This feature was first published in late September 2023 in My Mensa Weekly, our exclusive newsletter for Mensa members. Find out more about becoming a Mensa member here.

Words: Joanna Cummings 

Investigating the biology of bioluminescence – and some of the animals that wear it well 

Are you ready for your glow up? Bioluminescence, the emission and production of light by living creatures, occurs in thousands of species across the world. It is most prevalent in the open seas; it is thought that 75% of deep-sea animals in the pelagic zone – which occupies over 1,300 million cubic kilometres of the ocean with depths of up to 11km – are light-producing. 

Bioluminescence tends to happen through a chemical reaction, whereby a form of luciferin (a light-producing molecule) is oxidised by either the enzyme luciferase or photoprotein. This tends to produce a yellow or green light, though animals that produce luciferin on their own tend to give off a more blue-green hue. Some (as you’ll see with the bobtail squid, below) need a little help from their bacterial friends to produce light, and in some species, researchers still aren’t clear of the exact chemical mechanism. In all cases, less than 20% of the emitted light generates heat, meaning it is considered ‘cold light’.  

Why do animals emit light at all? One theory suggests it evolved in fish as they moved into deeper, darker waters, another posits that it evolved to eliminate oxygen from certain surroundings (as the chemical reaction consumes oxygen).  

Whatever the reason, many organisms certainly put its effects to good use. For some, it is a defence mechanism, for some a lure for prey. In other cases, it’s a form of disguise or a means of communication, and in others still, a way of attracting a mate. Take a closer look at the eight animals below that all ‘give off a glow’, each for their own distinct purposes… 

Fireflies 

Perhaps the most famous of all the glowing creatures in the world, fireflies – also known as glow-worms or lightning bugs – are actually a family of beetles that number more than 2,000 species. As larvae, they emit light as a warning to predators, but adults use it to attract mates. Some even have an array of communication styles at their disposal; as well as being able to emit a ‘steady glow’ they are able to create flashing lights too. Sadly, it’s not as romantic as it sounds – in at least one species, the female eats the male after mating…  

Railroad worms 

The humble railroad worm exhibits not just one, but two colours of bioluminescence – red and yellow-green – both of which act as a kind of ‘burglar alarm’ to keep away predators. The lights come from luminescent organs dotted along their body and on their head, which in the dark makes them look like a train with the internal lights on – giving them their distinctive name. 

Hawaiian bobtail squid 

The light emission from this cheerily named species is the result of quite a complex biological mechanism, and relies on the help of bioluminescent bacteria that live in the squid’s mantle (the pocket of skin covering its body). In exchange for an amino acid and sugar cocktail fed to them by the squid, the bacteria match and emit the amount of light visible on its body, a form of camouflage known as ‘counterillumination’. Now that’s teamwork. 

Motyxia millipedes 

There are very few examples of bioluminescent millipedes, but the motyxia genus carries it off with aplomb. While most are orangey-pink in colour in daylight, they all glow brightly in black light, thanks to the photoproteins in their exoskeleton – and they glow even more brightly when handled. If that isn’t enough to deter predators, they have another trick up their many sleeves: they can produce the poison cyanide. 

Tropical land snail 

Actually known as the quantula striata, these invertebrates are thought to emit light as a form of communication; they flash yellow-green while moving, and also while eating, though the latter is at half the intensity. The light comes from a group of giant cells within the front end of their ‘foot’, though as yet the substance that produces light has not been identified by scientists.  

Anglerfish 

These are the stuff that (bad) dreams are made of. When you hear the term ‘deep-sea fish’, it’s likely that you’ll picture this poster child of the underbite, a bony fish with a distinctive filament growing from between its eyes. It is the species living in the bathypelagic or ‘midnight’ zone (1,000-4,000 metres below the surface) that emits light from this appendage, which it wiggles to mimic prey, thereby luring in its own lunchtime snack. 

Cookiecutter shark 

You have to take your hat off to this little shark, which has a particularly cunning way of attracting prey. The light-producing organs on the underside of the cookiecutter – so named because it sucks out chunks of flesh from larger mammals – illuminates its whole body apart from the darker collar near its pectoral fins, which resembles a small fish. When predators approach the ‘small fish’, the cookiecutter then makes its move. 

Dragonfish 

Some species of these deep-sea fish give off a red glow, highlighting any red-pigmented prey that is usually invisible in the deep-sea environment. Rather chillingly, they have developed black stomach walls to hide the digestion of any light-emitting prey that forms part of their diet, keeping them safe from larger predators. These fish also give a whole new meaning to the insult ‘big mouth’; they can open their jaws to more than 100 degrees, which allows them to eat prey even larger than themselves.  

See bioluminescence at work 

Want to see some light-making in the wild? In this TED talk by biologist and deep-sea explorer Edie Widder, she shares photographs and footage of some of the weird and wonderful bioluminescent creatures she has encountered in her adventures.