A light at sea

An astonishing variety of ocean creatures can generate light, and there’s as much diversity in how they do it, as why, writes LISA-ANN GERSHWIN

Visitors to the Uffizi Gallery in Florence, Italy, may be so overcome with emotion in the presence of great art that they sometimes experience fainting, hallucinations, and palpitations – a cluster of symptoms known as Stendhal Syndrome. But these feelings of intense wonder and awe are not just limited to famous paintings and sculptures.

An eerie glow where the waves break. A flash beneath our feet at the water’s edge. Tiny dots of light on our skin. The balmy summer heat reminds us that it’s beach-time, and those who linger after dark are often rewarded with visions of great delight. Glows of light. Flashes so bright.

Bioluminescence, or “living light”, has intrigued humans for thousands of years. Aristotle wrote about sparks in the sea, marvelling at how these fires were not extinguished by water.

Bioluminescence is light made within the bodies of organisms. This cool light – in both meanings of the word – gives off very little heat, and it’s fascinating. The chemical reaction is similar in principle to that of a glow stick: two chemicals meet in the presence of oxygen, resulting in light. In organisms, these two substances are substrates generically known as luciferins and enzymes generically known as luciferases (both derive their names from the Latin lucifer: light-bearer).

Different colours of light are produced by changes in the polarisation and microclimate of the luciferins, as well as by mixing and matching with structurally different luciferases. These different colours are generally associated with habitat, with pelagic and deep-sea species generally emitting blue light, coastal marine species more often shifted toward green, and terrestrial and freshwater species typically shifted into yellow. There are deviations, however, such as the terrestrial railroad worm (Phrixothrix hirtus), which displays two rows of yellow-green lanterns along its body, plus two red lanterns on its head.

At least 11 different types of luciferins are known, along with dozens of associated luciferases, suggesting that bioluminescence has evolved independently many different times on the tree of life. In fact, recent studies have estimated this figure to be at least 94 times, and one study found that light production has evolved at least 27 different times in marine fishes alone. Luciferin-like proteins can even be found in some non-luminous species, and can be stimulated to produce light by exposure to luciferases.

Bioluminescence is more common than you might think. A 2017 study by the Monterey Bay Aquarium Research Institute, in California, found that bioluminescence occurred in more than 75% of the deep-sea animals observed over 17 years of video recordings. Indeed, in the inky darkness of this vast and alien realm – often called the twilight zone – bioluminescence is the dominant form of light.

Bioluminescence is found in more than 700 genera, including terrestrial plants, animals, fungi, and bacteria, and notably almost every marine phylum. So, if bioluminescence is so prevalent, it begs the question: why? And here is where the story becomes even more fascinating.

Many soft-bodied organisms give off a sudden flash when disturbed. The comb jellies (in the phylum Ctenophora) are flashers extraordinaire. There’s Beroe, a pocket-shaped creature with teeth, whose fiery luminescence lights up its internal organs like the strobe of a skeleton at a Halloween party. Cestum and its diminutive cousin Velamen are belt-shaped creatures that swim essentially sideways, undulating two ‘arms’. They, too, are brilliantly luminescent in split-second responses to threats. Even Mnemiopsis, one of the world’s most invasive species, reveals flashes through the body. Surprisingly, one of the phylum’s most delightful creatures, Pleurobrachia, which performs a series of twirls and somersaults to get food from its tentacles to its mouth, is one of the few incapable of glowing.

Flash luminescence is also common in cnidarian jellies (the ones that sting, stemming from the Greek cnida, meaning nettle). For example, many siphonophores – a group of strange colonial jellies including bluebottles – have evolved ways of using light as scare-tactics, lures, or mimics of other species. One of the most fascinating is the deep-dwelling species Erenna, which characteristically twitches red-glowing filaments as a lure for small fish.

