Bioluminescence explained: glowing seas, fireflies and fungi

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A wave breaking blue in the dark, a forest floor speckled with tiny lights, a beetle shining in the grass on a summer night: bioluminescence is the production of light by living things, and it is one of nature's most magical sights. Here is how it works, where to see it, and how to enjoy it without harming the organisms that make it.

How living things make light

Most bioluminescence comes from a chemical reaction between a light-emitting molecule, called a luciferin, and an enzyme, called a luciferase, in the presence of oxygen. The reaction releases energy as light rather than heat, which is why it is sometimes called cold light. Different groups of organisms have evolved their own versions of these chemicals, so the colour and behaviour of the light vary. In the sea, blue and green are most common, because those colours travel furthest through water.

Why glow?

Organisms use light for many purposes, and in some cases scientists are still working out why.

Attracting a mate. Fireflies and glow-worms use light signals to find partners.
Defence. Some plankton flash when disturbed, which may startle predators or attract bigger animals that eat whatever is attacking them.
Hunting. Some deep-sea fish use glowing lures to draw prey close.
Camouflage. Some sea creatures light their undersides to match the faint light from above, hiding their silhouette from predators below.
Spreading spores. One idea is that glowing fungi attract insects that carry spores away, though this is not settled for every species.

Glowing seas

The sparkling blue light in breaking waves is usually caused by microscopic plankton called dinoflagellates. One common species, Noctiluca scintillans, is sometimes called sea sparkle. When the cells are jostled by waves, oars, swimmers or footsteps on wet sand, they flash blue-green. Seeing it depends on luck and conditions:

Plankton blooms happen when water temperature, nutrients and calm weather line up, often in warmer months.
Darkness is essential. Go on a moonless or cloudy night, away from street lights, and give your eyes at least 20 minutes to adjust.
Sheltered bays with narrow openings can hold dense concentrations, which is why some places around the world are famous for glowing water.

Some bloom-forming plankton can produce toxins or irritate skin, so check local advice before swimming, and never rely on a glowing bay as a sign the water is clean.

Fireflies and glow-worms

Fireflies are not flies but beetles. In much of North America and Asia, males flash coded patterns while flying, and females answer from the ground. In Britain the best-known glowing insect is the common glow-worm, also a beetle. The adult female, which cannot fly, climbs a grass stem and glows a steady greenish light to attract flying males. Males, larvae and eggs give off much weaker light. To see glow-worms in the UK, look on summer nights in chalk and limestone grassland, hedgerows and woodland edges, typically starting after dusk. Local wildlife trusts often run guided glow-worm walks.

Glowing fungi

Some fungi glow faintly green, an effect long known as foxfire. The light often comes from the fungal threads inside rotting wood rather than from the mushrooms themselves, and it is easiest to see in very dark, damp woodland after your eyes have fully adjusted. It is usually too dim for the naked eye to appreciate quickly, and long-exposure photographs show it far more clearly than you will see it.

Enjoying it responsibly

Avoid bright torches. Use a red light and keep it pointed down.
Do not collect glow-worms or fireflies. Their light is for finding a mate, and a jar ends that.
Leave rotting wood where it is. It is home to many other species.
Keep artificial light down at home. Outdoor lighting is thought to interfere with glowing insects' courtship.
Stay safe on the coast at night. Check tides, go with someone and tell others where you are.

Have you ever seen glowing waves or glow-worms? Share where and when in the comments.