What is bioluminescence?
Glow-worms, along with their close relatives the fireflies, are perhaps the most famous members of a diverse array of creatures that glow in the dark – a phenomenon that is known as bioluminescence.
The world is lit up with bioluminescent jellyfish, insects, molluscs, crustaceans, millipedes, fish, fungi and micro-organisms. Australia is home to a 2m-long glowing earthworm, Terriswalkeris terraereginae. In fact, the only major groups without any glowing members are the flowering plants and the terrestrial vertebrates.
How does bioluminescence work?
Central to the chemistry are molecules called luciferins (from the Latin for ‘light-bearers’). These emit light in an oxidation reaction equivalent to ‘burning’ – but with virtually all of the energy released as light rather than heat.
Nature boasts a great variety of luciferins, each producing light of a different wavelength. Glow-worm luciferin emits green light, for example, while coelenterazine, the commonest such compound found in marine creatures, produces a blue glow. Others emit shades of yellow or red.
But luciferins are only part of the story. Despite using the same compound as glow-worms, fireflies produce yellow light rather than green. The difference seems to be due to an enzyme called a luciferase, which acts as a catalyst for the oxidation. Slight variations in the glow-worms’ and fireflies’ luciferases are thought to produce the different shades seen.
The glow of cephalopods and fish is often produced by bioluminescent bacteria with which the host has a symbiotic relationship. Several species of squid house bacteria in photophores – organs that may incorporate lenses, reflectors or even shutters to control when, how much and in which direction light is emitted.
As for glow-worms, we don’t know how they control light emission – perhaps by limiting the flow of oxygen to luciferins in their photogenic organs.
Why do glow worms, and other animals, glow?
Nature’s light shows are put to a variety of ingenious uses: for attack, defence, courtship or social cohesion. Glow-worms tick two of those boxes. The flightless female uses it as a beacon to attract winged males, while the toxic larvae flash to warn would-be predators to keep away.
Bioluminescence is also used aggressively by some fireflies. The females of Photuris spp. – known as ‘femme fatale fireflies’ – prey on the males of other species (notably Photinus spp. and Pyractomena spp.) by mimicking their courtship displays in order to lure them close on the false promise of mating. They then kill and eat the unsuspecting males.
Bioluminescence in fungi may serve to attract insects that disperse spores, while the glow of the Jack O’Lantern mushroom may simply be caused by waste products accumulating in its fruiting bodies.
Main image: a glowing firefly © Getty Images