Another remarkable medusa is the elegant crystal jelly Aequorea victoria, which flashes a brilliant blue along its internal canals and the margin of its bell. Aequorea is better known for tiny fluorescent spots at the base of its tentacles that glow almost radioactive green under ultraviolet (UV) light; research on these proteins led to the 2008 Nobel Prize in Chemistry. Intriguingly, instead of the typical luciferin–luciferase system, Aequorea uses a photoprotein to generate its bioluminescence. A photoprotein is a stable complex of a protein and a substrate, wherein the luminescent reaction is activated by calcium or iron ions rather than oxygen. More than 10 photoproteins are currently known.

A variation on this single flash is a propagation of flashes, such as in the pyrosomes (Latin for fire body), strange colonial tubular sea squirts. When one member flashes, it triggers its neighbours and so on, creating a ripple-on-a-pond effect. These flashers are thought to act as a burglar alarm, scaring away would-be predators. Other variations on these flashes include the deep-sea crown jellyfish Atolla, in which the flash races around the body in a circular pattern, like a Formula One car on a racetrack, or the Santa’s hat jelly, Periphylla, in which the whole body twinkles.

Crustaceans are also masters of bioluminescence, displaying either as a glowing cloud or as a steady internal or external light. For example, the sesame-seed-shaped ostracods, known as “fireflies of the sea”, vomit up bioluminescence, creating a distraction in the water.

During World War II, the Japanese army issued packets of ‘sand’ to soldiers (actually dried ostracods). The soldiers would pour a bit of this sand into the palm of their hand, spit on it to activate it, then rub their hands together to have enough light to read their maps without giving away their location.

Some prawns also create a glowing cloud, but this is less of a vomit and more of a spit. Either way, these clouds act as a smoke screen, allowing the organism under threat to escape undetected.

Another group of crustaceans, known as amphipods, are as dazzling in their luminescence as they are in their morphology. Forms range from little orange species the size of a pea, to ones the size of a wasp with a single elongated pointy eye that takes up half the body length, to ones even more elongated into the size and shape of a toothpick, to ones slightly larger than an avocado and so transparent that you can read newsprint through their body. Euphausiids, or krill, have tiny light organs called photophores distributed along the body.

Many fish are also able to glow. Some fish have photophores, which appear as lines of small silvery beads in species-specific patterns. Photophores are particularly common in the most abundant vertebrates on the planet – the fish-fry-sized bristlemouths – as well as in lanternfishes: big-eyed, charcoal-coloured, goldfish-sized, and of a disturbingly gelatinous consistency. If you’re a deep-sea fish, no good can come from spending time with a species that can eat you or that you can’t mate with, so photophore patterns may be one way to tell who’s who.

One particularly striking glow strategy is that of the anglerfishes. Like something out of science fiction, these have a stalk protruding from their forehead bearing a cluster of bioluminescent wormy-looking appendages. Smaller predatory species are attracted by the lure of these glowing ‘worms’, which look like food. The anglerfish gapes its enormous mouth, and the predator becomes the prey.

A variation on this is the monstrous-looking dragonfish, where the lure dangles from the chin, under a mouth so full of teeth that the fish can’t even close its grin to conceal its weapons.

In the flashlight fishes, light organs filled with luminescent bacteria blink on and off beneath the eyes, controlled by the fish to shut off the light at will with opaque ‘eyelids’. Bony fishes, however, aren’t the only ones who glow. Among the cartilaginous fishes, numerous types of sharks can also emit light, with at least four species able to do so spontaneously. The widely distributed kitefin shark (Dalatias licha), which inhabits sea-floor waters up to 600m deep and grows to nearly 2m in length, is the largest vertebrate – on land or in the sea – known to luminesce.

Many less glamorous species can also glow. For example, gossamer worms are segmented like earthworms, but their bodies are gelatinous and transparent, like jellyfish, with dozens of paddle-like legs they use for swimming. These worms emit yellow bioluminescence as a flurry of sparks from the tips of their legs. Yellow bioluminescence is very rare in the deep sea. We are still at the dawn of understanding the biochemistry and biology underlying this unusual colour production; however, it’s believed to occur through activation of nicotinic cholinergic receptors and calcium channels, and has been demonstrated to be under nervous control. The reason for this atypical expression is also unclear: the light has been interpreted by various authors as a deterrent, an attraction, an escape/defence response, or – as it’s thought that other deep-sea species are largely unable to see yellow – perhaps a means of private communication between worms.

