The Marine Creatures Lighting Up the Darkest Corners of the Ocean
When we picture the ocean, often all we see is a large expanse of blue with the only other splashes of colour coming from busy coral reefs, the shells which lie on the tideline or the odd shoal of fast-moving fish. When we think about the deepest parts of the ocean, all we picture is darkness. Yet the oceans that surround us are surprisingly colourful and even the most remote depths can be filled with light.
It is thought that 76% of all ocean animals have the power to become bioluminescent, whilst when we look at the deep sea alone, that number rises to 90% of creatures, with scientists frequently discovering more species which can glow in the dark. Bioluminescence is the ability to generate and emit light through chemical reactions which require oxygen and many animals and plants have evolved to develop this skill to help them camouflage, communicate, attract mates or scare off predators. Whilst we have bioluminescent creatures on land, such as fireflies, glow-worms and even mushrooms, the number of glowing species in the sea is much higher. This is thought to be due to several reasons including the fact that marine species have been living on Earth for millions more years than terrestrial species, giving them plenty of time to adapt and evolve.
Another reason why bioluminescence is so prevalent in the ocean is because its vast blue waters simply leave no place for species to hide. Near coastlines we have rocky outcrops, tangled seagrass beds and complex coral reef systems, but in the majority of the ocean, wide open space stretches as far as the eye can see. Some species have adapted to this by only emerging from their hiding places at night and use the cover of darkness to hunt for food. Others, such as jellyfish, have slowly evolved into transparent ghost-like creatures. Did you know that many transparent sea creatures actually have opaque guts? This is because if they snack on luminescent prey, their stomachs will start shining a brightly coloured neon light too! Others, like sardines, have evolved to have silvery sides which act as a mirror reflecting the water around them and help their silhouettes disappear. Many however, have chosen invisibility as their superpower and use light as a way to camouflage themselves into the surrounding sea.
Bioluminescent species can cleverly adapt when they shine, how brightly and sometimes they can even choose what colour to illuminate the water around them. This allows them to merge into the seawater no matter its colour which can change depending on location, weather, time of the day, depth and cloud cover. The Hawaiian bobtail squid does just this and in fact, uses bacteria living in its organs as a way of producing light. The bacteria and the squid have a mutual arrangement where the bacteria have a safe place to live and receive nutrients from the food eaten by the squid whilst the squid uses the bacteria’s light to camouflage into the ocean. The squid can control the direction of the light and its brightness which allows its underside to match the exact shade of light illuminating the water above it. This means that any predators lurking below struggle to see any tasty creatures floating above. Often, these squid use the light at night to camouflage into the moonlight and prevent their shadows being cast into the water below.
However, not all creatures shine to fit in. Others aim to dazzle and distract their predators so they can escape to safety whilst some other poor critter is eaten instead. Atolla wyvillei is the Latin name for the alarm jellyfish which uses light to escape predators. When caught by a hungry hunter, the jellyfish sets off what is known as a ‘burglar alarm’; an intensely bright light which can be seen for up to 300 feet! Many animals associate light with food and so upon seeing a bright light, larger animals rush to the area to feed on the original predator, hopefully distracting it for long enough so that the jellyfish can sneak away. This burglar alarm method is commonly used by smaller creatures which cannot make a quick getaway. It seems they opt for neither fight nor flight but light!
Other ocean-dwelling creatures opt instead to send out glimmering particles as a way of distracting their predators or sometimes even leave behind an entire glowing tentacle as a decoy. Whilst you may not recognise the name dinoflagellates, they are actually the most common species that uses bioluminescence and depending on where you live, you may have even seen them. Dinoflagellates are microscopic plant-like critters which are typically found in warm ocean bays in areas such as the Caribbean. Movement triggers these organisms to flash a blue light and because these creatures float in clusters of millions, it often causes large patches of the sea to glow a magical blue hue. Sometimes this spectacle is caused simply by a large wave or other times it can be a fish moving through the crowd. In order to create this light, the dinoflagellates use a naturally occurring molecule called Luciferin. This is named after Lucifer who was known as ‘the light bearer’. Luciferin is actually any molecule which reacts with oxygen and in doing so, emits energy in the form of a photon, or more simply, a flash of light. For this reaction to take place, Luciferase is also needed to kick start the chemical process.
Predators too use light in cunning ways and the anglerfish is possibly the most famous example. The anglerfish uses light as bait to attract other organisms which it then attacks with its spear-like teeth. Other hunters use searchlights and the clever ones use a red light as most deep sea creatures cannot see the colour red. This allows the predator to illuminate its prey whilst its unsuspecting meal has no idea anything is swimming towards it.
Finally, bioluminescence is also used for mating. The bigfin reef squid is one of the most common squid species that is served as calamari around the world. However, it is actually a very interesting animal as the male puts on a lightshow to protect his mate. When the female is laying eggs, the male hovers above her in the water and his underside is a calming white colour. Yet his upper body puts on a firework display of multicolour flashing lights in order to ward off any lurking predators. Tiny crustaceans known as ostracods also use bioluminescence in their mating practices but in a less romantic way. The male seeks to attract a partner by vomiting blobs of glowing mucus! Each species has a different display of vomit with some opting for spirals and others for zigzags. Similarly, many ocean species also use their luminous light to attract partners as each species has its own distinct light which acts almost like a name tag.
As you can see, there are so many weird and wonderful creatures living in our oceans that most of us have no idea about. In fact, scientists have really only explored a tiny fraction of our ocean and there are undoubtedly even more wacky critters living out there in the vast expanses of blue. Technology is now allowing us to move away from exploring shallow reefs and the odd deep sea vent as we send remotely operated vehicles (ROVs) out into the ocean. These vehicles are complete with cameras, lights, sensors, robotic arms, arms for digging the seafloor and small plastic buckets which are capable of keeping unusual specimens alive for the journey back to the surface so we can investigate them a lot more closely.
By studying bioluminescence in nature, scientists have actually learnt an incredible amount and are now using similar techniques within multiple disciplines. For example, bioluminescent bacteria are used to identify dangerous toxins in water supplies whilst bioluminescence can also create light proteins which are used in tracking the spread of cancer and HIV and also helping us understand cell damage caused by Alzheimer’s disease. If you want to look far far into the future, scientists are even using the pieces of DNA that cause bioluminescence in mushrooms to make normal plants glow in the dark. In an extraordinary vision for a sustainable planet, it is thought some of these plants could be used to line streets and pavements to help replace artificial light! Many scientists have said that we should take more examples from the environment as nature is able to create advanced systems which our technology still cannot and it seems the darkest corners of our oceans are only hiding even more incredible secrets which are waiting to be discovered.
By Neve McCracken-Heywood