The first time you spot a glowworm, you might do a doubletake. Its eerie yellow-green light seems so alien, it almost doesn’t look like something that belongs on planet Earth. In the UK, you’re most likely to see the common glow-worm, found across Europe and Asia. In the northern hemisphere, ‘glow-worms’ are not worms at all, but beetles from the family Lampyridae. The females don’t have wings and look similar to the larvae. During the day, the female hides underground. At night, she crawls up a plant stem and turns on the light in her abdomen in the hope of attracting a passing mate.
In Australasia, the term ‘glow-worm’ refers to the larvae of little flies that feed on fungi. Some of these fungus gnats are carnivorous, luring prey with a blue-green glow emitted from a light organ at the end of their body. The bioluminescent spectacle of the lethal Arachnocampa luminosa larvae draws tourists from around the world to sites such as Waitomo Cave in New Zealand.
Glowing in the dark is obviously useful for predation or communication for all sorts of species – whether on land at night or in the deep sea where there is little or no sunlight. But the majority of bioluminescent creatures live in the ocean depths – it is estimated that up to 90 per cent of life creates some form of light in the gloomy twilight zone, 500–1,000m down. Researchers think bioluminescence is more prevalent in the oceans because it evolved much earlier in marine species than terrestrial ones. Bioluminescence is also rare in freshwater environments, probably because these habitats developed later and so biodiversity is lower, and fresh water is often filled with sediment, so glowing lights are less visible in the murkier conditions.
Bioluminescent creatures produce light through a chemical reaction inside their bodies. The key molecule involved is luciferin. ‘Lucifer’ means ‘light bringer’ in Latin; the term ‘luciferin’ was coined by 19th-century chemist Raphaël Dubois, while he was working on clock beetles and bivalve molluscs.
Luciferin reacts with oxygen when catalysed by one of two enzymes – luciferase or photoprotein. When luciferin undergoes this reaction, an electron becomes ‘excited’ and moves to a higher energy level. When luciferin relaxes back to its normal state, it releases light. Different organisms contain different types of luciferin.
Bioluminescence has evolved independently many times throughout history and all across the tree of life – from fireflies to fungi to fish. “One of the most striking findings in fish in the deeper open ocean is that the species in this environment are overwhelmingly bioluminescent,” says biologist W Leo Smith, associate professor at the University of Kansas, who has researched the evolution of bioluminescence. “From our work, we can see that more than 80 per cent [of fish species] in this habitat are bioluminescent, so it is obvious that bioluminescence is practically a required adaptation.”
Many bioluminescent creatures synthesise luciferin themselves, such as tiny marine organisms known as dinoflagellates, which, gathered en masse, can make the sea glow at night. Other organisms absorb luciferin, either from their food or from symbiotic relationships with other creatures. These ‘symbionts’ have a close biological relationship with one another (either both creatures benefit; one creature benefits while the other comes to no harm; or one is a parasite that causes some harm to the host but doesn’t kill it). For example, the Hawaiian bobtail squid hosts the bioluminescent bacteria Vibrio fischeri in its ‘light organ’. The bacteria benefit by being nourished by the squid, while their glow camouflages their host, enabling it to blend in with the moonlit sea and cast no shadow on the seabed.
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