs called photophores. These organs can be highly complex and contain cells called photocytes which create the light, as well as components for reflecting and directing it to best achieve the desired effect.

Squid use their photophores for multiple functions. For example, the squid Taningia danae has photophores at the tips of one set of tentacles. Observations of this species have shown they use these photophores just before attacking prey, suggesting that the light provides a stunning effect or perhaps can be used to determine exactly how far away the prey is before they strike. T. danae also uses a long glow in a behaviour which appears to show curiosity, perhaps indicating that they also use bioluminescence to signal to each other.

Squid may also use bioluminescence for a clever process called countershading. At night, some squid, including Watasenia scintillans, migrate up from the depths to feed. The moonlight filtering down from above makes their shadowy outlines visible to predators below. This poses a danger as many predators at this depth have upward-facing eyes which they use to sneakily identify prey from the shadows. (For an absolutely amazing example of upward-facing eyes, take a look at the Barreleye fish, it deserves its own article!)

But, bioluminescence comes to the rescue for these vulnerable squid. Photophores on their undersides can be used to camouflage against overhead illumination. With just the right amount of bioluminescence, they can match the light from above so that predators can’t see them!

Angler Fish

As well as squid, there are many bioluminescent fishes in the deep ocean. Some can create their own luminescence, while others rely on a symbiotic relationship with bioluminescent bacteria. Anglerfish, the left-most fish in my drawing, use the latter approach to light up a self-made lure which juts right out of their forehead!

The particular anglerfish I’ve drawn is a black seadevil from the family Melanocetidae,  which is only one type of 11 different families that make up the 160 species of ceratioid anglerfish. These frightening-looking fish exist in the bathypelagic zone — the realm deeper than 1000 m — and the bioluminescent lure on their heads is called an esca. In addition to the esca, some anglerfish in the family Linophrynidae, also have a luminescent chin barbel.

Quite fascinatingly while the esca contains bioluminescent bacteria, the chin barbel of this genus is actually produced by the fish itself! 

Although the black seadevil is quite odd-looking, many other species of angler fish have an esca or barbel that is highly branched and intricate, making them look stranger yet. Complexity carries through to the structure of the photophore itself, which, like those in squid are intricate and can contain light-guiding tubes, reflectors, and pigmentation. The strength, direction and visibility of the light produced by the esca is controlled by these structures as well as surrounding muscle.

Lots of scientific research has been done to try and figure out how the bacteria get into the esca.  It seems that they come from the environment through a pore which links the inside of the light organ with the outside water. After the fish has metamorphosised into its adult form, the pore opens and bacteria make their way inside to a hospitable environment. Some species also have esca-like structures along their spine which can release luminescent bacteria into the surrounding water.

Another very interesting aspect of anglerfish is that all these unusual physical attributes are only observed in females because all anglerfish species exhibit extreme sexual dimorphism; males and females look very different and in fact, have quite different lives! In the most extreme case, seen in the species Ceratias holboelli, male anglerfish can be half a million times lighter than the female. The male of C. holboellis, as well as many species, melds with the female, becoming essentially an extension of her body.

It appears that in some anglerfish species, where the male has large eyes, bioluminescence is important for this extreme mating behaviour. However, in others, males have degenerate eyes and perhaps instead use well-developed ‘noses’ to track pheromones released by the females.

Dragonfish

The last animal in my illustration is a dragonfish — Idiacanthus fasciola –from the Stomiidae family. While it may have an impressive name this fish is fairly small, reaching a maximum of 35 cm in length.

I chose to draw this particular species because on first glance it’s the most interesting looking of the dragonfishes. However, I have since learned that the genera Aristostomias, Malacosteus, and Photostomias are perhaps yet more interesting. Species in these genera are unusual amongst deep-sea fishes in that they produce near-infrared, or red light, instead of the more common blue luminescence.

Blue light penetrates well through water (think of the blue of the ocean or a swimming pool), so most deep-sea fish both produce and see a very narrow band of blue or purple tones. By making red light, these unique dragonfish have a secret system for looking at their surroundings, allowing them to view other nearby fish without them knowing.

These special dragonfish species use their stealthy surveillance system to the fullest extent,  devouring prey using fascinating mouths which have earned them the name ‘loosejaw dragonfish’.  This is because their well-toothed jaws, which may take up 20% of their body length, can be opened very wide, greater than 100° in some cases.

On top of this, the bottom jaw doesn’t have a floor, instead, there’s just a gap, split by a narrow bone, which opens up the inside of the mouth to the outside water. But why you might ask? What advantage could be gained from having a hole in your mouth?

Researchers considered these questions by modelling how fast these loosejaw dragonfish are able to close their jaws, compared to the more customary jaws of other dragonfish which do not have an opening. As you might guess, drag forces make closing a very long jaw difficult to do quickly, and it’s even harder when the jaw begins its journey at such a large angle. But! By having a hole, the dragonfish is able to reduce drag and close its jaw fast, allowing it to catch prey that can be up to half its own length. (I recommend checking out this open-access article, not least for the great images.)

These interesting bioluminescence and feeding characteristics, however, are only seen in female dragonfish, which exhibit sexual dimorphism similar to the anglerfish. For example, I. fasciola males have large eyes and lack a functional gut, teeth, pelvic fins and a chin barbel.

These differences extend to dragonfish photophores; 17 out of the 24 dragonfish genera have sexual dimorphism of the photophores. While dragonfish have photophores over many parts of their bodies, sexual differences are restricted to the those around the eye and in some, the barbel as well. In all these cases but one, the photophores on the female are smaller, or not present at all.

Not just a pretty sight

Bioluminescence is a fascinating phenomenon, and while it is extremely beautiful, it’s also being used in the laboratory. The genetics which produces luciferase in animals such as the firefly can be genetically engineered into other organisms so that light is created in parallel with whatever scientists are investigating. This provides a way to actually see where and when certain processes take place in individual cells and even within living organisms.

One interesting use for this is to genetically engineer bioluminescent bacteria, viruses or other single-celled organisms to track the progress of infectious diseases like salmonella or malaria. This is useful because infectious diseases are usually monitored by killing infected animals to look inside them. With bioluminescence, the infection can be monitored while the animals are alive and overall far fewer animals need to be euthanised.

I began this article because I’d seen some pictures of bioluminescent fungi online and thought them beautiful and full of whimsy. But I’ve discovered that bioluminescence is far more common than I had thought, and plays a huge role in ecosystems, especially the deep sea. It’s also another natural phenomenon which scientists are taking great strides in mimicking to aid technological innovation; one of my favourite ways to do science. I’ve certainly come away from this article with a new appreciation and love of all things that glow!