Another strange glowing creature is Enypniastes, a swimming sea cucumber. Normal sea cucumbers spend their lives sitting or crawling on the seabottom, but Enypniastes drifts along, aided by an umbrella-like veil, occasionally dipping down to pick up a mouthful of sediment before rising back up into the water column, where it digests the edible particles from among the sand grains. With minor disturbance, Enypniastes luminesces at the site of irritation, but if broadly impacted, the animal sloughs off its whole skin in a glowing mass.

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Possibly the coolest of all cool-light species are the squids. Consider, for example, Vampyroteuthis infernalis (one of the best Latin names ever, translating to “vampire squid from hell”). Picture a quivering gelatinous mass of dark-red goo (appearing jet-black at depth), with huge hauntingly blue eyes and with a web-like cloak covering its eight arms, each bearing numerous sharp hooks instead of suckers.

Close up, Vampyroteuthis looks like a bloodthirsty villain, but in truth it is actually a timid little creature. It lives in the parts of the deep sea where oxygen is at its lowest, known as the Oxygen Minimum Zone. Here, with nowhere to hide, and more vulnerable to predation than it looks, it has two different bioluminescence modes. In response to mild stimulation, the animal responds with glowing or flashing bright blue light at the ends of all its arms simultaneously. If stimulated more strongly, a viscous fluid laced with glowing particles is emitted from the tips of the arms. Movement of the arms disperses the particles, creating a glowing cloud around the animal.

Amazingly, some squids have the ability to sense the downwelling light and match it quite perfectly with their own light, so that other organisms beneath them are unable to see their silhouette. They can even change their output on the fly, for example dimming slightly to match a cloud passing overhead. These squids are actually using bioluminescence for camouflage.

Most of us don’t have access to remotely operated vehicles or submarines to visit these denizens of the deep, but we can still see bioluminescence in action. Many zoos and aquariums exhibit flashlight fish with their blinky eyes or Aequorea in both its bioluminescence and fluorescence modes.

But the most wondrous experience is to play with sea sparkle at the beach. Not always, but often, summertime beaches come alive at night with blue vibrance akin to the signature hue of Avatar.

Sea sparkle is actually tiny spherical microalgae called Noctiluca scintillans – yet another splendid Latin name – nocti– meaning night, –luca meaning light, and scintillans meaning scintillating, glittering, or sparkling. The sparkling night light!

Noctiluca appears as unattractive pink slicks on the water during the day. Next time you see these slicks, come back at night. Bring a water bottle with a sports cap, so you can control the water flow. After dark, spritz some water on it, to elicit the magnificent blue flash. You may want to jump in and dance in the glow, and that’s okay too. It isn’t harmful to us.

Noctiluca embodies both beauty and beast. Beauty, in the sheer brilliance of its light. Unlike many types of nature’s luminescent displays, it’s easy to observe Noctiluca with the naked eye. And beast, in that Noctiluca is an oxygen-sucking, ammonia-exuding, voracious pest.

I’m not being unkind. Noctiluca is an unusual type of algae that doesn’t photosynthesise, so rather than making oxygen, it obtains it from the water. In bloom concentrations, Noctiluca can strip the water of oxygen, leaving heavy breathers like fish and crustaceans gasping. And without photosynthesis, it can’t make its own food, so it eats other organisms to survive. Again, in bloom conditions, it can vacuum up all the phytoplankton and even small zooplankton like microcrustaceans, fish eggs, and fish fry. Worse, as it ages it releases ammonia into the water. Often, these blooms result in mass fish deaths.

Nutrient pollution from urban runoff and aquaculture, combined with increasingly warmer waters, mean Noctiluca is doing more of what it does best. Don’t blame Noctiluca, however: it’s just responding to the world we’re creating.

And remember: if you are lucky enough to encounter sea sparkle on the type of low-energy beach where small waves swirl around rocks, its sheer beauty may cause Stendhal Syndrome.   

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