The ocean’s own light

Sunlight fades quickly in the ocean. A few hundred metres down, the water is dim blue; deeper still, it becomes darker than any night on land. And yet that darkness is not empty of light. Animals there make their own.

Light from a reaction

Bioluminescence is light produced by a living organism using a chemical reaction. NOAA Ocean Exploration explains that all bioluminescent organisms use a reaction between an enzyme and a substrate to make light. Different groups use different chemicals — a sign that the ability may have evolved independently many times.

It is worth separating bioluminescence from two other ways living things can glow. In fluorescence, an organism absorbs light and gives it off again in a different colour. Phosphorescence works in a similar way, but the glow lasts longer after the light that triggered it is gone — like the hands on a glow-in-the-dark watch. Bioluminescence needs no light to start with: the organism makes it from scratch.

Rare on land, everywhere in the deep

On land, bioluminescence is unusual: think of fireflies and some fungi. In the ocean it is common. NOAA reports that about 80 percent of the animals living in the open water between 200 and 1,000 metres deep are bioluminescent.

The list of glowing animals is long. It includes fish, squid and what scientists call gelatinous zooplankton — jellyfish, siphonophores and comb jellies. One striking example is the deep-sea shrimp Heterocarpus ensifer, which appears to "vomit" a cloud of light from glands near its mouth.

Why blue?

Most marine bioluminescence is blue. That is no accident. Blue is the colour of light that travels best through seawater, so it carries furthest in the deep. Some organisms produce light from near-violet to green-yellow, and, very rarely, red.

What the light is for

Making light costs energy, so it must be useful. Scientists think bioluminescence helps animals survive in several ways: finding food, helping with reproduction, and defending themselves. A sudden flash or cloud of light may startle or distract a predator, while a glowing lure can draw prey closer.

But NOAA is careful to note that the main purpose of bioluminescence is still not well understood. In a place as hard to reach as the deep ocean, watching animals behave naturally is extremely difficult — and many of their light signals remain a mystery.

The light at the surface

Not all bioluminescence is deep. On some coasts, waves breaking at night can glow blue, and footprints in wet sand can light up. Much of this light comes from tiny marine plankton that flash when the water around them is disturbed. It is one of the few ways most people can see the ocean’s own light without a submarine.

Why it matters

Bioluminescence turns our picture of the deep sea upside down. The largest living space on Earth is not completely dark: it is full of flashes, glows and signals we are only beginning to understand. Each expedition into that darkness is a reminder of how much of our own planet remains unexplored.